454 THE EVOLUTION OF LIFE. 



these. Observe the positions which were open to him and 

 the reasons which show them to be untenable. 



If it be said that the conditions of the ease necessitated 

 the derivation of all organisms from simple germs, and there- 

 fore necessitated a morphological unity in their primitive 

 states ; there arises the obvious answer, that the morphologi- 

 cal unity thus implied, is not the only morphological unity 

 to be accounted for. Were this the only unity, the various 

 kinds of organisms, setting out from a common primordial 

 form, should all begin from the first to diverge individually, 

 as so many radii from a centre ; which they do not. If, other- 

 wise, it be said that organisms were framed upon certain 

 types, and that those of the 'same type continue developing 

 together in the same direction, until it is time for them to 

 begin putting on their specialities of structure; then the 

 answer is, that when they do finally diverge they ought 

 severally to develop in direct lines towards their final forms. 

 No reason can be assigned why, having parted company, some 

 should progress towards their final forms by irregular or cir- 

 cuitous routes. On the hypothesis of design such deviations 

 are inexplicable. 



The hypothesis of evolution, however, while it pre-supposes 

 those kinships among embryos in fheir early forms which 

 are found to exist, also leads us to expect nonconformities in 

 their courses of development. If, as any rational theory of 

 evolution implies, the progressive differentiations of types 

 from one another during past times, have resulted from 

 the direct and indirect effects of external conditions — if 

 races of organisms have become different, either by imme- 

 diate adaptations to unlike habits of life, or by the mediate 

 adaptations resulting from preservation of the individuals 

 most fitted for such habits of life, or by both; and if 

 most embryonic changes are significant of changes that 

 were undergone by ancestral races; then these irregularities 

 must be anticipated. For the successive changes in modes of 

 life pursued by successive ancestral races, can have had no 



