292 GENERAL ZOOLOGY 



with the classification of similar animals in the same or different 

 species result sometimes in arbitrary decisions, which must be 

 changed later when additional specimens become available for 

 study. On a basis of special creation, every species should be 

 distinct from every other species, without wide variation within a 

 species and without overlapping between species. On the other 

 hand, variability within and among species is a natural predis- 

 posing cause or accompaniment of an evolutionary process. To 

 the degree, then, to which plasticity can be demonstrated as a 

 property of species, presumptive evidence of an evolutionary 

 process may be said to exist. 



The mutant conditions appearing in Drosophila and used as a 

 basis for numerous breeding experiments are examples of vari- 

 ations of a greater degree than normal. Little or nothing is 

 known as to the cause of mutations, but their occurrence pro- 

 vides a basis for further evolutionary change. Since mutations 

 occur naturally, the condition cannot be attributed to the 

 abnormal laboratory conditions under which the flies are bred. 

 A fuller discussion of the subject of variations will be considered 

 presently in connection with the work of Darwin and De Vries. 



Homology. — The concept of homology in parts or organs of 

 different animals implies a common evolutionary origin of cor- 

 responding parts. A study of the comparative anatomy of any 

 animal group, such as the vertebrates, reveals a basic pattern of 

 structure occurring throughout the group, modified in various 

 ways in the different subgroups, but always referable to a single 

 structural type. Thus the skeleton of the forelimb of the bird, 

 bat, whale, ox, horse, and man show the same parts of a common 

 type of structure, though in some cases the parts are distorted 

 almost beyond recognition (Fig. 172). The hand region of the 

 bird consists only of rudiments of three digits and three meta- 

 carpal elements. In the bat the digits are enormously elongated, 

 exceeding in length the remainder of the forelimb skeleton. In 

 the whale the forelimb is greatly shortened in all its parts to 

 form a paddlelike structure. In the ox there is a reduction of 

 parts in the hand region to two fused metatarsals and two digits. 

 This reduction is carried even farther in the horse. Curiously 

 enough the human forelimb is the least modified of the lot. 

 Such limbs are said to be homologous because they are believed 

 to be derived by evolution from a common type of ancestral 



