No. 4.] CROSSING OF PLANTS. 25 



and they seldom 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 experiment 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 follow that the more unlike the parents are the greater 

 will 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 present 

 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 it is to cross with a 

 peach or a pear. 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 pandemonium, 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 nat- 

 ural selection ; and this proposition that intermixing must 

 constantly tend to obliterate all differences between plants 

 and to prevent the establishment of well-marked types, has 

 been called the "swamping effects of intercrossing." It is 

 exceedingly important that we consider this question, for it 



