CHAP. xxi. EMBRYOLOGICAL DEVELOPMENT. 415 



known, the newly discovered fossils serve to fill up gaps 

 between specific or generic types previously familiar to us, 

 supplying often the missing links of the chain, which, if 

 transmutation is accepted, must once have been continuous. 



One of the most original speculations in Mr. Darwin's 

 work is derived from the fact that, in the breeding of 

 animals, it is often observed that at whatever age any varia 

 tion first appears in the parent, it tends to reappear at a 

 corresponding age in the offspring. Hence the young in 

 dividuals of two races which have sprung from the same 

 parent stock are usually more like each other than the 

 adults. Thus the puppies of the greyhound and bull-dog 

 are much more nearly alike in their proportions than the 

 grown-up dogs, and in like manner the foals of the cart and 

 racehorse than the adult individuals. For the same reason 

 we may understand why the species of the same genus, 

 or genera of the same family, resemble each other more 

 nearly in their embryonic than in their more fully developed 

 state, or how it is that in the eyes of most naturalists the 

 structure of the embryo is even more important in classifica 

 tion than that of the adult, 6 for the embryo is the animal in 

 its less modified state, and in so far it reveals the structure 

 of its progenitor. In two groups of animals, however much 

 they may at present differ from each other in structure and 

 habits, if they pass through the same or similar embryonic 

 stages, we may feel assured that they have both descended 

 from the same or nearly similar parents, and are therefore in 

 that degree closely related. Thus community in embryonic 

 structure reveals community of descent, however much the 

 structure of the adult may have been modified.'* 



If then there had been a system of progressive develop 

 ment, the successive changes through which the embryo of a 



* Darwin, Origin, &c., p. 448. 



