136 REPRODUCTION 



and hogs, we get new strains. Sometimes these are of no 

 value, sometimes they result in a mingling of the charac- 

 ters of both parents, and sometimes they give a result 

 decidedly more perfect for our uses than either of the 

 parents. Very much of the improvement in most of our 

 cultivated animals and plants has been possible because 

 of this mixing of breeds, together with the selection of 

 what suits us best out of the results. 



7. The Limits of the Crossing. We have seen that in 

 mingling gametes there seems to be some advantage in 

 having the gametes different, as in male and female 

 parents. Where nature lays so much emphasis we have 

 come to feel there must be something worth while. There 

 is, however, a limit to the difference that is possible in 

 gametes and in parents. If we select parents of different 

 varieties, or particularly of different species, the offspring 

 are quite certain to be different from either and often are 

 not so good as either. 



The following rules may help the reader appreciate this: 



a. The more different the parents are the less likely 

 their gametes are to unite. We may cross different 

 species of lilies or different species of roses, but we 

 cannot cross lilies with roses. 



b. If fertilization succeeds, the more different the 

 parents are, the more likely is the offspring to be 

 imperfect. For example, we can cross the different 

 varieties of horses readily enough. We can also 

 cross the horse with the ass, and the result is the 

 mule. The mule has qualities that make it a better 

 animal for some human uses than either the horse 

 or the ass. The mule, however, is entirely unable 

 to breed. It cannot breed with either parent or with 

 other mules. Its gametes seem to be imperfect. 

 The horse and ass are of different, but related, 

 species. The individual plants or animals which 

 result from crossing different species are called 

 hybrids. Hybrids are not all sterile, as is the mule. 



