ARTIFICIAL SELECTION 



89 



traits become more important in the more highly specialized 

 organisms. Among animals, related species often cannot be 

 crossed at all; the germ cells refuse to intermingle. Sometimes 

 there is a very imperfect mingling and the resultant animal is 

 divided within itself and does not live long. An example of this 

 is seen in Dr. Moenkhaus's cross of the silverside (Menidia) 

 with the killifish (Fundulus). The unmixed chromosomes of the 

 germ-cell nucleus are seen unblended, through several segmen- 

 tations of the egg. 



In the case of the mule, the cross of the horse with the ass, 

 the hybridization is readily effected, but the resultant offspring 

 is sterile. Presumably the hereditary difference in the repro- 



FIG. 55. Wild boar contrasted with modern domestic pig. (After Romanes.) 



ductive organs in the two parental strains is too great to allow 

 the normal development of generative organs in the progeny. 



In general, crosses between closely related species are fertile, 

 the degree of fertility being less as the parent species are more 

 widely differentiated. Among animals, any great difference 



