98 Plant- Breeding 



and, therefore, little adapted to 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 degree of differences. 



Swamping effects of inter-crossing. Now, essentially 

 this reason has been advanced to combat the evolution 

 of plants and animals by means of natural selection ; and 

 this proposition that inter-mixing 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 inter-crossing." It is exceed- 

 ingly important that we consider this question, for it 

 really lies at the foundation of the improvement of cul- 

 tivated plants by means of crossing, as well as the persist- 

 ence and evolution of varieties and species under wholly 

 natural conditions. 



What determines the limits of crossing f 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 ovule to be impregnated 

 by the pollen of another plant. The pollen will not 

 "take." For instance, if we apply the pollen of a Hub- 

 bard squash to the flower of a common field pumpkin, 

 there will 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 sterility of the cross or hybrid : the pollen 

 may "take" and seeds may be formed and the seeds 



