6 The Rural Library. 



of any sexual union between plants, whether of different species, or varie- 

 ties, or even different flowers upon the same plant. It is a general term. 

 And the word is also sometimes used to denote the operation of performing 

 or bringing about the sexual union. There are different kinds of crosses. 

 One of these is the hybrid. A hybrid is a cross between two species, as a plum 

 and a peach, or a raspberry and a blackberry. There has lately been some 

 objection urged against this term because it is often impossible to define the 

 limitations of species, to tell where one species ends and another begins. 

 And it is a fact that this difficulty exists, for plants which some botanists re- 

 gard as mere varieties others regard as distinct species s But the term 

 hybrid is no more inaccurate than the term species, upon which it rests, and 

 so long asimen talk about species, so long have we an equal right to talk about 

 hybrids. Here, as everywhere, terms are mere conveniences, and they sel- 

 dom express the whole truth. In common speech, the word hybrid is much 

 misused. Crosses between varieties of one species are termed half-breeds 

 or cross-breeds, and those between different flowers upon the same plant are 

 called individual crosses. 



If crossing is good for the species, which philosophy and direct experi- 

 ment abundantly show, it is necessary at once to find out to what extent it 

 can be carried. Does the good increase in proportion as the cross becomes 

 more violent, or as the parents are more and more unlike ? Or do we soon 

 find a limit beyond which it is not profitable or even possible to go — a point 

 at which we say that ■ ■ An inch is as good as an ell " ? If great variability 

 is good for the species in the struggle for existence, and if crossing induces 

 variability because of the union of unlike individuals, it would seem to fol- 

 low that the more unlike the parents are the greater would be the variation 

 in offspring and the more the species would prosper ; and carrying this 

 thought to its logical conclusion we should expect to find that the most 

 closely related plants would constantly tend to refuse to cross, because the 

 offspring of them would be little variable and, therefore, little adapted to 

 the struggle for existence, while, the most widely separated plants would 

 constantly tend to cross more and more, because their offspring would preseMt 

 the greatest possible degrees of differences. We should expect, for instance, 

 that a Baldwin apple would be less likely to cross with a Greening than 

 with a Norway spruce or Indian corn ! And if we should carry our 

 thought a step farther, we should at once see that this crossing between 

 different species would soon fill in all differences between those species, and 

 that definite specific types would cease to exist. This would be pandenao* 

 nium, and crossing would be the cause of it ! 



Now, essentially this reasoning has been advanced to combat the evolution 

 of plants and animals by means of natural selection, and this proposition 



