Conclusion 263 



In fertilization the nuclei of father and mother simply 

 touch each other. In the course of development the con- 

 tact becomes gradually closer, bringing their equivalent 

 elements as near to each other as possible, in such a way 

 that the latter finally all lie side by side in pairs. But the 

 pronuclei by no means lose their independence thereby, 

 and for the purpose of every nuclear division they sepa- 

 rate their component parts more or less distinctly. Shortly 

 before their separation, their leave-taking, they are still 

 the same as before. But now they exchange their indi- 

 vidual units, and thus cause the creation of those countless 

 combinations of characters, of which nature is in need in 

 order to make species as plastic as possible, and to em- 

 power them to adapt themselves in the highest degree to 

 their ever changing environment. 



This increase of variability and of the power of indi- 

 vidual adaptation is the essential purpose of sexual repro- 

 duction. It can be attained only by a mutual combination 

 in all conceivable forms of the peculiarities developed in 

 different individuals in different directions and degrees. 

 To this end the pronuclei mutually exchange their units 

 from time to time, and by assuming, on the ground of ex- 

 periments with hybrids, that this takes place, on the whole, 

 according to the laws of chance, that is, according to the 

 theory of probability, we have gained a basis which al- 

 lows us to probe to its very bottom this most significant 

 and mysterious process. 



