10 INTRODUCTORY REMARKS. CHAP. I 



Mimulus luteus and Ipomoea purpurea, both of which, 

 unlike the Linaria and Dianthus, are highly self- 

 fertile if insects are excluded. Some flowers on a 

 single plant of both species were fertilised with their 

 own pollen, and others were crossed with pollen from 

 a distinct individual ; both plants being protected by a 

 net from insects. The crossed and self-fertilised seeds 

 thus produced were sown on opposite sides of the same 

 pots, and treated in all respects alike ; and the plants 

 when fully grown were measured and compared. With 

 both species, as in the cases of the Linaria and 

 Dianthus, the crossed seedlings were conspicuously 

 superior in height and in other ways to the self- 

 fertilised. I therefore determined to begin a long 

 series of experiments with various plants, and these 

 were continued for the following eleven years ; and we 

 shall see that in a large majority of cases the crossed 

 beat the self-fertilised plants. Several of the excep- 

 tional cases, moreover, in which the crossed plants 

 were not victorious, can be explained. 



It should be observed that I have spoken for the 

 sake of brevity, and shall continue to do so, of crossed 

 and self-fertilised seeds, seedlings, or plants ; these 

 terms implying that they are the product of crossed or 

 self-fertilised flowers. Cross-fertilisation always means 

 a cross between distinct plants which were raised from 

 seeds and not from cuttings or buds. Self-fertilisation 

 always implies that the flowers in question were im- 

 pregnated with their own pollen. 



My experiments w,ere tried in the following manner. 

 A single plant, if it produced a sufficiency of flowers, or 

 two or three plants were placed under a net stretched 

 on a frame, and large enough to cover the plant 

 (together with the pot, when one was used) without 

 touching it. This latter point is important, for if 



