v EXPERIMENTS ON CROSSING 243 



both parental forms. In natural conditions, crossing 

 occurs both among animals and plants, although, 

 generally speaking, among animals at least, there is 

 a marked tendency against the mating of unlike, and 

 towards that of similar animals. It has even been 

 noticed that in the same town I have heard of the 

 fact in Florence the pigeons congregate in flocks 

 according to their colour, and keep together, and mate 

 together, while they seldom do either with the pigeons 

 of a different flock. In many cases, crossing gives 

 origin to hybrids which are unable to reproduce their 

 own form, so that the process is not of much use if 

 new varieties are required. In other cases, however, 

 the varieties produced by crossing are fertile inter se, 

 and it must be noticed, that when crossing does not 

 seriously impair or totally destroy fertility, it increases 

 it in a marked manner. It is on account of this 

 increase of fertility that crossing is often resorted to by 

 breeders or agriculturists, and this increase has been 

 demonstrated by Darwin for plants, and observed 

 by many persons in animals. Cornevin quotes an 

 instance when a flock of sheep yielding 6/ twin 

 births had its percentage increased to 1 3 through the 

 introduction of a ram of different breed. The same 

 is true of hogs, of pigeons, and other animals. 



If crossing is to be used in experiments on Evolu- 

 tion some points must be particularly attended to, 



R 2 



