Growth Variations 179 



course of the life-history, though it is difficult to see how this 

 has occurred. It is much more likely, if we may judge from 

 available evidence, that every stage has had its counterpart in the 

 ancestral form from which it has been derived by descent with 

 modification. Just as the adult phase of the living form differs, 

 owing to evolutionary modification, from the adult phase of the 

 ancestor from which it has proceeded, so each larval phase will differ 

 for the same reason from the corresponding larval phase in the life- 

 history of the ancestor. Inasmuch as the organism is variable at 

 every stage of its independent existence and is exposed to the action 

 of natural selection there is no reason why it should escape modifica- 

 tion at any stage. 



If there is any truth in these considerations it would seem to 

 follow that at the dawn of life the life-cycle must have been, either 

 in posse or in esse, at least as long as it is at the present time, and 

 that the peculiarity of passing through a series of stages in which new 

 characters are successively evolved is a primordial quality of living 

 matter. 



Before leaving this part of the subject, it is necessary to touch 

 upon another aspect of it. What are these variations in structure 

 which succeed one another in the life-history of an organism ? I am 

 conscious that I am here on the threshold of a chamber which 

 contains the clue to some of our difficulties, and that I cannot enter 

 it. Looked at from one point of view they belong to the class of 

 genetic variations, which depend upon the structure or constitution 

 of the protoplasm; but instead of appearing in different zygotes 1 , 

 they are present in the same zygote though at different times in its 

 life-history. They are of the same order as the mutational variations 

 of the modern biologist upon which the appearance of a new character 

 depends. What is a genetic or mutational variation ? It is a genetic 

 character which was not present in either of the parents. But these 

 "growth variations" were present in the parents, and in this they 

 differ from mutational variations. But what are genetic characters ? 

 They are characters which must appear if any development occurs. 

 They are usually contrasted with "acquired characters," using the 

 expression "acquired character" in the Lamarckian sense. But 

 strictly speaking they are acquired characters, for the zygote at first 

 has none of the characters which it subsequently acquires, but only 

 the power of acquiring them in response to the action of the environ- 

 ment. But the characters so acquired are not what we technically 

 understand and what Lamarck meant by "acquired characters." 

 They are genetic characters, as defined above. What then are 



1 A zygote is a fertilised ovum, i.e. a new organism resulting from the fusion of an 

 ovum and a spermatozoon. 



12—2 



