392 THE SELF-FERTILIZATION OF PLANTS. 



test, when individuals of the same species are frequently 

 sterile with each other ! How safely make structural 

 difference, per se, a test, when there are many cultivated 

 varieties, and many natural races, of the same species, 

 distinguished from each other, by differences, greater 

 than those which mark even genera from each other ! 

 It has been popular faith in this idea, of mere structural 

 difference, which has hitherto secured Darwin's theory 

 against confutation. As soon as the idea is broached, 

 that structural difference is of little account, compared 

 with physiological difference, the main stay of his 

 theory is gone. 



The result of the test of structural difference, and of 

 the ignorant application of the test of sterility, has 

 been, to make the science of classification, chaos worse 

 confounded. The true test, of a species of plant, is 

 the capacity for long-continued self-fertilization, with- 

 out abatement in fertility, vigor, or size. All plants 

 capable of indefinitely-continued reproduction with 

 one capable of such self-fertilization, belong to the 

 same species, however structurally different such plants 

 may be. Each plant, which is self-impotent, or of 

 lessened fertility, falls short of the full structure of the 

 species. Plants, of the same, and plants of different 

 species, may be discriminated, by breeding them each 

 with another. If they are capable of long-continued 

 fertilization, they are of the same species. If, how- 

 ever, they are sterile with each other, it is not conclu- 

 sive evidence of their being of distinct species. For, 

 that may occur with individuals of the same species; 

 in fact, those most alike in structure, if wanting in any 



