Chap. XH. GENERAL RESULTS. 457 



or may not be subjected to a change of climate, but it 

 is always grown in ground broken up, and more or less 

 manured ; it is also saved from competition with other 

 plants. The paramount importance of this latter 

 circumstance is proved by the multitude of species 

 which nourish and multiply in a garden, but cannot 

 exist unless they are protected from other plants. 

 When thus saved from competition they are able to 

 get whatever they require from the soil, probably 

 often in excess ; and they are thus subjected to a great 

 change of conditions. It is probably in chief part 

 owing to this cause that all plants with rare excep- 

 tions vary after being cultivated for some generations. 

 The individuals which have already begun to vary 

 will intercross one with another by the aid of insects ; 

 and this accounts for the extreme diversity of character 

 which many of our long cultivated plants exhibit. 

 But it should be observed that the result will be 

 largely determined by the degree of their variability 

 and by the frequency of the intercrosses ; for if a plant 

 varies very little, like most species in a state of nature, 

 frequent intercrosses tend to give uniformity of 

 character to it. 



I have attempted to show that with plants growing 

 naturally in the same district, except in the unusual 

 case of each individual being surrounded by exactly 

 the same proportional numbers of other species having 

 certain powers of absorption, each will be subjected to 

 slightly different conditions. This does not apply to 

 the individuals of the same species when cultivated in 

 cleared ground in the same garden. But if their 

 flowers are visited by insects, they will intercross ; and 

 this will give to their sexual elements during a 

 considerable number of generations a sufficient amount 

 of differentiation for a cross to be beneficial. More- 



