OX VARIATION. 165 



Sexually-produced individuals very generally pass 

 in development from a lower to a higher grade ; but 

 this can hardly be said to occur in certain forms, 

 such as Aphis, etc. 



The differences between the two forms of repro- 

 duction being thus much less than at first sight 

 appears, we are led to inquire for the reason why the 

 more complex and difficult process is so universal. 

 Sexual reproduction appears to confer two benefits 

 on organisms (1) " When species are rendered highly 

 variable by changed conditions of life, the free inter- 

 crossing of the varying individuals will tend to keep 

 each form fitted for its proper place in nature, and 

 crossing can be effected only by sexual generation " ; 

 (2) Many experiments tend to show that free 

 and wide inter - crossing induces vigour in the 

 offspring. 



Darwin concludes that the reason why the germ- 

 cell perishes if it does not unite with another from 

 the opposite sex is simply because it includes " too 

 little formative matter for independent existence and 

 development." He was led to this conclusion by 

 the fact that the male and female germ-cells " do 

 not in ordinary cases differ in their power of giving 

 character to the embryo," and also from experiments 

 which seemed to show that a certain number of 

 pollen grains or of spermatozoa may be required to 

 fertilise a single seed or ovum. "The belief that it 

 is the function of the spermatozoa to communicate 

 life to the ovule seems a strange one, seeing that the 



