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 isimportant 
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 Fs. 
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. Mcenkhaus’ 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. 
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