866 EXPEHIMENT STATION RECORD. I Vol. 41 



la the earlier generations malnutrition delayed the tiuie of puberty. In 

 the latter generations considerably more than 25 per cent cast their lirst litter 

 before the age of 90 days, while less than 10 per cent were more than 120 days 

 old at the time of first parturition. Of the .594 females used for breeding, 

 68.5 per cent produced the required four litters. The majority of the others 

 died of pneumonia. A few remained barren after one or two litters; post- 

 mortem examinations revealed inflamed ovaries or uterus in these cases. 

 These facts are held to Indicate that the inbreeding did not induce sterility. 

 The mortality records indicate that the span of life was longer in the inbred 

 animals than in ordinary stock. 



The behavior of some of the inbred animals used in this experiment has been 

 studied by A. W. Yerkos^ and by N. Utsurikawa.^ These investigators found 

 that the inbred rats learned their way out of a maze more slowly than stock 

 rats, and were more timid, more savage, and less active. It is suggested that 

 perhaps nervousness is an hereditary trait that is intensified through inbreed- 

 ing and that the nervousness of many pure bred domestic animals, notably 

 race horses, is associated with the inbreeding (in a broad sense) frequently 

 practiced by breeders of registered stock. 



III. The effects of inbreeding, with selection, on the sex ratio of the albino 

 rat. — The ratio of males to females among the 2,818 individuals of the 424 lit- 

 ters of stock animals used as controls was 104.6 : 100. The ratio among 

 the 3,256 inbred animals in the six generations before selection began was 

 108.6:100. The young of the A series (male line) in generations 8 to 25 

 that were produced in first and second litters (sired by inbred males) showed 

 a ratio of 122.3 : 100. Among young in the third and fourth litters of the A 

 series females (sired by stock males) the ratio was 115.6:100. Breeding 

 females of the B series (female line) in generations 8 to 25 produced male 

 and female young in the proportion of 81.1 : 100 when mated to inbred males, 

 and in the proportion of 9:! .1 : 100 when mated to stock males. 



It is concluded that by selection the inbred rats have been separated into 

 two distinct strains showing radically different sex ratios. 



IV, A further study of the effects of inbreeding on the groioth and variahiUti/ 

 in the body tceight of the albino rat. — This paper completes study I by pre- 

 senting the growth data for 296 males and 310 females of generation 16 to 25. 

 These rats were not so heavy at any age period as those of the earlier gen- 

 erations, but they were much superior in weight to stock albinos reared under 

 similar conditions of environment and nutrition. Variability did not tend to 

 decrease with the advance of inbreeding. There were practically no differences 

 in adult weights between the A and the B series. 



By way of general conclusions, the author points out that selection for vigor 

 is seemingly able to check any tendency for inbreeding to bring out undesirable 

 latent traits, and that adverse housing and feeding conditions are capable of 

 producing more detrimental effects on growth and fertility in the albino rat 

 than is inl)reeding. 



A bibliography of nearly 200 titles on inbreeding, selection, sex ratios, and 

 related topics is appended to the fourth study. 



On a back cross in mice involving three alleloniorphic pairs of characters, 

 .T. A. Detfelsen and E. Roberts (Genetics, 3 (1918), No. 6, pp. 573-598).— To 

 discover (1) any linkage that might exist between three dominant hereditary 



' Comparison of the Behavior of Stock and Inbred Albino Rats. .Tour. Anim. Be- 

 havior, 6 (191G), No. 4, pp. 267-269, flgs. 4. 



- Temperamental Difforeuccs between Outln-ed and Inbred Strains of the Albino Rat. 

 Jour. Anim. Behavior, 7 (1917), No. 2, pp. 111-129. 



