THE PRACTICAL VALUE OF GENETICS 349 



tribution to the new individual. In no case can we conclude 

 that chromosomal insufficiency is responsible, for the haploid 

 complex of either germ cell may contain a complete set of deter- 

 miners for the production of a new individual. Hybrids have 

 been produced many times between species with different numbers 

 of chromosomes. The general reason for infertility between 

 species may therefore be stated as some lack of harmony in the 

 accessory phenomena of reproduction. 



Since varieties and strains of the same species are similar in 

 all fundamental structures and processes this difficulty cannot 

 hinder the production of hyl^rids between such groups. 



Infertility of Hybrids. Although difference in the chromosome 

 complexes of the parents need not affect the production of 

 the hybrid, it may well be expected to have serious results in 

 the production of the F2 generation. Since the process of matura- 

 tion of the germ cells hinges upon synapsis and the resulting 

 reduction of the chromosomes, it is easy to see that any asymmetry 

 which affects the consummation of this delicately adjusted series 

 of events may prevent the formation of normal germ cells. The 

 number of chromosomes found in ova of the horse is said to be 

 nineteen, and the number in the spermatozoa of the ass thirty-two 

 or thirty-three. The mule therefore has an asymmetrical chromo- 

 some complex. Male mules are not known to produce functional 

 germ cells. Cases are on record of fertile female mules, and they 

 may occasionally occur, although they are open to doubt. The 

 rarity of even doubtful cases is in itself suggestive. 



When the species are closel}^ related and have similar chromo- 

 somes there is no reason to expect infertility in their hybrids, but 

 the actual occurrence of infertility even in such hybrids forces us 

 to the conclusion that physiological differences in the chromosomes 

 may exist even when visible morphological differences are lacking. 

 The production by some species of fertile female hybrids and 

 infertile males is another puzzling complication of the problem. 

 This is true of the hybrids of domestic cattle with the bison. 



For practical purposes it is evident that the infertility of hybrids 

 is not insurmountable. It is, indeed, no more serious than the 

 inconstancy of desirable heterozygous strains. Either demands 

 the maintenance of pure parent stocks and the production of new 

 hybrids solely by repetition of the cross unless the breeder, in 

 the case of heterozygotes, is willing to breed from hybrids and 



