242 EXPERIMENTAL EVOLUTION LECT. 



animals may vary in a large number of directions, that 

 variability increases with variation, and that any 

 desired variation is more apt to occur among plants 

 whose tendency to variability generally has been 

 considerably increased through the process which has 

 been mentioned. And then also, it is well to 

 remember that while proceeding in this manner, 

 though we may not meet what we desire, we may 

 meet with very unexpected variations which may 

 prove of even greater interest than that which we are 

 seeking. 



In the second place one must not forget that in 

 experiments of this kind, especially with wild or 

 uncultivated plants, a long time is sometimes required 

 before any important variations occur ; the species 

 seems for a long period to resist all inducements to 

 variation, and then, all of a sudden, it begins to vary 

 considerably and in many different directions. 



So much for experiments on selection. While 

 speaking of selection, a word may also be said of the 

 method which is in some sense exactly the reverse of 

 selection ; I refer to crossing. While in the course of 

 selection progenitors are chosen among the animals 

 which are the most similar to each other, in that of cross- 

 ing, on the contrary, the progenitors are of different 

 type, and crossing is performed in order to obtain 

 animals or plants which combine the character of 



