214 GENERAL EMBRYOLOGY 



the presence, indeed the origin, of variations and fluctuations, 

 the "raw materials" of evolution. 



Both of these views are based upon the more fundamental 

 and underlying hypothesis of the representative particle nature 

 of the elements of the chromosomes, or perhaps of other portions 

 of the germ cells, which themselves vary in their structure or 

 their combinations. Here again the occurrence, among the 

 Metazoa, of fertilization only when the organisms are in the 

 form of single cells, grows out of the fact that complete nuclear 

 fusion can occur only when in this state. 



There is little direct factual evidence for or against these 

 views, either one of which can be maintained upon theoretical 

 grounds. In a few cases it is known that the amount of 

 variability is not significantly different among sexually (gamet- 

 ically) and asexually (parthenogenetically) produced indi- 

 viduals of the same species. And from the standpoint of more 

 recent studies upon heredity and variation the evidence is 

 chiefly either negative or opposed to the idea that this relation 

 constitutes an important element in the origin or present func- 

 tion of fertilization. The present aspects of this relation 

 between fertilization and variation merge in the larger question 

 of the relation with heredity which we may refer to next. 



Whatever the significance of fertilization may prove to have 

 been originally, its relation to the phenomena of heredity is 

 to-day undoubtedly its most important aspect, at any rate 

 among the Metazoa. The general subject of the relation of the 

 structure of the germ cells to the main facts of heredity is 

 reserved for consideration in Chapter VII, but we should point 

 out here some of the underlying conditions involved in the 

 fundamental fact of the union of the two germ cells derived in 

 nearly all cases from two different individuals of the same group 

 or species. 



As pointed out in the introductory chapter, the germ cells 

 are not to be regarded as the material links between successive 

 generations of specific organisms, for organismal specificity is 

 not discontinuous, but continuous, and the germ cells are no 

 less specific, no less the organism, than are the mature indi- 



