CHAP. I. INTRODUCTORY REMARKS. 27 



fore well-fitted to throw light on the nature and extent 

 of the good effects of crossing, and on the evil effects 

 of close interbreeding or self-fertilisation. The most 

 important conclusion at which I have arrived is that 

 the mere act of crossing by itself does no good. The 

 good depends on the individuals which are crossed 

 differing slightly in constitution, owing to their pro- 

 genitor^ having been subjected during several genera- 

 tions fo slightly different conditions, or to what we 

 call in our ignorance spontaneous variation. This 

 conclusion, as we shall hereafter see, is closely con- 

 nected with various important physiological problems, 

 such as the benefit derived from slight changes in the 

 conditions of life, and this stands in the closest con- 

 nection with life itself. It throws light on the origin of 

 the two sexes and on their separation or union in the 

 same individual, and lastly on the whole subject of 

 hybridism, which is one of the greatest obstacles to the 

 general acceptance and progress of the great principle 

 of evolution. 



In order to avoid misapprehension, I beg leave to 

 repeat that throughout this volume a crossed plant, 

 seedling, or seed, means one of crossed parentage, that 

 is, one derived from a flower fertilised with pollen 

 from a distinct plant of the same species. And that 

 a self-fertilised plant, seedling, or seed, means one 

 of self-fertilised parentage, that is, one derived from 

 a flower fertilised with pollen from the same flower, 

 or sometimes, when thus stated, from another flower 

 on the same plant. 



