384 ANIMAL BIOLOGY 



'created,' while others who are more conservative prefer to con- 

 sider the new forms as 'artificial species.' Probably all would 

 agree that some of the new types would be regarded as true species 

 were their origin not actually known. The question, however, is 

 not so important as it may, at first glance, appear. The essential 

 fact is that we now understand, at least to a considerable extent, 

 the mechanism of inheritance and variation that is at the basis of 

 similarity and dissimilarity of individuals, parents and offspring — 

 the mechanism that surely is crucially involved in the differentia- 

 tion of groups of similar individuals, or species. 



To epitomize — these facts from genetics, taken in connection 

 with the wealth of data from geographical distribution, the suc- 

 cession of types in the geologic past, and so on, give us the modern 

 background for attempting to form an opinion of the method of 

 evolution. The opinion of most biologists is that natural selection 

 in general is a guiding principle underlying the establishment 

 of the adaptive complexes of organisms. Evolution is the result 

 of mutations, germinal variations, largely, though not entirely, 

 independent of environing conditions. Many of these variations 

 give rise to characters which neither increase nor decrease the 

 adaptation of the organism, and consequently are neutral from 

 the standpoint of its survival. With regard to such characters 

 natural selection is essentially inoperative. Other germinal changes 

 occur, some of which produce adaptive and others unadaptive 

 characters, and here natural selection is effective. It may elim- 

 inate the unadaptive and leave the adaptive variations and so 

 make possible the survival value of the latter in the struggle for 

 existence. The germ plasm never ceases to experiment, or natural 

 selection to discover. Variability affording opportunity for adapta- 

 bilit y is expressed in evolvability — perhaps the most profoundly 

 significant characteristic of life. 



So, it will be noted, this is essentially a clarified view of Darwin's 

 idea of natural selection that has been made possible by recent 

 intensive studies of the intrinsic nature and the origin of varia- 

 tions. Natural selection still affords the most satisfactory explana- 

 tion of that coordinated adaptation which pervades every form of 

 life: it shows how nature can be self-regulating in establishing 

 adaptations. But it is probable — indeed, positive — that there 

 are more factors involved than are dreamt of in our biology. 



In the words of Thomson: "The process of evolution from in- 



