21)6 



GENERAL ZOOLOGY 



the gill clefts never become functional parts of a respiratory 

 system. Long before the adult stage is reached they disappear 

 or, as in the case of the first gill cleft, enter into the forma- 

 tion of entirely different structures. Embryonic rudiments 

 in higher forms must always be compared with embryonic 

 rudiments of lower forms and not with the corresponding organs 

 of the adults of the latter. Facts of development of this sort are 

 none the less significant as evidences of evolutionary relationships. 

 The homology assumed to exist between various types of 

 vertebrate limbs, previously referred to, on a basis of the com- 

 parative morphology of the adult limbs, is supported by develop- 

 ment to the extent that in each case the limb arises from a 



Fig. 174. — Domestic pig with its wild-boar ancestor. {After Romanes, Darwin 

 and after Darwinism, Open Court Publishing Company.) 



similar rudiment. Vertebrate limb rudiments are strikingly 

 alike, particularly those of a single group, such as the mammals. 

 Soon, however, each type of limb develops along its own par- 

 ticular lines into the type characterizing the adult. Thus the 

 human limb does not pass through stages found in adults of 

 lower mammals — instead it develops from a limb-bud stage, 

 common to all of them, into a human limb. The origin of the 

 limb in all cases from the same sort of rudiment is interpreted as 

 indicating a common evolutionary origin. Enough of the com- 

 mon plan of structure survives in the adults to make the com- 

 munity of origin recognizable. The facts of comparative 

 anatomy find their explanation in the facts of development. 



Artificial Selection. — It has been pointed out that the members 

 of the same species are not identical. Animal and plant breeders 



