CROSSING BREEDS 135 



separate sexes, a flower produces only pollen or only- 

 ovules. In other cases, as in the hermaphrodite animals, 

 the two kinds of gametes in an individual flower do not 

 ripen at the same time. All of these facts show that nature 

 appears to favor cross-fertilization over self-fertilization. 



5. Evidence from Experiment. Charles Darwin was the 

 first to prove by actual experiment that cross-fertilization 

 has, at least in some forms, advantages over self- 

 fertilization. He took plants of one species and subjected 

 them to the same general conditions. Some he forced to 

 be self-pollinated if at all. He cross-pollinated others. As 

 a result he found that those that were cross-pollinated set 

 more seed, the seed weighed more, and the plants coming 

 from them were more thrifty, on the average, than those 

 produced by self-pollinated flowers. Other observers have 

 found the same thing to be true. There is thus some 

 advantage in crossing plants and animals, and the devices 

 that we have seen that aid in crossing are expressions of 

 this advantage. Some plants, however, seem to do quite 

 as well with self-fertilization as with cross-fertilization. 



6. Why is Cross-Fertilization Advantageous? This 

 question is not easy to answer. We have not yet reached 

 the place where the biologist can say just what is gained 

 by mixing different strains in this way. It seems in some 

 way related to this mingling of protoplasms, nuclei, 

 and chromosomes, which have different history and 

 composition. It is possible that new and different 

 protoplasm coming in from the outside sperm is more 

 stimulative to the ^gg than that from the same parent. It 

 seems also that there is more chance for variety in the 

 offspring when two different strains are mixed than when 

 both cells are of the same strain. 



From this last mentioned fact, whether or not this can be 

 regarded as an advantage to the plant or animal species, 

 we can readily see how it is of advantage to man. By 

 crossing different apples, or peaches, or daisies, or cattle 



