378 FOUNDATIONS OF BIOLOGY 



the data from geographical distribution, the succession of 

 types in the geologic past, and the great diversities in 'breeds' 

 in nature, etc., give us the modern background for attempt- 

 ing to form an opinion of the method of evolution. The 

 consensus of opinion seems to be that natural selection in 

 some form is the guiding principle in the establishment of the 

 'adaptive complexes' of organisms. Evolution is the result 

 of germinal variations, largely 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 variations 

 arise which produce adaptive structures and here natural 

 selection is effective it sifts them out, as it were, from the 

 unadaptive and neutral variations and in this way makes 

 possible their survival value in the struggle for existence. 



So, it will be noted, this is essentially a clarified Darwinism. 

 Instead of all variations being heritable some are inherited 

 and some are not. Instead of all heritable variations being 

 important some are and some are not. The important 

 ones are the heritable adaptive variations and these form 

 the raw materials for natural selection. Natural selection 

 is still the only natural explanation of that coordinated 

 adaptation which pervades every form of life, but it is prob- 

 able indeed, positive that there are more factors involved 

 than are dreamt of in our philosophy. 



