Developmental	Systems	Theory	as	a	Process	Theory1 Paul	Griffiths	and	Karola	Stotz Abstract Griffiths	and	Russell	D.	Gray	(1994,	1997,	2001)	have	argued	that	the	fundamental	unit	of	analysis	in developmental	systems	theory	should	be	a	process	–	the	life	cycle	–	and	not	a	set	of	developmental	resources and	interactions	between	those	resources.	The	key	concepts	of	developmental	systems	theory,	epigenesis	and developmental	dynamics,	both	also	suggest	a	process	view	of	the	units	of	development. This	chapter explores	in	more	depth	the	features	of	developmental	systems	theory	that	favour	treating	processes	as fundamental	in	biology	and	examines	the	continuity	between	developmental	systems	theory	and	ideas	about process	in	the	work	of	several	major	figures	in	early	20th	century	biology,	most	notable	C.H	Waddington. 1.	Introduction Developmental	Systems	Theory	(DST)	builds	on	a	long	tradition	of	ideas	about	the	systems	nature	of development	amongst	biologists	and	psychologists,	predominantly	from	workers	in	the	field	of developmental	psychobiology.	Elements	of	DST	are	derived	from	C.	H.	Waddington's	ideas	about developmental	systems	(Waddington	1952)	and	the	'epigenotype'	(Waddington	1942),	from	Daniel Lehrman's	influential	critique	of	the	idea	of	innate	behavior	(Lehrman	1953),	from	Gilbert	Gottlieb's theory	of	probabilistic	epigenesis	(Gottlieb	2001),	and	from	Susan	Oyama's	The	Ontogeny	of	Information (Oyama	1985).	Developmental	psychobiologists	Donald	Ford	and	Richard	Lerner	integrated	many	of these	ideas	into	a	formal	framework	in	their	book	Developmental	Systems	Theory	(Ford	and	Lerner	1992). DST	analyses	development,	heredity	and	evolution	in	a	way	that	avoids	dichotomies	of	nature	versus nurture,	genes	versus	environment,	and	biology	versus	culture.	In	this	framework	development (ontogeny)	is	the	reconstruction	of	a	life	cycle	using	resources	passed	on	by	previous	life	cycles.	DST takes	heredity	to	encompass	both	the	stability	and	plasticity	of	biological	form,	which	are	complementary aspects	of	the	recurrence	and	modification	in	each	generation	of	a	system	of	genetic,	epigenetic	and exogenetic	developmental	resources	(Stotz	and	Griffiths	In	Press).	The	prime	focus	of	a	DST	account	of evolution	is	the	life	cycle,	the	series	of	events	that	occurs	in	each	generation	of	a	lineage.	The	process	of evolution	is	the	differential	reproduction	of	variant	life	cycles.	The	end	of	one	life	cycle	and	the	beginning of	the	next	is	marked	by	the	reconstruction	of	the	various	mechanisms	that	allow	the	life	cycle	to 1	To	appear	in:	Daniel	J.	Nicholson	and	John	Dupre	(eds)	Everything	Flows:	Towards	a	Processual Philosophy	of	Biology.	Oxford,	NY:	Oxford	University	Press reproduce	itself	from	relatively	simple	resources.	The	replication	of	genes	is	simply	one	aspect	of	the replication	of	a	life	cycle.	Many	classes	of	developmental	resource	are	replicated:	genes,	methylation patterns,	membrane	templates,	cytoplasmic	gradients,	centrioles,	nests,	parental	care,	habitats	and cultures	are	all	at	least	partly	constructed	by	past	generations,	and	interact	to	construct	future generations. Developmental	systems	theory	attracted	the	interest	of	philosophers	of	biology	in	the	1990s,	mostly	in response	to	the	work	of	Susan	Oyama	(Godfrey-Smith	2000;	Gray	1992;	Griffiths	and	Gray	1994;	Moss 1992;	Robert,	Hall,	and	Olson	2001).	However,	whilst	most	scientific	work	in	the	developmental	systems tradition	was	on	behavioral	development,	including	child	development,	philosophical	discussion	of	DST focused	on	its	implications	for	'gene	centered'	views	of	molecular	developmental	biology	and evolutionary	biology.	In	this	vein,	Kim	Sterelny,	Michael	Dickison,	and	Kelly	Smith	(1996)	proposed	to assimilate	developmental	systems	theory	to	the	replicator/interactor	view	of	evolution	of	Richard Dawkins	(1976)	and	David	Hull	(1988).	They	suggested	that	the	evidence	and	arguments	used	to	support DST	could	be	accommodated	by	the	concession	that	there	are	some	non-genetic	replicators	in	addition	to the	genetic	replicators.	Evolution	is	the	result	of	competition	between	an	extended	class	of	replicators. The	'extended	replicator'	approach	was	rejected	by	developmental	systems	theorists	Griffiths	and	Gray (1997),	who	pointed	to	some	paradoxical	consequences	of	trying	to	describe	developmental	systems	and their	evolution	in	a	replicator	framework.	A	developmental	system	includes	a	'developmental	niche'	that contains	reliably	inherited	developmental	resources	needed	to	reconstruct	that	developmental	system	– or	to	modify	it	in	the	case	of	phenotypic	plasticity.	Some	of	the	resources	that	make	up	the developmental	niche	are	actively	constructed	by	the	parents,	such	as	breast	milk	or	the	incubation mounds	built	by	male	Brush	Turkeys	(Goth	2004).	Others	are	constructed	by	the	activity	of	many conspecifics,	and	not	only	the	parents,	like	the	flock	structure	required	for	normal	behavioural development	in	Cowbirds	(West	and	King	2008).	But	some	merely	persist,	independent	of	the	activities of	previous	generations	of	the	developmental	systems,	like	the	territories	inherited	by	male	Scrub	Jays from	their	fathers	(West,	King,	and	Arberg	1988,	49).	All	these	resources	are	potentially	part	of	the evolved	developmental	system:	"There	is	a	fundamental	similarity	between	building	a	nest,	maintaining one	built	by	an	earlier	generation,	and	occupying	a	habitat	in	which	nests	simply	occur	(for	example,	as holes	in	trees).	In	all	three	cases,	there	may	be	an	evolutionary	explanation	of	the	interaction	of	the	nest with	the	rest	of	the	developmental	system"	(Griffiths	and	Gray	1994,	291). Griffiths	and	Gray	used	examples	of	habitat	and	host	imprinting	to	show	that	one	lineage	can	outcompete another	even	when	the	feature	in	which	the	two	lineages	differ	falls	into	the	category	of	'merely persistent'	resources	(Griffiths	and	Gray	1994,	288-290).	They	reiterated	this	point	in	1997	to	rebut	the objection	that	habitat	and	host	associations	themselves	do	not	evolve	by	natural	selection,	only	the behaviors	that	produce	habitat	and	host	associations.	This	objection	fails	because	populations	with exactly	the	same	behavioral	mechanism	for	habitat	or	host	imprinting	may	differ	in	their	level	of	fitness because	of	the	specific	persistent	resource	to	which	they	(re)establish	a	relationship:	"In	cases	of	host imprinting	in	parasitic	insects	or	cuckoos...	much	of	the	rest	of	the	parasite	group's	evolution	may	result from	the	success	of	lineages	with	one	relationship	rather	than	another"	(Paul	E	Griffiths	and	Gray	1997, 485). The	relationship	between	a	developmental	system	and	a	persistent	resource	can	explain	aspects	of	both development	and	evolutionary	success.	But	the	persistent	resource	itself	is	not	replicated	in development,	nor	does	it	increase	its	representation	relative	to	alternatives	as	a	result	of	the evolutionary	success	of	the	system,	so	the	persistent	resource	cannot	be	treated	as	a	replicator.	What	is replicated,	and	may	increase	in	representation,	is	the	relationship	between	system	and	resource. Griffiths	and	Gray	(1997)	argued	that	treating	these	relationships	as	replicators	independent	of	their relata	would	be	a	reductio	ad	absurdum	of	the	replicator	concept. DST	offers	a	less	paradoxical	treatment of	persistent	resources: "We	conceive	of	an	evolving	lineage	as	a	series	of	cycles	of	a	developmental	process,	where	tokens of	the	cycle	are	connected	by	the	fact	that	one	cycle	is	initiated	as	a	causal	consequence	of	one	or more	previous	cycles,	and	where	small	changes	are	introduced	into	the	characteristic	cycle	as ancestral	cycles	initiate	descendant	cycles.	The	events	which	make	up	the	developmental	process are	developmental	interactions	events	in	which	something	causally	impinges	on	the	current state	of	the	organism	in	such	a	way	as	to	assist	production	of	evolved	developmental	outcomes." (Griffiths	and	Gray	1994,	291)2 A	process	ontology	for	DST	allows	it	to	reconcile	two	otherwise	paradoxical	facts:	some	components	of the	evolved	developmental	system	persist	without	reference	to	the	rest	of	the	system,	but	the	presence of	these	components	in	the	system	can	be	explained	by	natural	selection.	These	facts	cease	to	seem paradoxical	if	we	focus	on	how	life	cycles	–	processes	rather	than	systems	of	entities	reproduce themselves	and	on	how	variant	life	cycles	reproduce	themselves	more	or	less	effectively. It	is	the developmental	process	that	replicates	itself	across	the	generations,	making	use	of	persistent	resources	as well	as	resources	created	by	earlier	cycles	of	that	process.	Re-establishing	or	breaking	a	relationship	to	a 2	This	proposal	has	a	parallel	within	the	replicator	tradition,	namely	G.	C.	Williams'	view	of	the	organism as	a	region	of	space-time	structured	by	evolved	information	(Williams	1992).	By	this	stage	of	his	career Williams	had	abandoned	the	idea	of	objects	competing	to	replicate	themselves	in	favour	of	the differential	replication	of	information,	an	approach	that	lends	itself	more	easily	to	a	process	perspective, and	which	would	seem	to	accommodate	the	replication	of	relationships. persistent	resource,	for	example	by	becoming	imprinted	on	a	new	habitat,	is	an	event	that	is	part	of	a developmental	process. 2.	Process	biology Griffiths	and	Gray	argue	that	the	fundamental	unit	of	analysis	in	DST	is	a	developmental	process.	This process	is	better	described	as	a	life	cycle,	since	it	encompasses	the	entire	period	between	conception	and death.	Does	this	mean	that	DST	is	a	form	of	process	biology?	In	this	chapter	we	will	argues	that	it	does.	As a	first	step	we	discuss	the	inspiration	that	many	early	20th	century	biologists	drew	from	process philosophy,	and	ask	which	if	any	of	their	ideas	correspond	to	those	found	in	DST.	A	direct	link	between DST	and	these	earlier	thinkers	is	the	embryologist,	developmental	geneticist	and	theoretical	biologist Conrad	Hal	Waddington	(1905-1975)	whose	ideas	have	been	cited	by	many	advocates	of	DST.	In	The Evolution	of	Developmental	Systems	(1952)	Waddington	describes	his	understanding	of	physiology, developmental	and	evolution	as	nested	processes: Biologists	have	always	been	forced	by	their	subject	matter	to	take	time	seriously.	But	it	is	only gradually	that	they	have	realised	to	the	full	the	necessity	always	to	consider	living	things	as essentially	processes,	extended	in	time,	rather	than	static	entities. ... The	day-to-day	activities	of	living	things	are	carried	on	by	processes	which	occur	anything	from	a fraction	of	a	second	to	an	hour	or	two...	[the	organism]	is	gradually	carried	along	through	another series	of	processes,	those	of	development,	each	phase	of	which	occupies	a	time	which	is	fairly	long compared	with	the	life-cycle.	Each	life-cycle	is	again	nearly	circular,	and	gives	rise	to	new	animals, the	descendant	generation...	The	accumulation	of	such	changes	gives	rise	to	the	slow	process	of evolution,	which	is	on	a	still	larger	scale	than	either	the	physiological	or	developmental	ones. (Waddington	1952,	155) Waddington	was	deeply	influenced	by	the	process	philosophy	of	Alfred	North	Whitehead,	as	were	many of	his	British	contemporaries,	such	as	biochemist	Joseph	Needham,	theoretical	biologist	J.H	Woodger, and	the	leading	Cambridge	behavioral	biologist	W.H.	Thorpe	(Abir-Am	1987;	for	Thorpe's	views,	see Thorpe	1956).	Whitehead's	influence	on	biology	was	equally	strong	in	Australia,	including	the	Nobel prize	winning	immunologist	Frank	Macfarlane	Burnet	(Anderson	and	Mackay	2014),	geneticist	and ecologist	L.	Charles	Birch	(Birch	1965)	and	geneticist	Wilfrid	E.	Agar.	It	was	Agar	(1882-1951)	who introduced	Australian	biologists	to	process	philosophy,	through	works	such	as	'Whitehead's	Philosophy of	Organism:	an	Introduction	for	Biologists'	(Agar	1936).	Agar	sought	to	convince	his	fellow	biologists that	process	philosophy	was	a	good	framework	for	biological	research	and	so	his	work	is	particularly useful	when	we	ask	what	aspects	of	this	older	enthusiasm	for	process	thought	are	relevant	to	DST. There	are	some	aspects	of	Whitehead's	thought	that	were	important	to	these	biologists,	but	which	have little	relevance	in	the	current	context.	Whitehead	rejected	the	idea	that	the	mental	must	be	explained	in physical	terms,	insisting	instead	on	a	monistic	'panexperientialism'.	Agar	noted	that	"The	conception	that the	world,	including	the	physical	world,	is	composed	of	entities	which	are	"drops	of	experience"	or feelings	will	seem	to	many	people	a	strange	one"	(1936,	18).	He	hastened	to	reassure	biologists	that,	"We must	bear	in	mind	that	'feeling'	is	here	used	throughout	as	the	purely	general	term	for	any	kind	of	acting or	being	acted	upon,	in	such	a	way	that	the	make-up	of	the	subject	is	affected"	(1936,	18;	quoting	Emmet 1932,	142).	Through	these	aspects	of	his	system	Whitehead	offered	biologists	a	way	to	make	room	for consciousness	in	the	physical	world,	as	well	as	a	route	to	reconcile	science	and	religion,	and	for	many biologists	this	was	an	important	source	of	their	attraction	to	Whitehead	(Bowler	2014). However,	as	historian	Peter	Bowler	has	noted, "Waddington	had	no	interest	in	encouraging	scientists	to revive	an	interest	in	religion"	(Bowler	2014,	80).	His	interest	in	Whitehead's	metaphysics	was	strictly biological.	The	lessons	he	derived	from	Whitehead	were	that	biological	phenomena	should	be	explained at	the	level	of	the	whole	system,	and	that	the	reconstruction	of	form	in	development	should	be	explained dynamically,	not	by	the	transmission	of	something	that	concretely	embodies	that	form.	In	his autobiography	he	outlined	some	specific	impacts	on	his	work	in	embryology	and	genetics: "In	the	late	'30s	I	began	developing	the	Whiteheadian	notion	that	the	process	of	becoming	(say)	a nerve	cell	should	be	regarded	as	the	result	of	the	activities	of	a	large	number	of	genes,	which interact	together	to	form	a	unified	"concrescence".	...	Again	a	few	years	later	it	had	become apparent	that	"gene-concrescence"	itself	undergoes	processes	of	change;	at	one	embryonic	period a	given	concrescence	is	in	a	phase	of	"competence"	and	may	be	switched	into	one	or	other	of	a small	number	of	alternative	pathways	of	further	change	but	the	competence	later	disappeared and	if	you've	missed	the	bus	the	switch	won't	work.	...	If	I	had	been	more	consistently Whiteheadian,	I	would	probably	have	realized	that	the	"specificity"	[the	fact	that	the	switch	sends the	cell	down	a	particular	developmental	pathway]	involved	does	not	need	to	lie	in	the	switch	at	all but	may	be	a	property	of	the	"concrescence"	and	the	way	in	which	it	can	change.	Because	of course	what	I	have	been	calling	by	the	Whiteheadian	terms	[sic]	"concrescence"	is	what	I	have later	called	a	chreod	[canalised	developmental	trajectory]"	(Waddington	1975,	9-10). As	we	will	see	below,	what	makes	DST	a	process	theory	is	that	it	seeks	to	explain	developmental outcomes	as	the	result	of	a	dynamic	process	in	which	some	of	the	interacting	factors	are	products	of earlier	stages	of	the	process,	rather	than	as	the	result	of	the	arrangement	of	pre-existing	factors	into	a static	mechanism.	Even	when	factors	exist	independently	of	the	developmental	process,	they	are	drawn into	it	and	made	part	of	a	developmental	'system'	by	the	unfolding	process,	as	we	have	already	discussed above.	It	is	the	process	that	defines	the	system.	In	these	respects	DST	is	the	direct	inheritor	of Waddington's	process	biology. Another	reason	biologists	were	drawn	to	Whitehead	was	his	organicism	–	the	idea	that	collectives	have an	enduring	identity	that	cannot	be	reduced	to	the	continuity	of	their	parts.	This	was	at	the	heart	of Agar's	interest	in	Whitehead: "It	is	in	[Whitehead's]	conception	of	the	unity	of	a	nexus	that	we	strike	the	main	idea	of	theories	of organism	as	usually	understood	by	biologists,	namely,	the	idea	that	the	whole	is	more	than	the sum	of	its	parts,	and	indeed	imposes	its	own	character	on	its	parts.	As	Ritter	puts	it,	the	whole	acts causally	on	its	parts,	as	well	as	being	acted	on	causally	by	its	parts.	This	is	only	understandable	if we	get	away	from	the	idea	of	substance	and	fix	our	attention	on	process.	We	must	not	think	of	the molecule	as	composed	of	ultimate	particles	of	matter	in	motion.	But	the	molecule	is	a	pattern	of processes,	and	each	constituent	process	conforms	to	its	place	in	the	pattern,	and	resists	factors tending	to	alter	it."	(Agar	1936,	29;	see	also	Agar	1943) Agar	finds	in	Whitehead	an	account	of	the	identity	of	an	organism	that	is	reassuringly	Darwinian: 'It	is	cardinal	to	Whiteheads'	philosophy	that	the	subjective	aim	of	an	actual	entity	is	not	merely	at self-realization,	but	at	self-realization	as	an	agent	creative	of	other	entities	like	itself,	or	at	least	of the	production	in	other	actual	entities	of	feelings	like	its	own."	(Agar	1936,	22) Elsewhere,	Agar	identifies	'subjective	aim'	with	final	causation,	so	part	of	what	he	is	saying	here	is	that the	telos3	of	an	organism	is	to	reproduce	itself. We	do	not	think	that	the	details	of	Agar's	Whiteheadean	organicism	contain	much	of	relevance	to	DST, but	the	issue	he	is	addressing	–	the	identity	of	processes	through	time	and	their	distinctness	from	one another	–	is	a	vital	one	for	DST.	A	biological	individual	is	a	process	which	may	intersect	with	other organismic	processes,	but	which	has	a	principle	of	identity	that	marks	just	this	series	of	events	out	as	one biological	individual.	DST	thus	requires	an	account	of	what	is	known	in	process	philosophy	as 'genidentity'	or	identity	as	continuity	of	organization.	This	principle	of	identity	also	determines,	by identifying	a	process,	the	boundaries	of	the	developmental	system	–	the	matrix	of	resources	required	for that	process	to	proceed	to	completion. We	return	to	this	issue	in	section	6,	where	we	will	see	that	the identity	of	a	developmental	process,	and	its	distinction	from	other	individual	processes	is	indeed	given by	its	telos,	though	doubtless	not	in	the	exact	sense	envisaged	by	Agar. 3	Its	end	or	purpose;	that	for	the	sake	of	which	the	organism	exists	and	acts. 2.	Process	in	the	developmental	systems	tradition "An	animal	is,	in	fact,	a	developmental	system,	and	it	is	these	systems,	not	the	mere	adult	forms which	we	conventionally	take	as	typical	of	the	species,	which	becomes	modified	during	the	course of	evolution."	(Waddington	1952,	155) Waddington's	idea	of	a	developmental	system	and	his	early	attempt	to	explain	development	as	the	result of	the	dynamical	structure	of	that	system	is	an	important	precursor	of	DST	(Griffiths	and	Tabery	2013). However,	he	remained	a	profoundly	gene-centered	thinker.	The	dynamical	structure	is	an	'epigenotype'	the global	expression	of	all	the	organism's	genes	and	it	explains,	through	the	presence	of	'creods',	the resistance	of	some	developmental	trajectories	to	environmental	perturbation.	Waddington	does	not embrace	the	idea	that	the	evolved	developmental	system	actually	includes	aspects	of	the	environment rather	than	merely	being	designed	to	function	in	an	environment	and	cope	with	its	variations. For	this	central	theme	in	DST	we	must	turn	to	the	comparative	psychologist	Daniel	S.	Lehrman,	perhaps the	single	most	important	figure	in	the	development	of	DST	and	indeed	of	the	scientific	field	of developmental	psychobiology: 'Natural	selection	acts	to	select	genomes	that,	in	a	normal	developmental	environment,	will	guide development	into	organisms	with	the	relevant	adaptive	characteristics.	But	the	path	of development	from	the	zygote	stage	to	the	phenotypic	adult	is	devious,	and	includes	many developmental	processes,	including,	in	some	cases,	various	aspects	of	experience'	(Lehrman	1970, 36) Lehrman	was	not	a	Whiteheadean	and	so	it	is	all	the	more	significant	that	his	efforts	to	place development	at	the	heart	of	the	study	of	animal	behavior	led	him	to	adopt	a	process	view	of	the organism: The	use	of	"explanatory"	categories	such	as	"innate"	and	"genetically	fixed"	obscures	the	necessity of	investigating	developmental	processes	in	order	to	gain	insight	into	the	actual	mechanisms	of behavior	and	their	interrelations.	The	problem	of	development	is	the	problem	of	the	development of	new	structures	and	activity	patterns	from	the	resolution	of	the	interaction	of	existing	ones, within	the	organism	and	its	internal	environment,	and	between	the	organism	and	its	outer environment.	At	any	stage	of	development,	the	new	features	emerge	from	the	interactions	within the	current	stage	and	between	the	current	stage	and	the	environment.	The	interaction	of	which	the organism	develops	is	not	one,	as	is	so	often	said,	between	heredity	and	environment.	It	is	between organism	and	environment!	And	the	organism	is	different	at	each	stage	of	its	development. (Lehrman	1953,	435,	italics	in	original.) Lehrman	is	not	merely	being	pedantic	in	insisting	that	organisms,	not	genes,	interact	with	environments. The	impact	of	a	genetic	or	environmental	factor	at	some	point	in	development	depends	on	how	the organism	has	developed	up	to	that	point. It	is	an	organism	at	some	stage	of	development	that	interacts with	both	genes	and	environment	to	produce	the	next	stage	of	development.	Development	is	essentially	a dynamic	process	in	which,	as	Waddington	insisted,	we	need	to	take	account	of	time,	as	well	as	a	list	of ingredients. The	historical	contingency	of	individual	development	was	at	the	heart	of	what	developmental psychobiologist	Gilbert	Gottlieb,	another	major	source	for	DST,	called	the	'developmental psychobiological	systems	view'	(Gottlieb	1970).	Gottlieb	contrasted	what	he	called	'probabilistic epigenesis'	with	what	he	saw	as	the	prevailing	view	of	'predetermined	epigenesis'.	The	latter	concept,	he argued,	covered	up	the	persistence	of	preformationist	thought	in	modern	biology	(Gottlieb	2001,	see	also Robert	2004).	According	to	Gottlieb,	"the	cause	of	development	what	makes	development	happen	is the	relationship	of	the	components,	not	the	components	themselves"	(Gottlieb	1997,	91).	The	impact	of any	causal	factor	depends	on	the	order	in	which	the	system	is	exposed	to	that	and	other	factors.	This places	limits	on	our	ability	to	predict	the	results	of	development	from	a	list	of	measured	factors. Gottlieb's	influence	lives	on	in	DST's	emphasis	on	contingency.4 The	idea	of	a	formal	Developmental	Systems	Theory	is	due	to	Donald	Ford	and	Richard	Lerner	(1992), who	identify	two	core	theses	of	DST.	The	first,	which	they	call	"developmental	contextualism",	is	derived from	Gottlieb's	concept	of	probabilistic	epigenesis.	Development	proceeds	at	several	levels	–	for	example, gene	expression,	the	formation	of	tissues,	and	the	state	of	the	environment	and	the	interactions between	levels	are	the	prime	focus	of	research,	rather	than	treating	one	level	as	focal	and	the	others	as background	against	which	it	unfolds.	Developmental	contextualism	is	a	modern	version	of	the	epigenetic, as	opposed	to	predeterminist,	view	of	development Their	second	core	thesis	is	"dynamic interactionism",	which	they	contrast	to	conventional,	"static	interactionism".	This	reflects	Lehrman's distinction	between	organism–environment	and	gene–environment	interaction	quoted	above.	Ford	and Lerner	regard	interaction	as	an	ongoing	process	that	can	transform	the	interactants	themselves.	In	other words,	the	parts	that	interact	with	each	other	and	with	the	developmental	system	are	products	of	the developmental	system.	Overall,	Ford	and	Lerner	present	a	thoroughly	processual	view	of	the 4	To	the	best	of	our	knowledge,	Gottlieb	was	not	influenced	by	Whitehead,	but	he	did	acknowledge	John Dewey	as	a	major	source	of	inspiration	(Griffiths	and	Tabery	2013),	and	Dewey	was	acknowledged	by Whitehead	as	an	important	influence. developmental	system	in	which	we	can	see	the	same	ideas	that	we	encountered	in	Waddington	and which	he	claimed	to	have	derived	from	Whitehead. This	process	view	of	development	led	to	the	radical	reformulation	of	the	distinction	between	nature	and nurture	proposed	by	Susan	Oyama	(2002).	In	the	conventional	picture	of	(static)	gene–environment interaction	nature	and	nurture	are	simply	interacting	causes.	Genes,	or	genes	plus	'epigenes',	represent nature.	The	environment	represents	nurture.	Added	together	they	cause	development.	In	DST,	however, nature	and	nurture	are	product	and	process.	Nurture	is	the	interaction	between	the	current	state	of	the organism	and	the	resources	available	to	it	–	environmental	and	genetic.	The	nature	of	the	organism	at each	stage	is	simply	the	state	of	the	organism	–	including	the	modified	state	of	its	genome	and	of	its developmental	niche,	both	of	which	have	been	transformed	by	earlier	processes	of	nurture.	Oyama rejects	the	very	idea	that	nature	exists	separate	from	and	before	nurture.	One	way	she	expresses	this	is by	insisting	that	the	developmental	information	expressed	in	the	organism	is	not	present	in	the	starting point	of	development,	but	is	itself	created	by	the	process	of	development,	through	feedback	from	the current	state	of	the	organism	to	the	states	of	the	resources	that	will	influence	future	development.	This	is what	she	means	by	the	'ontogeny	of	information'. In	The	Ontogeny	of	Information	(1985)	Oyama	pioneered	the	parity	argument	or 'parity	thesis' concerning	genetic	and	environmental	causes	in	development	(see	also	Griffiths	and	Gray	1994;	Griffiths and	Gray	2005;	Griffiths	and	Knight	1998;	Stotz	2006;	Stotz	and	Allen	2012).	Oyama	relentlessly	tracked down	failures	of	parity	of	reasoning	in	earlier	theorists.	The	same	feature	is	accorded	great	significance when	a	gene	exhibits	it	only	to	be	ignored	when	a	non-genetic	factor	exhibits	it.	When	a	feature	thought to	explain	the	unique	importance	of	genetic	causes	in	development	is	found	to	be	more	widely distributed	across	developmental	causes,	it	is	discarded	and	another	feature	is	substituted.	Griffiths	and Gray	(1994)	argued	in	this	spirit	against	the	idea	that	genes	are	the	sole	or	main	source	of	information	in development.	Other	ideas	associated	with	'parity'	are	that	the	study	of	development	does	not	turn	on	a single	distinction	between	two	classes	of	developmental	resources,	and	that	the	distinctions	useful	for understanding	development	do	not	all	map	neatly	onto	the	distinction	between	genetic	and	non-genetic. Ulrich	Stegmann	has	argued	that	because	DST	has	not	identified	a	single,	essential	way	in	which	genetic and	non-genetic	resources	must	be	treated	with	parity	of	reasoning	the	idea	of	parity	is	too	vague	to	be useful	(Stegmann	2012).	It	is	hard	to	know	what	to	make	of	this	criticism. Other	critics	of	DST	have dismissed	parity	as	the	wildly	holistic	view	that	no	distinctions	of	any	kind	can	be	made	amongst developmental	causes:	"Parity	arguments	then	claim	that	picking	out	one	cause,	when	in	fact	there	are many,	cannot	be	justified	on	ontological	grounds	because,	after	all,	causes	are	causes"	(Waters	2007, 533).5	Developmental	systems	theorists	have	repeatedly	rejected	this	interpretation	and	provided examples	of	ways	in	which	one	developmental	cause	can	be	more	significant	than	another	in	ways consistent	with	DST	(Oyama	2000;	Griffiths	and	Knight	1998;	Griffiths	and	Gray	2005). In	recent	work, Griffiths	et	al	have	constructed	a	quantitative	measure	of	relative	causal	contribution	and	used	it	to assess	the	parity	thesis	in	specific	cases	(Griffiths	et	al.	2015). So	the	ideas	that	Waddington	derived	from	process	philosophy	can	be	found	in	all	the	major	figures	who inspired	and	developed	DST.	In	the	next	section	we	discuss	in	more	detail	two	core	ideas	that	define	DST as	a	distinctive	approach	to	development	and	argue	that	both	support	the	idea	that	the	fundamental	unit of	analysis	for	DST	is	the	developmental	process	or	life	cycle. 3.	Core	ideas	in	DST:	Epigenesis	and	developmental	dynamics Two	ideas	recur	in	all	the	authors	described	above,	and	are	at	the	heart	Developmental	Systems	Theory: epigenesis	and	developmental	dynamics	(Griffiths	and	Tabery	2013).	Both	of	them	support	the	idea	that the	central	focus	of	DST	should	be	on	developmental	processes	rather	than	on	sets	of	objects (developmental	resources). Epigenesis The	term	'epigenetics'	is	derived	from	the	process	of	epigenesis.	"As	a	continuation	of	the	concept	that development	unfolds	and	is	not	preformed	(or	ordained),	epigenetics	is	the	latest	expression	of epigenesis"	(Hall	2011,	12).	It	was	invented	by	Waddington	as	a	fusion	of	'epigenesis'	and	'genetics'	to refer	to	the	processes	by	which	genotype	gives	rise	to	phenotype	and	to	the	study	of	those	processes (Waddington	1942).	Waddington	suggested	that	existing	knowledge	from	experimental	embryology supported	a	view	of	how	genes	were	connected	to	phenotypes	broadly	in	line	with	the	older	idea	of epigenesis.	The	interaction	of	many	genes	produces	an	emergent	level	of	organization	that	Waddington called	the	'epigenotype'	and	development	is	explained	by	the	dynamics	of	the	developmental	system	at this	level. Waddington's	'epigenotype'	was	a	global	expression	of	the	genetic	causal	factors	that	influence development.	The	effect	of	changing	any	one	gene	depended	on	how	it	interacted	with	the	rest	of	the system.	The	epigenotype	as	a	whole	interacted	with	the	environment	to	determine	the	phenotype.	DST 5	For	other	examples,	see	(Weber	2006,	607;	Thornhill	2007,	206;	Rosenberg	and	McShea	2008,	174;	Okasha	2009,	724; Woodward	2011,	249;	French	2012,	197).	Okasha	calls	this	kind	of	wild	holism	'causal	democracy',	a	term	introduced	by Philip	Kitcher	(2001).	Kitcher's	principle	of	causal	democracy	states	that	biology	should	not	assume	that	genes	are	the	most significant	causes,	but	assess	the	issue	empirically	on	a	case-by-case	basis. It	is	thus	very	similar	to	Oyama's	parity	thesis,	and neither	is	committed	to	any	kind	of	holism! expands	this	vision	to	include	non-genetic	factors	that	influence	development.	The	epigenotype	is replaced	by	a	more	inclusive	vision	of	a	developmental	system,	a	global	expression	of	all	the	causal factors	that	influence	development.	The	developmental	system	still	does	not	determine	a	unique phenotype,	both	because	development	is	a	probabilistic	process,	as	Gottlieb	emphasized,	and	because development	is	plastic	by	design.	The	environment	provides	many	requirements	for	normal	genome expression	(the	"ontogenetic	niche"	of	West	and	King	1987)	and	thus	partly	constitutes	the developmental	system,	but	the	environment	also	determines	the	specific	values	of	variables	in	an individual	life	cyele	and	so	determines	the	particular	course	development	will	take	out	of	those	available to	the	system.	The	genome	also	plays	these	two	roles,	as	some	of	these	variables	are	determined	by genetic	individual	differences	(see	Tabery	2009). In	1958	the	biologist	David	L.	Nanney	introduced	another	sense	of	'epigenetics',	the	sense	in	which	it	is primarily	used	in	molecular	biology	today	(Haig,	2004).	Epigenetics	in	this	sense	is	the	study	of mechanisms	that	determine	which	genome	sequences	will	be	expressed	in	the	cell.	These	mechanisms control	cell	differentiation	and	give	the	cell	an	identity	that	is	often	passed	on	through	mitosis.	Writing	in the	year	that	Francis	Crick	first	stated	his	'sequence	hypothesis'	that	the	order	of	nucleotides	in	DNA determines	the	order	of	amino	acids	in	a	protein	and	thus	encodes	the	biological	specificity	of	the	protein (Crick,	1958),	Nanney	wrote	that,	"On	the	one	hand,	the	maintenance	of	a	'library	of	specificities,'	both expressed	and	unexpressed,	is	accomplished	by	a	template	replicating	mechanism.	On	the	other	hand, auxiliary	mechanisms	with	different	principles	of	operation	are	involved	in	determining	which specificities	are	to	be	expressed	in	any	particular	cell.	...	they	will	be	referred	to	as	'genetic	systems'	and 'epigenetic	systems'"	(Nanney	1958,	712). Epigenetics	in	this	narrower,	modern	sense	allows	a	major	role	for	the	environment	in	development: "As the	past	70	years	made	abundantly	clear,	genes	do	not	control	development.	Genes	themselves	are controlled	in	many	ways,	some	by	modifications	of	DNA	sequences,	some	through	regulation	by	the products	of	other	genes	and/or	by	[the	intraor	extra-cellular]	context,	and	others	by	external	and/or environmental	factors"	(Hall	2011,	12).	The	regulated	expression	of	the	coding	regions	of	the	genome depends	on	mechanisms	that	differentially	activate	and	select	the	information	in	coding	sequences depending	on	context.	Biological	information	is	distributed	between	the	coding	regions	in	the	genome and	regulatory	mechanisms,	and	the	specificity	manifested	in	gene	products	is	the	result	of	a	process	of 'molecular	epigenesis'	(Stotz	2006;	Griffiths	&	Stotz	2013). So	the	idea	of	epigenesis	is	alive	and	well	in	contemporary	biology.	As	Waddington	argued, developmental	outcomes	are	to	be	explained	at	the	level	of	the	whole	system	and	not	by	single	causes that	'encode'	or	'instruct'	that	outcome	(for	an	influential	restatement	of	this	view,	see	Noble	2006). Developmental	outcomes	are	also	explained	dynamically,	as	trajectories	in	a	space	of	possible	states	of genome	expression.	The	role	of	epigenetic	marks	in	development	is	to	successively	differentiate	cells	as	a result	of	earlier	stages	of	development,	making	genome	expression	in	a	tissue	at	a	time	a	function	of	the history	of	these	cells.	The	complexity	of	biological	networks	makes	it	plausible	that	in	many	cases	this process	will	display	emergent	dynamics	that	can	only	be	studied	through	simulation,	a	point	we	expand on	in	the	next	sub-section. DST	adds	to	this	modern	epigenetic	vision	of	development	the	same	thing	that	it	added	to	Waddington's original	vision	of	epigenetics,	namely	a	constructive	role	for	the	environment.	The	networks	that	regulate gene	expression	extend	outside	the	cell	and	outside	the	organism.	Evolution	designs	developmental processes	that	draw	these	wider	resources	into	the	developmental	system	by	reestablishing relationships	to	them.	The	presence	of	suitable	external	resources	is	in	many	cases	explained	by	the activity	of	parents	and	of	conspecifics	more	generally	("developmental	niche	construction"	(Griffiths	and Stotz	2013),	and	sometimes	by	the	feed-forward	effects	of	earlier	stages	of	the	developmental	process itself,	as	discussed	immediately	below. Developmental	dynamics The	idea	that	development	is	a	dynamic	process	is	central	to	DST.	Ford	and	Lerner	contrast	'dynamic interaction'	with	a	more	conventional	conception	of	interaction	associated	with	analysis	of	variance techniques,	such	as	those	used	in	behavioral	genetics	(Ford	and	Lerner	1992).	In	that	'static	interaction' the	values	of	two	variables	measured	before	development,	such	as	shared	genes	and	shared	environment, are	shown	to	interact	with	one	another.	In	contrast,	dynamic	interaction	must	be	studied	as	a	temporally extended	process.	For	example,	in	Celia	Moore's	iconic	work	on	sexual	development	in	male	rat	pups, male	sexual	development	depends	on	differential	licking	of	the	genital	area	of	male	and	female	pups	by the	mother.	But	this	response	to	male	pups	depends	on	differences	in	their	urine,	which	are	the	result	of earlier	processes	of	sexual	differentiation	(Moore	1984,	1992).	The	presence	of	this	environmental influence	is	a	feed-forward	from	earlier	development	in	the	pup	itself.	The	patterns	of	gene	expression that	underlie	sexual	development	in	the	rat	arise	through	interaction	with	an	environment	that	has	been partially	structured	by	an	earlier	stage	of	the	rat's	development.	The	idea	of	developmental	dynamics embodies	one	of	the	basic	ideas	of	process	biology,	namely	that	the	developmental	system	is	defined,	and in	part	physically	produced,	by	the	process	of	development. If	interaction	is	a	dynamic	process,	then	the	temporal	dynamics	of	the	interaction	may	play	an independent	role	in	explaining	the	outcome.	This	explains	why	many	DST	advocates	have	also	been attracted	to	explanations	of	developmental	which	draw	on	dynamical	systems	theory	(abbreviated	as DyST	to	avoid	confusion).	Griffiths	and	Tabery	(2013)	argue	that	there	is	nothing	about	the	basic	idea	of dynamical	interaction	found	in	DST	that	requires	the	use	of	DyST.	The	example	of	rat	development	just given,	for	example,	is	a	sequential	mechanism	that	can	be	described	without	using	DyST.	But	in	other cases,	DyST	provides	additional	explanatory	resources	(Thelen	and	Smith	1994;	Bechtel	and Abrahamsen	2013).	Dynamical	systems	theory	exemplifies	even	more	strongly	than	the	bare	notion	of developmental	dynamics	the	idea	that	developmental	outcomes	should	be	explained	at	the	systems	level. In	this	section	we	have	argued	that	the	core	ideas	of	DST,	epigenesis	and	developmental	dynamics, embody	the	very	same	ideas	that	featured	in	Waddington's	process	biology.	Developmental	outcome	are explained	at	the	systems	level,	and	in	identifying	the	components	of	a	developmental	system	we	start with	the	developmental	process,	not	the	other	way	around.	In	the	next	section	we	develop	these	ideas further	by	examining	how	DST	has	conceptualized	the	constituents	of	developmental	systems. 4.	An	ontology	for	DST:	genomes,	epigenomes	and	developmental	niches One	of	the	most	controversial	features	of	DST	is	its	conceptualization	of	the	developmental	system	as	an organism-environment	system.	Rather	than	an	organism	developing	in	an	environment,	aspects	of	the developmental	environment	are	part	of	the	developmental	system.	As	well	as	talking	of	developmental systems,	advocates	of	DST	have	talked	of	sets,	collections	or	matrices	of	developmental	resources	and more	recently	of	ontogenetic	or	developmental	niches	that	provide	the	developmental	context	for organisms	or	genomes. DST	has	always	resisted	the	idea	that	there	is	a	single	way	to	divide	the	inputs	to	development	that	will be	useful	for	every	scientific	question	about	development	(Hinde	1968;	Johnston	1987;	Oyama	1985). Instead,	distinctions	should	be	introduced	locally	to	suit	the	question	at	hand. For	some	purposes,	as	an alternative	to	'organism	and	environment'	or	'genes	and	environment',	the	resources	that	make	up	a developmental	system	can	be	partitioned	into	three:	genome,	the	epigenome	(chemical	modifications	of DNA	that	are	transmitted	through	meoisis),	and	the	developmental	niche.	Since	the	fundamental	unit	of analysis	for	DST	is	the	complete	developmental	process,	or	life	cycle,	we	can	think	of	that	process	as occurring	within,	and	feeding	forward	into	the	construction	of,	a	developmental	systems	with	these	three components.	Or	we	can	think	of	the	life	cycle	as	consisting	of	the	regulated	expression	of	an epigenetically	modified	genome	through	its	interaction	with	a	developmental	niche. The	genome	is	a	familiar	idea,	and	the	epigenome	increasingly	so.	The	idea	of	a	developmental	niche	will be	less	familiar	to	many	readers.	Developmental	psychobiologists	Meredith	West	and	Andrew	King (1987)	introduced	the	term	'ontogenetic	niche'	to	capture	the	idea	that	environmental	resources	form	a social	and	ecological	legacy	inherited	by	a	developing	organism.	We	have	used	'developmental	niche'	as	a synonym	for	West	and	King's	term	(Stotz	2008;	Stotz	2010;	Griffiths	and	Stotz	2013).	Species-specific phenotypes	depend	on	species-typical	environments	of	development.	These	are	often	the	result	of parental	activities,	but	their	construction	can	also	involve	other	conspecifics	past	and	present	and, importantly,	the	offspring	itself.	The	idea	of	the	construction	of	a	developmental	niche	answers	a fundamental	question	about	inheritance	–	how	do	parents	reliably	influence	the	phenotype	of	their offspring	and	promote	their	healthy	development?	Organisms	do	not	rely	on	chance	to	provide	their offspring	with	the	resources	for	normal	development:	they	actively	intervene	to	modify	environments	for this	end. West	and	King	described	the	ontogenetic	niche	as	an	'information	centre'	in	the	sense	that	it makes	the	interaction	of	organism	and	environment	more	specific	than	it	would	otherwise	be.	The	idea	of an	information	centre	was	developed	initially	to	capture	the	experiences	necessary	for	species-typical learning	(Galef	and	Wigmore	1983).	These	are	the	'aspects	of	experience'	that	Lehrman	identified	as	part of	the	developmental	system	(Lehrman	1970,	36	and	quoted	above).	However,	the	idea	can	be	applied	to the	much	broader	category	of	any	environmental	stimulus	that	acts	as	a	specific	cause	of	normal development	(Griffiths	&	Stotz	2013). Dividing	the	developmental	system	into	genome,	epigenome	and	developmental	niche	may	be	useful	in the	study	of	evolution,	because	it	parallels	one	way	to	divide	mechanisms	of	heredity.	It	is	now	fairly conventional	to	recognize	epigenetic	heredity	mechanisms	as	a	genuine	form	of	heredity	alongside genetic	inheritance,	although	arguments	about	whether	these	mechanisms	have	equal	evolutionary significance	continue.6	But	DST,	like	other	recent	theorists	(e.g.	Jablonka	and	Lamb	2005),	recognizes	a wider	range	of	heredity	mechanisms.	It	is	unfortunate	that	this	wider	class	of	mechanisms	is	often	also referred	to	as	'epigenetic	inheritance',	making	that	term	ambiguous,	as	it	is	used	more	narrowly	to	refer only	epigenetic	marks	inherited	through	meiosis.	In	earlier	work	we	have	suggested	keeping	'epigenetic inheritance'	for	the	narrower	class	of	mechanisms	and	using	West	and	King's	term	'exogenetic inheritance'	(West	&	King	1987,	5)	for	the	broader	class	of	mechanisms.	It	is	this	broader	class	of heredity	mechanisms	that	constructs	the	developmental	niche: "Organisms	construct	their	life	cycles through	the	interaction	of	the	contents	of	the	fertilized	egg,	the	genome	and	its	narrowly	epigenetic surroundings,	with	a	'developmental	niche'	which	is	the	result	of	epigenetic	inheritance	in	a	wider	sense ...	'exogenetic	inheritance'..."	(Griffiths	and	Stotz	2013,	5). 6	It	if	often	asserted	that	epigenetic	change	will	only	affect	evolution	if	the	changes	themselves	persist	for	more	than	one generation	(e.g.	Wilkins	2011).	But	in	conventional	quantitative	genetics	the	evolutionary	significance	of	genetics	does	not result	from	tracking	individual	alleles	from	one	generation	to	the	next	–	quantitative	genetics	does	not	do	this.	Instead, Mendelian	assumptions	let	us	work	out	what	phenotypes	(and	hence	fitnesses)	will	appear	in	the	next	generation	as	a function	of	the	phenotypes	in	the	last	generation. Epigenetic	and	exogenetic	inheritance	both	change	this	mapping	from parental	phenotype	to	offspring	phenotype,	and	therefore	affect	evolution.	Both	epigenetic	and	exogenetic	inheritance	appear in	quantitative	genetics	as	'parental	effects':	correlations	between	parent	and	offspring	phenotypes	above	and	beyond correlations	between	parent	and	offspring	genotypes,	which	are	also	not	the	result	of	a	shared	environment	independently influencing	both	parent	and	offspring.	It	has	long	been	understood	that	one-generation	parental	effects	can	substantially	alter the	dynamics	of	evolutionary	models,	and	change	which	state	a	population	will	evolve	to	as	an	equilibrium	(Lande	and	Price 1989;	Wade	1998).	The	argument	that	epigenetic	inheritance	needs	to	be	stable	for	several	generations	to	have	evolutionary significance	appears	to	be	a	non-sequitur. It	is	worth	noting	that	this	broader,	exogenetic	form	of	inheritance	may	be	more	stable	than	narrow epigenetic	inheritance.	Some	exogenetic	inheritance	occurs	through	the	induction	of	epigenetic modifications	in	offspring	by	parental	behavior.	This	can	have	long-term,	often	lifelong,	effects	on offspring	phenotype.	In	some	known	cases	these	offspring	phenotypes	include	the	very	parental behavior	that	induced	them,	so	that	the	offspring	reproduce	the	effect	in	the	next	generation,	and	so	forth (Champagne	and	Curley	2009).	These	behaviorally	transmitted	but	epigenetically	mediated	effects contribute	to	the	long-term	stabilization	of	aspects	of	the	developmental	niche,	and	hence	may	be	more long-lived	than	meiotic	epigenetic	inheritance. It	is	important	not	to	conflate	the	developmental	niche	with	the	'niche'	of	niche-construction	theory (Odling-Smee,	Laland,	and	Feldman	2003).	Niche	construction	theory	concerns	the	influence	of	past generations	on	the	selective	pressures	that	act	on	future	generations.	This	activity	partially	constructs	a selective	niche,	the	set	of	parameters	that	determine	the	relative	fitness	of	competing	types	in	the population.	The	developmental	niche,	however,	is	the	set	of	parameters	that	must	be	within	certain bounds	for	an	evolved	life	cycle	to	occur	(or	in	more	traditional	terms,	for	the	organism	to	develop normally).	The	two	niches	will	often	share	many	parameters.	They	are,	however,	conceptually	quite distinct.	For	example,	signals	from	parent	to	offspring	that	induce	transgenerational	adaptive	phenotypic plasticity,	as	when	Daphnia	signal	their	offspring	to	grow	additional	defenses	against	predators,	are	a clear	example	of	developmental	niche	construction:	the	parent	Daphnia	is	structuring	the	developmental environment	of	its	offspring.	But	this	is	no	more	a	case	of	selective	niche	construction	than	is	the inheritance	of	an	advantageous	mutation!	The	Daphnia	embryo	is	alters	itself	to	fit	the	selective environment	rather	than	altering	the	selective	environment. 5.	DST	as	a	process	theory	of	the	organism One	reason	early	20th	C	biologists	were	drawn	to	process	philosophy	was	that	it	offered	a	'theory	of	the organism'	–	an	account	of	the	unity	of	living	systems.	Recent	interest	in	process	ontologies	for	biology has	revived	interest	in	the	concept	of	'genidentity'	or	identity	as	continuity	of	organization (Guay	& Pradeu	2015).	Distinct	stages	are	stages	of	the	same	thing	because	one	developed	from	the	other,	rather than	because	they	share	some	common	properties: [genidentity]	says	that	the	identity	through	time	of	an	entity	X	is	given	by	the	continuous connection	of	states	through	which	X	goes.	...	In	this	view,	the	individual	X	is	never	presupposed	or given	initially,	because	the	starting	point	is	the	decision	to	follow	a	specific	and	appropriate	process P,	and	the	individual	X	supervenes	on	this	process. ...	In	other	words,	for	the	genidentity	view,	what we	single	out	as	an	"individual"	is	always	the	by-product	of	the	activity	that	is	being	followed,	not its	prior	foundation	(not	a	presumed	"thing"	that	would	give	its	unity	to	this	activity).	(Guay	and Pradeu	2015,	317-18) Guay	and	Pradeu	here	exemplify	themes	familiar	from	our	earlier	discussion	of	Waddington	and	Agar. The	persistence	of	biological	form	should	be	explained	dynamically,	not	by	the	transmission	of something	that	concretely	embodies	that	form.	The	identity	of	an	individual	through	time	is	a	dynamic continuity	of	form.	If	the	fundamental	unit	of	analysis	in	DST	is	the	developmental	process,	or	life	cycle,	if heredity	in	DST	is	a	relation	between	one	life	cycle	and	another,	and	natural	selection	occurs	in populations	of	life	cycles	(Griffiths	and	Gray	2001),	then	DST	needs	to	give	an	account	of	the	genidentity of	these	processes.	It	needs	to	say	where	one	developmental	process	ends	and	the	next	begins.	This problem	arises	in	a	dramatic	form	when	organisms	have	alternating	haploid	and	diploid	phases	of comparable	length	(Godfrey-Smith	2015).	Is	each	phase	a	life	cycle,	or	is	a	life	cycle	the	combination	of	a haploid	and	diploid	phase? The	principle	of	genidentity	of	a	life	cycle	also	needs	to	explain	how	a	life	cycle	can	consist	of	a	different series	of	events	from	one	generation	to	the	next.	This	problem	arises	in	a	dramatic	form	when	a	species have	a	range	of	substantially	different	ways	to	get	from	conception	to	death.	Some	newts,	for	example, exhibit	facultative	paedomorphosis	in	which	individuals	respond	to	differences	in	their	environment	by either	retaining	the	morphology	of	their	acquatic,	larval	stage	and	becoming	reproductively	mature	in that	state,	or	going	through	metamorphosis	to	become	a	terrestrial	'adult'	reproductive	form	(Denoël, Joly,	and	Whiteman	2005).	The	same	issue	arises	in	principle,	however,	whenever	an	organism	exhibits adaptive	phenotypic	plasticity,	so	that	successive	life	cycles	in	a	single	lineage	do	not	contain	the	same developmental	events. DST	has	often	been	criticized	for	replacing	the	commonsense	idea	of	an	individual	organism	with	a	novel and	nebulous	'system'	(Sterelny,	Dickison,	and	Smith	1996;	Merlin	2010;	Pradeu	2010;	and	see	the references	in	fn.	4.	).	This	criticism	has	become	increasingly	unfair	over	the	past	twenty	years.	It	is	no overstatement	to	say	that	conventional	theories	of	biological	individuality	are	in	a	state	of	crisis	brought on	by	new	empirical	and	theoretical	developments	in	biology.	These	include	research	on	evolutionary transitions	in	individuality,	the	realization	of	the	extent	to	which	core	physiological	processes	in	multicellular	organisms	are	carried	out	by	microbial	commensals,	the	discovery	of	ever	more	complex	and highly	integrated	functional	associations	between	microbes	themselves,	as	well	as	increased	attention	by philosophers	and	theoretical	biologists	to	the	full	diversity	of	life,	in	all	it's	glorious	weirdness!	The	result has	been	a	wave	of	new	work	in	philosophy	and	theoretical	biology	on	the	nature	of	individuality,	a literature	that	shows	little	sign	of	reaching	consensus	(Calcott	and	Sterelny	2011;	Pradeu	2012; Ereshefsky	and	Pedroso	2013;	Bouchard	and	Huneman	2013;	Guay	and	Pradeu	2015).	Statements	like the	following	are	not	hard	to	find	in	the	recent	literature: Individuals	can	be	defined	anatomically,	embryologically,	physiologically,	immunologically, genetically,	or	evolutionarily...	each	stems	from	the	common	tenet	of	genomic	individuality:	one genome/one	organism.	As	such,	all	classical	conceptions	of	individuality	are	called	into	question by	evidence	of	all-pervading	symbiosis.	(Gilbert,	Sapp,	and	Tauber	2012,	325). It	is	not	very	reasonable	to	complain	that	DST	has	a	more	problematic	conception	of	a	biological individual	than	the	traditional	organism	when	that	traditional	conception	of	a	population	of physiologically	integrated	cells	with	a	single	genotype	is	itself	so	widely	regarded	as	problematic! In	the light	of	this,	the	initial	response	of	Griffiths	and	Gray	to	this	line	of	criticism	continues	to	be	effective. They	argued	that	the	idea	of	an	individual	organism	was	in	fact	quite	problematic	even	at	that	time,	and that	DST	did	not	need	to	offer	a	watertight	account	of	the	individuality	of	developmental	processes	in order	to	render	itself	a	viable	competitor	to	conventional	accounts	of	the	units	of	evolution	and development	(Griffiths	and	Gray	1994;	Griffiths	and	Gray	2001). Griffiths	and	Gray	sketched	how	DST	would	approach	the	problem,	using	much	of	the	same	apparatus that	biologists	were	already	using	to	address	problems	with	the	traditional	conception	of	an	individual organism.	They	argued	that	a	DST	account	of	the	individuality	of	developmental	processes	–	what	we	are now	calling	genidentity	would	define	individuality	in	terms	of	the	ability	to	act	as	a	unit	of	selection (Griffiths	and	Gray	1994,	292-298;	Griffiths	and	Gray	2001,	209-214).	They	drew	on	what	were	then current	accounts	of	evolutionary	individuality	within	the	emerging	framework	of	multi-level	selection theory	to	suggest	that, "an	individual	is	a	life	cycle	whose	components	cannot	reconstruct	themselves when	decoupled	from	the	larger	cycle"	(Griffiths	and	Gray	2001,	213)	and	to	recognize	that,	just	like	cells, organisms	and	superorganisms,	life	cycles	might	exist	at	several	different	levels	of	biological organization. Looking	back	at	theories	of	the	organism	in	early	20th	century	process	biology,	we	can	see	a	distinct similarity	between	Griffiths	and	Gray's	ideas	about	the	identity	of	developmental	processes	and	Agar's idea	that	an	organismic	process	is	united	by	its	'subjective	aim'	or	telos.	A	series	of	developmental	events is	a	single	process	because	they	serve	a	common,	evolutionary	goal,	namely	to	maximize	the representation	of	cycles	descended	from	them	in	future	generations,	relative	to	variant	cycles	with	which they	compete.	We	can	draw	on	conventional	evolutionary	theory	to	make	this	suggestion	a	little	more precise	–	an	individual	life	cycle	is	a	token	of	a	life-history	strategy,	and	that	strategy	is	it	telos	and	its principle	of	genidentity. Life	history	theory	is	a	powerful	and	remarkably	general	framework	for	addressing	many	basis	questions about	organismic	design	(Stearns	1992;	Roff	2002).	In	life-history	theory,	the	goal	of	an	organism	is	to find	the	optimal	way	to	parcel	the	resources	available	to	it	into	offspring.	This	problem	is	modelled	as	the simultaneous	optimisation	of	two	parameters,	the	probability	of	surviving	to	each	age	class	and	the number	of	offspring	produced	in	each	age	class,	integrated	across	all	age	classes.	The	primary	constraint on	this	optimisation	problem	is	the	quantity	of	resources	available	to	the	organism.	But	it	is	also constrained	by	multiple	trade	offs	between	the	two	key	parameters:	an	overall	trade-off	between survival	and	reproduction;	a	trade-off	between	reproduction	in	the	current	age	class	and	in	later	age classes;	another	between	current	reproduction	and	growth,	between	growth	and	survival	to	later	age classes,	and	so	forth.	Solving	this	complex	optimization	problem	under	different	sets	of	constraints	and in	different	ecological	settings	leads	to	the	many	different	life	history	strategies	observed	in	nature.	Since life	history	theory	already	conceives	of	an	organism	as	a	series	of	events	(age	classes)	it	is	readily applicable	to	a	life	cycle	consisting	of	a	series	of	developmental	interactions,	each	of	which	moves	the lifecycle	forward. Life-history	theory	embodies	a	powerful	principle	of	genidentity,	because	the	evolutionary	rationale	for the	choice	of	strategy	at	each	life	history	stage	is	conditional	on	what	choices	have	been	or	will	be	made at	the	other	stages.	A	life	cycle	conceived	of	as	the	implementation	of	a	life-history	strategy	is	held together	by	the	trade-offs	between	its	stages.	If	these	events	were	not	part	of	a	single	life	history	serving a	single,	Darwinian	telos,	then	they	would	not	trade	off	against	one	another	in	this	way. It	makes	sense for	me	to	accept	an	elevated	risk	of	cancer	in	later	life	in	return	for	my	increased	reproductive	success, but	not	for	your	reproductive	success	unless	that	is	discounted	by	our	coefficient	of	relationship.	The	life history	strategy	also	defines	where	one	process	ends	and	another	begins,	namely	at	the	points	between which	a	single	set	of	such	trade	offs	exists.7	Life	history	theory	also	explains	how	life	cycles	that	do	not contain	the	same	developmental	events	can	nevertheless	constitute	a	single	lineage	of	cycles	that succeeds	in	reproducing	themselves.	Adaptive	phenotypic	plasticity	is	part	of	a	life-history	strategy,	and individuals	who	exhibit	different	developmental	outcomes	as	a	result	of	this	plasticity	are	individuals who	shared	the	same,	plastic	strategy. Introducing	a	life-history	perspective	makes	it	clear	why	it	is	legitimate	for	developmental	systems theorists	to	help	themselves	to	whatever	is	currently	the	best	evolutionary	account	of	biological individuality8	and	to	'process'	that	account.	Evolutionary	accounts	of	individuality	seek	to	identify collections	of	biological	material	which	are	evolving	as	one:	they	are	more	or	less	successful	in reproducing	themselves	as	a	whole,	and	that	success	cannot	be	reduced	to	the	successes	of	each	part	of 7	Life	history	theory	in	practice	is	conducted	as	a	branch	of	population	genetics,	and	we	anticipate	the objection	that	the	implicit	definition	of	the	limits	of	an	individual	that	we	have	made	use	of	in	this	section is,	in	fact,	derived	from	genetic	identity.	But	this	cannot	be	the	case,	as	the	theory	applies	perfectly	well	to asexual	organisms	whose	parents	and	offspring	are	genetically	identical	to	themselves.	Moreover,	so	far as	we	can	see,	life-history	theory	could	be	extended	unproblematically	to	cases	in	which	heredity	is epigenetic	and	exogenetic	as	well	as	genetic. 8	For	the	current	views	of	one	author,	see	(Bourrat	and	Griffiths	2016) the	whole,	or	the	success	of	some	larger	whole	of	which	this	is	a	part.	Admittedly,	many	discussions	of this	problem	make	it	seem	a	matter	of	finding	which	spatial	parts	make	a	spatial	whole.	But	this	is	an illusion	–	any	such	unit	will,	in	fact,	be	extended	in	time	and	will	embody	a	life-history	strategy.	DST	will use	this	strategy	to	identify	the	events	that	make	up	a	single,	processual	biological	individual. 6.	Conclusions Developmental	Systems	Theory	has	a	natural	affinity	with	process	views	of	the	organism.	The	theorists who	inspired	and	created	DST	all	shared	the	view	that	development	is	a	dynamic	process	whose	study requires	an	investigation	of	it's	dynamic	form	as	well	as	the	static	constituents	on	which	it	draws.	In Waddington's	case	this	conviction	was	directly	inspired	by	process	philosophy.	The	idea	of	epigenesis, perhaps	the	single	most	important	idea	in	the	developmental	systems	tradition,	is	fundamentally processual.	In	development,	something	new	comes	into	being	that	is	not	prefigured	in	any	of	the	inputs to	development.	Dynamic	interaction,	another	idea	that	has	been	central	to	all	the	major	contributors	to DST,	is	also	essentially	processual.	The	impact	of	a	genetic	or	environmental	factor	at	some	point	in development	depends	on	how	the	organism	has	developed	up	to	that	point.	Development	is	essentially	a dynamic	process	and	cannot	be	reduced	to	a	list	of	ingredients	and	their	interactions.	The	entities	that make	up	a	developmental	system,	which	we	can	divide	for	some	purposes	into	a	genome,	an	epigenome and	a	developmental	niche,	are	picked	out	as	elements	of	a	single	system	by	the	unity	of	the	process	to which	they	contribute,	and	not	vice-versa.	That	principle	of	unity	–	the	genidentity	of	a	life	cycle	–	we have	argued,	is	simply	its	Darwinian	telos	a	life-history	strategy. Acknowledgments "This	project/publication	was	made	possible	through	the	support	of	a	grant	from	the	Templeton	World Charity	Foundation.	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