8 MORPHOLOGICAL DEVELOPMENT. 



phenomena the process of evolution; and to show how, as 

 displayed in them, it conforms to those first principles which 

 evolution in general conforms to. Two sets of factors have 

 to be taken into account. Let us look at them. 



The factors of the first class are those which tend directly 

 to change an organic aggregate, in common with every other 

 aggregate, from that more simple form which is not in equi- 

 librium with incident forces, to that more complex form which 

 is in equilibrium with them. We have to mark how, in corre- 

 spondence with the universal law that the uniform lapses into 

 the multiform, and the less multiform into the more multi- 

 form, the parts of each organism are ever becoming further 

 differentiated; and we have to trace the varying relations to 

 incident forces by which further differentiations are entailed. 

 We have to observe, too, how each primary modification of 

 structure, induced by an altered distribution of forces, becomes 

 a parent of secondary modifications — how, through the neces- 

 sary multiplication of effects, change of form in one part 

 brings about changes of form in other parts. And then we 

 have also to note the metamorphoses constantly being induced 

 by the process of segregation — by the gradual union of like 

 parts exposed to like forces, and the gradual separation of like 

 parts exposed to unlike forces. The factors of the second 



class which we have to keep in view throughout our interpret- 

 ations, are the formative tendencies of organisms themselves 

 — the proclivities inherited by them from antecedent organ- 

 isms, and which past processes of evolution have bequeathed, 

 We have seen it to be inferable from various orders of fact- k s 

 (§§ 65, 84, 97-97</), that organisms are built up of certain 

 highly-complex molecules, which we distinguished as physio- 

 logical units [or constitutional units as they might otherwise 

 be called] — each kind of organism being built up of units pe- 

 culiar to itself. We recognized in these units, powers of ar- 

 ranging themselves into the forms of the organisms to which 

 they belong; analogous to the powers which the molecules of 

 inorganic substances have of aggregating into specific crystal- 



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