No. 4.] CROSSING OF PLANTS. 27 



Crosses would be made between its own offspring and per- 

 haps between those offspring and itself in succeeding years ; 

 and it is fair to suppose that some of the crosses would be 

 particularly well adapted to the conditions in which the 

 parent grew, and these would constantly tend to perpetuate 

 themselves, while less adaptive forms would constantly tend 

 to disappear. Now, the same thing would take place if this 

 individual or its adaptive offspring were to cross with the 

 main stock of the parent species ; for all the offspring of 

 such a cross which are intermediate in character and there- 

 fore less adapted to the new conditions would tend to dis- 

 appear, and the two branches would, as a result, become 

 more and more fixed, and the tendency to cross would con- 

 stantly decrease. The refusal to cross, therefore, becomes a 

 positive character of separation, and the "missing links" 

 which resulted from crossing are no more or no less inex- 

 plicable than the "missing links" due to simple selection; 

 or, to put the case more accurately, natural selection weeds 

 out the tendency to promiscuous crossing, when it is hurtful, 

 in just the same manner that it weeds out any other injurious 

 tendency. It makes no difference in what way this tendency 

 expresses itself ; whether in some constitutional refusal to 

 cross, — if such exists, — or in infertility of offspring, or in 

 different times of blooming, — all equally come under the 

 power of natural selection. We are apt to look upon infer- 

 tility as the absence of a character, a sort of a negative fea- 

 ture which is somehow not the legitimate property of natural 

 selection ; but such is not the case. We are perhaps led the 

 more to this feeling because the word infertility is itself 

 negative, and because we associate full productiveness with 

 the positive attributes of plants. But loss of productiveness 

 is surely no more a subject of wonder than loss of color or 

 size, if there is some corresponding gain to be accomplished. 

 In fact, we see, in numerous plants which propagate easily 

 by means of runners and suckers, a very low degree of pro- 

 ductiveness. 



Now, if this reasoning is sound, it leads us to conclusions 

 quite the reverse of those held by the advocates of the 

 swamping effects of intercrossing, and these conclusions are 

 of the most vital importance to every man who tills the soil. 



