FERTILITY 141 



development by the other. The progeny thrived and became 

 highly productive. The success of this result is believed to have 

 been due to the complementary action of two dominant factors 

 rather than an unintelligible stimulation effect brought about by 

 the cross. The general evidence as to the effect of inbreeding 

 is that it promotes homozygosity of genes, and this produces 

 uniformity of characters. Unfortunately for the practical 

 breeder, the homozygosity is very often for genes of harmful or 

 undesirable characters, and that is why the process must be 

 accompanied by careful selection. On this view, if the undesir- 

 able genes can be eliminated and only useful ones are allowed 

 to remain so as to be perpetuated, the final result should be all 

 to the good. It is suggested that it was the practical adoption 

 of this principle by certain of our famous breeders, such as Bake- 

 well and Cruikshank, which led to some of the most highly 

 successful results that have been attained in the breeding 

 of domestic animals. 



Cross Sterility between Species 



The sterility of hybrids between species is a very common, 

 not to say usual, phenomenon, but its cause is still largely 

 an open question. Such infertility, however, is by no means- 

 invariable. Thus, species of the dog genus (Canis) will breed 

 together exceptionally and produce fertile offspring. The various 

 members of the family of Bovidse or cattle are known to do the 

 same, even though the parents belong to what are ordinarily 

 regarded as different genera (e.g. the cow. Bos taurus, and the 

 bison. Bison americanus). 



In some hybrid animals the generative organs are imperfectly 

 developed, and in most the gametes are not formed. It has been 

 suggested that the sterility is duetto irregularities in the mechanics 

 of division in the germ cells, and in some hybrids the number of 

 the chromosomes derived from each parent is different, but this 

 is not invariable. 



Blakeslee has shown that in the plant Datum a new variety 

 or mutation may arise with double the number of chromosomes 

 (Polyploidy, see p. 115), and that this form is sterile with its 

 parent. This di^overy is of great interest in relation to the 

 problem of the origin of new species— the central problem which 



