ORIGIN OF THE INDIVIDUAL 251 



two gametes. Careful studies show, in Paramecium for in- 

 stance, that variation is greater after than before fertilization, 

 and therefore that the chief significance of the process is to 

 afford new combinations, some of which will more effectually 

 meet be better adapted to the exigencies of the environ- 

 ment, and so have a survival value for the organism in the 

 struggle for existence. So whatever the primary meaning of 

 fertilization may be, its importance in establishing the essen- 

 tially dual nature of every sexually produced organism is 

 settled beyond dispute, and it is the cardinal fact of heredity. 

 It may seem strange that such a fundamental phenomenon 

 and one so generally distributed throughout the animal and 

 vegetable kingdom should so long have eluded solution. The 

 truth probably is that therein lies the secret of the difficulty. 

 Whatever fertilization may have been originally, it is no 

 longer a simple process, but has undergone evolutionary 

 specialization hand in hand with that of other functions and 

 with the structure of organisms. To-day one or another of 

 its various aspects rejuvenation, stimulus to development, 

 control of variation, or basis of biparental inheritance 

 may assume the chief role or, at any rate, loom largest in the 

 mind of the student. The popular idea that fertilization is 

 reproduction is solely due to the fact that in higher organ- 

 isms, if fertilization is to occur at all, it must take place at 

 that period in the life history when the individual is but a 

 single cell detached from the parent that is, at repro- 

 duction. 



E. ORGANIZATION OF THE ZYGOTE 



The new individual, established by the orderly merging of 

 a cell detached from each parent in sexually reproducing 

 species, has before it first of all the problem of assuming the 

 adult form by a complicated developmental process. As we 



