THE ORIGIN AND HISTORY OF ANIMAL LIFE 



613 



Seal flipper 



Bird wing 



Baf wing 



Horse 

 forelimb 



2 34^ 

 Human arm 



Figure 433. Homology and adaptation in bones of the forelimbs of vertebrates. The limbs 

 are homologous in being of the same fundamental structure, based on common descent, but in 

 each kind of animal they are adapted for special functions by modifications of an ancestral 

 appendage. 



tions. The eye of man, for example, has a 

 vestigial nictitating membrane (Fig. 434), 

 and the modern horse possesses splints in 

 place of what were in its ancestors func- 

 tional metacarpals and metatarsals (Fig. 

 437). Many organs during the course of 

 evolution have changed in function; for ex- 

 ample, the salivary glands in certain snakes 

 have become modified into poison glands 

 (Fig. 288). 



Embryology 



The biogenetic law was formulated many 

 years ago by a German biologist (Haeckel) 

 who observed that the early stages of verte- 

 brate embryos were remarkably similar (Fig. 

 435). He thought that animals recapitulate 

 in their developmental stages the phylo- 

 genetic history of the race. This conclusion 

 was euphoniously stated as ontogeny repeats 

 phylogeny. However, this broad generaliza- 



tion was not entirely justified. A more ac- 

 curate statement of this law is that an ani- 

 mal may repeat in its embr)'onic develop- 

 ment some of the corresponding stages of its 

 ancestors. This modification of the original 

 statement of the biogenetic law is necessary 

 because it is evident that structural changes 

 have taken place in the embryonic and 

 larval stages. For example, certain stages 

 may be omitted; others may be added, the 

 latter of course, have no ancestral signifi- 

 cance. Despite such criticism of the bio- 

 genetic law, there can be no doubt that a 

 study of the developmental stages of an ani- 

 mal give important clues regarding its an- 

 cestry. 



Comparative biochemistry 



Within recent years the iliscovery has 

 been made that the biochemical characters 

 of animals furnish convincing evidence in 



