SEX AND EVOLUTION 323 



characters. Now if we consider the case of a species which 

 occasionally produces, by mutation, an individual with a 

 new character, and if we imagine that this species has lost 

 the power of sexual reproduction, it will be clear that each 

 new character will be confined to the individual in which 

 it originated, and to the descendants of that individual ; 

 and that a new character appearing in one individual will 

 never have a chance of combining with another new cha- 

 racter in a second individual. Let us suppose that in the 

 course of time ten new characters have appeared in ten 

 different individuals ; we have then ten new types with no 

 power of intercombination. If, however, the species 

 possesses the power of sexual reproduction and the new 

 types are free to cross, then combination becomes possible 

 and with it the possibility of the origin of a vast number 

 of new types from a small number of individual changes. 

 With 10 distinct characters there is a possibility of 1023 

 distinct combinations. The chance of new and successful 

 races being produced is enormously increased and the rate 

 of evolution must be accelerated. The importance of 

 hybridisation in the production of garden varieties, and 

 improved races of cultivated plants, needs no emphasis, 

 and may indicate a similar importance in natural evolution. 

 If it does nothing else, sexual reproduction would therefore 

 seem to have an evolutionary function ; indeed, hybridisa- 

 tion has been made by Lotsy (19 16) the basis of a complete 

 theory of evolution. 



This is the best explanation we have of the significance 

 of sexual reproduction, but we must keep in mind a diffi- 

 culty which stands in the way of its acceptance. Every 

 character of an organism must stand the test of natural 

 selection through competition with its fellows and the 

 action of adverse conditions, and, if it be definitely dis- 

 advantageous to the individual, the race which shows it 

 may succumb. We must believe that sexual reproduction, 

 especially in its lower grades, is a precarious process ; we 

 do not know that it in any way benefits the individual 

 possessing it or arising through its action ; the role that we 



