STUDIES OF ADULT WEIGHT. 43 



STUDIES OF ADULT WEIGHT. 



GROWTH CURVES. 



The growth curves mentioned on page 25 were plotted on sheets of 

 coordinate paper 20X24 inches from weights that were taken in most 

 cases every week, giving an average of about 56 observations for each 

 curve. Curves were made for nearly all of the animals. When this 

 investigation was started it was supposed that weight would be the 

 main character studied. It was believed that the growth curves would 

 clearly show the adult weights, and that a comparison of the curves of 

 a litter of FI animals with those of a litter of back-cross animals might 

 picture their relative variability. A study of about 300 growth curves 

 has shown that this is not the case. To obtain rabbit curves that would 

 be smooth enough to determine the adult weights with accuracy would 

 require more perfect experimental conditions than have been possible. 

 But unfavorable conditions are by no means the only causes of trouble 

 in dealing with these curves. In many cases the animals have begun 

 to accumulate fat soon after maturity and at fifteen months their 

 curves have shown no signs of flattening. In other cases the accumu- 

 lation of fat has begun after a year, so that the fat curve and the growth 

 curve proper are easily separable. Pregnancies and nursing occasion 

 much uncertainty in estimating the adult weights of FI females. For 

 a period of ten weeks pregnant females were weighed three times a week 

 that the form of a normal pregnancy curve might be found, which 

 would assist in disentangling the curves of breeding females. No uni- 

 formity could be found in these curves. 



The most important information the growth curves afford is in 

 vouching for the recovery of animals that have been sick ; for, in spite 

 of fluctuations, the trace of a regular curve can be seen in nearly every 

 one. It soon becomes possible to determine whether at a certain point 

 an animal is below or above its normal, through a recognition of the 

 normal type of curve. Based on this element of regularity, which (when 

 shown by a part of a litter) gives the type of curve the others would be 

 expected to have followed, adult weights for the animals were estimated. 

 Approximations were made within 100 grams, in a few cases within 

 50 grams. 



COEFFICIENTS OF VARIABILITY. 



Since absolute means can be found and the coefficients of variabil- 

 ity calculated, weights have an advantage; but this can only slightly 

 offset the uncertainty of their exactness. Coefficients of variability 

 are given in table 8 for the adult weights of the various FI and back- 

 cross fraternities. Most of the coefficients of the back-cross animals 

 are higher than those for FI, but some are as low as the coefficients of 

 the FI fraternities. It should be stated that the two litters in the FI 

 fraternity of family 1493 wore found to have very different coefficients, 



