62 HEREDITARY CHARACTERS 



between individuals is, in fact, a matter of training where 

 small differences are in question. Though in the very great 

 majority of instances these variations are comparatively 

 small, occasionally we come across cases where the indi- 

 vidual varies in some very striking manner from its parent. 

 In the case of the higher animals, for example, an individual 

 is occasionally born with a greater number of digits than 

 usual, a human child with six fingers or six toes. Such 

 considerable differences between parent and offspring are, 

 however, comparatively rare. It is also generally known 

 that monstrosities appear occasionally, such as a calf with 

 two heads. We may dismiss monstrosities, however, as 

 there is no evidence that they are ever perpetuated, and 

 so need not be considered in dealing with heredity. 



There are then two kinds of variations which we have 

 to consider. Innumerable small differences between parents 

 and offspring and between the offspring of the same parents, 

 which are found to occur always and continuously ; and 

 more considerable differences appearing in new individuals, 

 which are of comparatively rare occurrence. It is necessary, 

 however, to point out that there seems to be a regular 

 gradation without any definite line of demarcation between 

 these small and universal variations and the large and 

 comparatively rare variations. Biologists are divided with 

 regard to the question as to whether evolution is brought 

 about by the selection of the small universal variations, 

 or whether it is entirely due to selection acting upon the 

 larger variation. Darwin himself expressed it very definitely 

 as his opinion that evolution depended entirely upon the 

 selection of small differences, but more recently de Vries 

 has propounded the theory that the production of new 

 species depends entirely upon the selection of large differ- 

 ences. He and his followers claim most emphatically that 

 they are the true exponents of the Darwinian theory carried 

 a little further than it was carried by Darwin himself, and 

 that all their teachings are perfectly compatible with all 

 that was written in the earlier editions of the Origin of 



