General Results of Mating 193 



made for but few cases, this tendency for like to mate with 

 like, and to refuse to mate with unlike, probably exists in 

 considerable degree throughout the world of organisms. 

 This is one of the important facts to be reckoned with in 

 attempting to get any general picture of the results of 

 mating. 



General Results of Mating: To a picture of the general 

 results of mating we now turn. In what respect does the 

 world of organisms, or any particular group of organisms, 

 differ from the condition which we would find if no mating 

 occurred? We leave out of account here the results of the 

 replacement of the old active nucleus by the reserve nucleus, 

 since this is not a distinctive feature of mating, occurring as 

 it does in the infusoria without mating; and in most organ- 

 isms not occurring even at mating. 



We hear it maintained on the one hand that mating pro- 

 duces variation; some assert, indeed, that it is the great 

 source of variation. On the other hand, some maintain that 

 the result of mating is to prevent or destroy variation ; to 

 keep the species of organisms uniform. Facts can be ad- 

 duced that support both these propositions. 



The difficulty here is that the expressions "increase varia- 

 tion" or "decrease variation" are ambiguous, and that 

 neither of them precisely touches the essential point. The 

 increase and decrease of variation are mere diverse aspects 

 of what really occurs ; sometimes one of these may result 

 from mating, sometimes the other. The really fundamental 

 thing that mating does is to produce new combinations of 

 hereditary characters. And in so doing it quite changes the 

 face of the world of organisms. 



We may illustrate this most simply by noticing again what 

 happens in the case of the primary hereditary characters, 

 the chromosomal packets ; we know that the secondary char- 



