26 BOARD OF AGRICULTURE. [Pub. Doc. 



really lies at the foundation of the improvement of cultivated 

 plants by means of crossing, as well as the persistence and 

 evolution of varieties and species under wholly natural condi- 

 tions. 



We find, however, that distinct species, as a rule, refuse 

 to cross ; and the first question which naturally arises is, 

 What is the immediate cause of the refusal of plants to 

 cross? How does this refusal express itself? It comes 

 about in many ways. The commonest cause is the positive 

 refusal of a plant to allow its ovules to be impregnated by 

 the pollen of another plant. The pollen will not "take." 

 For instance, if we apply the pollen of a Hubbard squash to 

 the flower of the common field pumpkin, there will simply 

 be no result, — the fruit will not form. The same is true of 

 the pear and the apple, the oat and the wheat, and most 

 very unlike species. Or the refusal may come in the ster- 

 ility of the cross or hybrid. The pollen may "take " and 

 seeds may be formed and the seeds may grow, but the plants 

 which they produce may be wholly barren, sometimes even 

 refusing to produce flowers as well as seeds, as in the 

 instance of some hybrids between the wild-goose plum and 

 the peach. Sometimes the refusal to cross is due to some 

 difference in the time of blooming or some incompatibility 

 in the structure of the flowers. But it is enough for our 

 purpose to know that there are certain characters in widely 

 dissimilar plants which prevent intercrossing, and that these 

 characters are just as positive and just as much influenced by 

 change of environment and natural selection as are size, 

 color, productiveness and other characters. Here, then, is 

 the sufficient answer to the proposition that intercrossing 

 must swamp all natural selection, and also the explanation of 

 the varying and often restricted limits within which crossing 

 is possible. That is, the checks to crossing have been devel- 

 oped through the principle of universal variability and natural 

 selection, as has been shown by Darwin and Wallace. Plants 

 vary in their reproductive organs and powers just the same 

 as they do in other directions ; and when such a variation is use- 

 ful it is perpetuated, and when hurtful it is lost. Suppose 

 that a certain well-marked individual of a species should find an 

 unusually good place in nature, and it should multiply rapidly. 



