612 



COLLEGE ZOOLOGY 



FISH {Shark)V:ii 



Convergence 

 for aquatic 

 life 



;••; REPTILE 



.••■(ExMncl Ichthyosour) 



MAMMAL (Porpoise) 



Figure 432. Adaptive convergence or convergent evolution. Although the fish, reptile, and 

 mammal shown are not closely related, they have a marked superficial similarity because they 

 are all adapted to living in the same environment. 



revealed an abundance of intergrades for 

 which the terms suborder, subfamily, sub- 

 genus, subspecies, etc., have been coined. 

 Classification, therefore, expresses the de- 

 gree of kinship that exists among animals 

 as required by the theory of organic evolu- 

 tion. 



Domesticated animals 

 and plants 



Many plants and animals have been main- 

 tained under domestication for centuries 

 by man. During this time various breeds 

 have developed. In certain cases, we know 

 that these breeds have all arisen from a sin- 

 gle wild species; in other cases, they may 

 have evolved under domestication from sev- 

 eral wild species. That changes have taken 

 place is undeniable. For example, the fan- 

 tail, pouter, tumbler, and other types of 

 pigeons may all have evolved from the blue 

 rock pigeon. Horses, cattle, sheep, dogs, and 

 fowls appear to have undergone similar 



changes. If these various breeds had been 

 found in nature, no one would hesitate to 

 consider them distinct species; yet we know 

 that they have evolved from one or several 

 wild species. Darwin wrote two large vol- 

 umes on this subject, entitled The Varia- 

 tion of Animals and Plants under Domesti- 

 cation (1868), which make the idea of 

 fixity of species seem very improbable in- 

 deed. 



Comparative anatomy 



The study of comparative anatomy brings 

 out similarities and dissimilarities in struc- 

 ture (Fig. 433) and offers much evidence 

 in favor of organic evolution. For example, 

 the flipper of the seal, the wing of the bird, 

 the wing of the bat, the leg of the horse, 

 and the arm of man have evidently all 

 evolved from the same type of ancestral ap- 

 pendage. 



Vestigial organs, which are especially evi- 

 dent among vertebrates, furnish striking 

 evidence of changes from ancestral condi- 



