io The Rural Library. 



or two volumes ; but a list of all the -individual plants of the world aoaldjnot 

 be compressed into ten thousand volumes. There are a few genera in which 

 the species are not well denned, or in which some character of inflorescence 

 favors promiscuous crossing, in which hybrids are conspicious ; but even here 

 the number of individual hybrids is very small in comparison with the whole 

 number of individuals. That is, the hybrids are rare, whilethe parents may 

 be common. This is well illustrated even in the willows and oaks, in which, 

 perhaps, hybrids are better known than in any other American plants. The 

 great genus carex or sedge, which occurs in great numbers and many species 

 in almost every locality in New England, and in which the species are par- 

 ticularly adapted to intercrossing by the character of their inflorescence, 

 furnishes but few undoubted hybrids. Among 167 species and prominent 

 varieties inhabiting the northeastern states, there are only nine hybrids re- 

 corded, and all of them are rare or local, some of them having been collected 

 but once. Species of remarkable similarity may grow side by side for years, 

 even intertangled in the same clump, and yet produce no hybrid. These 

 instances prove that nature avoids hybridization, a conclusion at which we 

 have already arrived from philosophical considerations. And we have reason 

 to infer the same conclusion from the fact that flowers from different species 

 are so constructed as not to invite intercrossing. But, on the other hand, 

 the fact that all higher plants habitually propagate by means of seeds, which 

 is far the most expensive to the plant of all methods of propagation, while 

 at the same time most flowers are so constructed as to prevent self-fertiliza- 

 tion, prove that some corresponding good must come from crossing within 

 the limits of the species or variety ; and there are purely philosophical rea- 

 sons, as we have seen, which warrant a similar conclusion. But experiment 

 has given us more direct proof of our proposition, and we shall now turn 

 our attention to the garden. 



Darwin was the first to show that crossing within the limits of the species 

 or variety results in a constant revitalizing of the offspring, and that this is 

 the particular ultimate function of cross-fertilization. Kolreuter, Sprengel, 

 Knight, and others, had observed many, if, indeed, not all the facts obtained 

 by Darwin, but they had not generalized upon them broadly and did not 

 conceive their relation to the complex life of the vegetable world. Darwin's 

 results are, concisely, these : Self-fertilization tends to weaken the offspring, 

 as compared with its natural condition ; crossing between different plants of the 

 same variety gives stronger and more productive offspring than arises from 

 self-fertilization ; crossing between stocks of the same variety grown in different 

 places or under different conditions gives better offspring than crossing between 

 different plants grown in the same place or under similar conditions ; and his 

 researches have also shown that, as a rule, flowers are so constructed as to f awpr 



