FERTILITY IN ANIMALS 553 



may be summed up in the trite statement, Nature abhors inbreeding. 

 From his extensive investigations Darwin concluded that all organic 

 beings benefit from an occasional cross and that the inevitable effect of 

 continued inbreeding is loss of size and decreased constitutional vigor 

 and fertility, and at times unusual tendency toward the production of 

 malformations. Since Darwin's evidence was drawn largely from do- 

 mesticated animals, and since other serious detrimental features of in- 

 breeding are pointed out in addition to loss of fertility, it is important 

 that enquiry be made into the reasons why inbreeding should result in 

 decreased fertility. It is, also, important to note that we are attempting 

 to harmonize in this treatment Darwin's conclusions with a theory of 

 heredity unknown to him. 



Inbreeding not in Itself Harmful. Although supposed evidence of 

 harmful effects of inbreeding has been presented by a number of inves- 

 tigators, there is nothing in this evidence which necessarily throws the 

 blame upon inbreeding in itself. A single contrary case is all that is 

 necessary for establishing the negative interpretation, and there are a 

 number of such cases. Thus investigations on the effects of inbreeding 

 in the fruitfly have been carried out on a much more extensive scale than 

 would ever be possible with any of the higher domestic animals. For 

 example, Castle and his associates inbred the fruit fly for fifty-nine 

 generations, mating brother with sister throughout the investigations. 

 They reached the general conclusion that inbreeding unaccompanied 

 by selection generally results in decreased productiveness, but that proper 

 selection for high productiveness results in maintaining the original 

 fertility of the race. They found further that low productiveness is 

 sometimes inherited like a Mendelian recessive, as shown by its appear- 

 ance in alternate generations, and that in crosses between strains of 

 high and low productiveness there was evidence of segregation in F 2 . 



Castle further comments upon a polydactylous race of guinea-pigs 

 which was descended from a single individual. They have been inbred 

 for over 10 years, yet despite this fact they show no signs of diminished 

 fertility; on the contrary, they are superior in size and in constitutional 

 vigor to most races. Moenkhaus' results with Drosophila also seem 

 to indicate that a high degree of fertility may be maintained in successive 

 generations of inbreeding if sufficient care be taken to select from the 

 most fertile individuals. Hyde, on the other hand, found a decrease in 

 fertility consequent on continued inbreeding. The experimental results, 

 therefore, show that sometimes inbreeding does not result in diminished 

 fertility. The fact, however, that there are so few cases in which in- 

 breeding has not been followed by measurably harmful results calls for 

 some explanation. In the rest of this chapter some reasons for this 

 fact will be pointed out. 



