2 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



may suppose that all the stages of the lower form are passed through 

 and a further condition reached. Thus we may get a new form E 

 which shows in its development the stages A, B, C, D and E. Were 

 the course of evolution as simple as this Ave can readily see that " the 

 development of the individual would be a recapitulation of that of 

 the race." Species, however, arise by the modification of old ones in 

 more than one way. We may distinguish : — 



(1) Sjjecies which have advanced in organization. 



(2) Species which have retrograded. 



(3) Species which are simple transformations of old ones, not higher 



or lower in organization, but possessing new characters. 



In the two first-mentioned groups we may suppose a species 

 ABODE to have arisen from an older one A B C D, E being 

 either a more complex, or a more simple form than D : in these cases 

 the development (in the later stages at any rate) indicates the source 

 of the race. In the third case, however, instead of the series A B C I) 

 we should get a new series A B C E, the stage D being omitted, 

 owing to the selection of variations which appear as entirely new 

 characters, and not merely as higher or lower conditions of the cha- 

 racters seen in the stage D. In this case, it is clear the development 

 is only an incomplete recapitulation of the phylogeny. 



In spite of this, were the adult the only variable stage, the task 

 of tracing the ancestral history of a form would be much easier than 

 it actually is. As a matter of fact, however, variations affect not 

 merely the adult, but also the earlier stages. 



If we compare the larva? of two very similar animals, we shall 

 not unfrequcntly find considerable differences ; these can only be 

 explained by supposing the \&vvee to vary. It would, indeed, seem 

 true that any variation in the adult, must, to a certain extent, be 

 represented by a corresponding difference in all earlier stages, for 

 each of the latter must possess all the potentialities of the adult ; but 

 independently of this the variations are considerable. One can readily 

 convince oneself of this by comparing the very different kinds of 

 larvae seen in any one of a number of animal groups, such as the 

 Insecta, Crustacea, Mollusca, &c. We may, then, represent a species 

 derived from a form A B C D, not by the symbols ABODE, but 

 as Ai Bi Ci T>i E (supposing for the sake of simplicity that all the 

 stages become thus modified) : we might, however, get such forms as 

 Ai Bi CDi Ei t Ai B x Ci DE, the stages C or D being supposed to 

 remain unaltered. Further modification would lead to the successive 

 forms A 2 B 2 C 2 D 2 E x F, A 3 B 3 C 3 D 3 E 2 F x G and so on. The later 

 phylogenetic history of the last-mentioned form is D E F G : but D 3 , 



