b MORPHOLOGICAL DEVELOPMENT. 



phenomena tlie 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 a necessary inference from various orders 

 of facts (§§ 65, 84, 97,) that organisms are built up of certain 

 highly-complex molecules, which we distinguished as physio- 

 logical units — each kind of organism being built up of phy- 

 siological units peculiar to itself. "We found ourselves obliged 

 to recognize in these physiological units, powers of arranging 

 themselves into the forms of the organisms to which they be- 

 long ; analogous to the powers which the molecules of inor- 

 ganic substances have of aggregating into specific crystalline 



