a1 (From the National Institute for Medical Research, Hampstead, N. W. 3, and the London School of Hygiene and Tropical Medicine, W.C.1.)
1. The average number of guinea-pigs born in albino litters is 3·000 as against 3·097 in the case of the mixed group, but the difference between the mean values is not statistically significant. Hence, it will be seen that there is little or no relationship between the colour of guinea-pigs and the number of their progeny.
2. It may be said that there is a relationship between fertility and the period of the year. There is a tendency for fewer litters to be born during the quarter, January to March, and likewise for the fertility to be lowest during this period, as the mean number of births per litter is 2·65 for albino guineapigs and 2·67 for the mixed class, both values being significantly below the mean for the whole period.
3. The mean weight of the albino guinea-pigs at birth is 81·2 ± 0·36 grm., the corresponding value for the cream, cream and white class is 82·6 ± 0·43, but the difference is of no statistical importance. Hence we conclude that the weight of a guinea-pig at birth is not affected by its colour. Once again attention is centred on the January–March quarter as the most unfavourable period, since there is a tendency for guinea-pigs of either colour born in these months to be below the normal weight.
4. When allowance was made for the effects of selection on our data, there was no material difference between the rates of growth for the two types of guinea-pigs and, furthermore, the period of the year at which littering occurred exercised no apparent influence.
5. The rate of mortality during the first thirteen days of life amongst albino guinea-pigs is 5·58 per 1000 per day, and amongst cream, cream and white guinea-pigs 4·73 per 1000 per day, but the difference probably represents nothing fundamental because, when the mortality is studied according to the size of the litter, the rates are sometimes in the reverse direction.
6. Finally, there is, in the present data, nothing to suggest that albino guinea-pigs are as regards fertility, growth and mortality, significantly different from cream, cream and white guinea-pigs.
(Received April 02 1930)