CAUSES OF VARIATION 



the proper number. In this case the second polar body appears 

 in the role of a male element, so we may speak of this as a kind 

 of "fertilization by the second polar body!' 



In the other form of parthenogenesis, however, there is little 

 suggestion of a second polar body ; certainly no formal separa- 

 tion and later fusing of nuclear matter takes place. On the con- 

 trary, development takes place directly upon the extrusion of 

 the first polar body, and it is significant that individuals arising 

 in this way possess but half the number of chromosomes as com- 

 pared with those arising by sexual reproduction or by the method 

 just described. Some species (as in Artemia) : reproduce par- 

 thenogenetically by both methods, giving rise to two distinct 

 varieties, one with half the number of chromosomes character- 

 istic of the other. 2 



Mutation as related to reduction and fertilization. Mutants 

 seem to be departures characterized by a sudden loss of some 

 racial character or by its possession in some unusual degree. 

 They do not appear to be endowed with characters new to the 

 race, except when artificially produced by hybridization. 



If the process of reduction means the loss of hereditary 

 material, and if fertilization means its restoration, and if either 

 means in any sense new combinations, then we can see in the 

 two phenomena taken together, or even singly, abundant oppor- 

 tunity for the most profound variations ; indeed, admitting their 

 possibility through these causes, the wonder is that they are not 

 yet more common and infinitely more remarkable. 



If there is in any sense, however slight, a qualitative loss 

 through reduction, then by the law of chance the time is certain 

 to come when something unusual will appear. Is it not more 

 than likely that here lies a fruitful source of sweeping changes, 

 as well as of the more obscure differences that are everywhere 

 about us ? And is it not likely that still greater and more fre- 

 quent changes would present themselves were it not that fertili- 

 zation is for the most part restricted to comparatively narrow 

 lines ? 



1 Wilson, The Cell, pp. 282-283. 



3 Artemia thus varies from 84 to 168, according to the particular method 

 observed. 



