Cross- Breeding and Hybridizing. 7 



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 really lies at the foundation of the 

 improvement of cultivated plants by means of crossing, as well as of the 

 persistence and evolution of varieties and species under wholly natural 

 conditions. 



We find, however, that distinct species, as a rule, refuse to cross, and the 

 first question which naturally arises in this discussion is, What is the imme- 

 diate cause of this refusal of plants to cross ? How does this refusal express 

 itself ? It comes about in many ways. The commonest cause is the posi- 

 tive 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 sterility 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 be- 

 tween the Wild Goose plum and the peach. Sometimes the refusal to cross 

 is due to some difference in the time of blooming or some incompatability 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 inter- 

 crossing, and that these characters are just as positive and just as much in- 

 fluenced 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 developed 

 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 useful it is perpetuated, and when hurtful it is lost. Sup- 

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

 usually good place in nature and it should multiply rapidly ; crosses would be 

 made between its own offspring and perhaps 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 



