EFFECTS OF INBREEDING AND CROSSBREEDING. 35 
simply to the impossibility of such a formulation until more was 
known of the principles of ordinary heredity. 
This same remark applies to a number of experiments on inbreeding 
of animals which were made before the rediscovery of Mendel’s law. 
Crampe (1883) and Ritzema-Bos (1894) inbred rats and obtained 
marked degeneration in fertility and vitality. Im Crampe’s strain 
there was also considerable decline in weight and many abnormalities 
appeared, effects which Ritzema-Bos did not obtain. Weismann and 
Von Guaita bred mice brother and sister for many generations and 
noted a decline in fertility. Similar results were obtained by Fabre- 
Domengue with pigeons. 
MENDELIAN HEREDITY AND THE PROBLEM OF INBREEDING. 
With the rediscovery of Mendel’s law, the explanation of at least 
one class of effects attributed to inbreeding at once became clear. 
It had often been noted that in the human race certain rare abnor- 
malities, of which albinism is a good example, most frequently 
appeared among the progeny of consanguineous marriages. Good 
reasons were soon found for believing that albinism in man is a simple 
Mendelian recessive. A recessive factor can come into bodily expres- 
sion only if it is received from both parents. A rare recessive trait 
is transmitted by many more people than actually show it. Never- 
theless, these transmitters are relatively uncommon in the total popu- 
lation and the chances of union between them are not great. If, 
however, a given individual can transmit the trait, the chances are by 
no means small that a close relative will also have received the factor 
from the common ancestor and be a transmitter. Thus it is clear 
why consanguineous matings should frequently bring to lght such 
traits as albinism. We see that inbreeding can not cause abnormali- 
ties of this kind to appear in a stock from which the genetic basis is 
absent. It is, however, a system of mating which is likely to reveal 
any abnormalities carried out of sight in the stock by recessive factors. 
We can see how inbreeding could lead to the frequent appearance of 
abnormalities in Crampe’s stock of rats and yet fail to do so in the 
stock of Ritzema-Bos. 
Castle and students (1906) inbred the fruit fly Drosophila melano- 
gaster for 59 generations of brother-sister mating. While much steril- 
ity and low fertility appeared in the early generations, it was found 
possible to maintain high fertility by selection of lines. The segrega- 
tion of recessive factors for low fertility was clearly indicated. These 
results have been confirmed as far as the main features are concerned 
in later experiments with the same fly by Moenckhaus, Hyde, and 
Wentworth. 
Davenport (1908) called attention to the fact that in most known 
cases the dominant character in a pair of Mendelian allelomorphs was 
