The T^^end of Evolutionary Thought. 11 



Darwin regarded as the basis of evolution. Not a few Avorkers 

 have found it difficult to imagine the success of such ; and 

 Herbert Spencer, in particular, held that it was incredible that 

 without transmission of acquii'ed characters there should be 

 harmonious variation of the different parts that co-oi)erate to 

 produce one physiological result. 



But de Vries and the Mendelians go further, for they 

 challenge the heritability of the Darwinian variations. De Vi-ies 

 maintains that casual variations or fluctuations such as one finds 

 in every family, are swamped in cross-breeding ; and he argues 

 that new species appear all at once, by what he calls "mutation." 

 The Mendelians provide a reason for this : they say that each 

 individual characteristic of a species is represented by an 

 individual determinant in the germ-plasm, and the transition from 

 one form to another is only possible through the addition or dis- 

 appearance of one or more of the characteristics. They hold that 

 " the conception of evolution as proceeding through the gradual 

 transformation of masses of individuals is one that the study of 

 genetics shows immediately to be false." 



So there is a considerable body of opinion hostile to the 

 Darwin- Weismann view of the basis of evolution. 



But, secondly, the Natural Selection theory does not explain 

 the origin of the distinctively new. As Arthur Thomson says, 

 biology seems justified in holding "that there has been a frequent 

 epigenesis or new formation, a frequent outcrop of genuine 

 novelties. . . . There Avas a time when there were no insects ; 

 they came into being, and they were new ideas."! But Darwin 

 offers no explanation of the new, and Weismann has not carried 

 conviction by suggesting that the new is merely a combination of 

 the old. T. H. Morgan puts the case fairly+ as follows : "There 

 is evidence to show that the germ-plasm does sometimes change 



* Hugo de Vries : Species and Vaineties : Their Origin bv Mutation. 

 1905. 



t " The System of Animate Nature,"" vol. II,, p. 867. 

 X " Sex and Heredity," p. 17. 



