MULTIPLE FACTORS IN MENDELIAN INHERITANCE 191 



c. Body weights 



When the work was taken over by the writer it was believed 

 that body weight could be taken as a measure of size. Ac- 

 cordingly weights of the rabbits were recorded weekly. From 

 these records growth curves were plotted by which the adult 

 weight for each animal was to be determined. That this could 

 not be done with any nicety, was clearly demonstrated by the 

 study of some 300 curves. Rabbits' weights are very sensitive 

 to changes in conditions; and to obtain curves that would be 

 smooth enough to determine adult weights with accuracy would 

 require more perfect experimental conditions than it has been 

 possible to obtain in raising large numbers of animals. In many 

 cases fat is deposited in such a way that there is no flattening of 

 the curves at about 150 days, as in most cases ; instead it may con- 

 tinue to rise for a year without flattening. Pregnancies and 

 nursing disturb the curves of the Fi females. 



The most important information the growth curves afford is 

 their vouching for the recovery of animals that have been sick. 

 In spite of fluctuations, one can see in nearly every one the trace 

 of a regular curve. Through a recognition of the normal type 

 of curve it soon became possible to determine whether at a cer- 

 tain point in a curve an animal was above or below its normal. 

 Based on this element of regularity, which, when shown by a 

 part of a litter, gives the type of curve the other would be expected 

 to have followed, adult weights were estimated. Approxima- 

 tions were made within 100 grams; in a few cases within 50 

 grams. Coefficients of variability were calculated for the Fi and 

 back cross fraternities (table 4) . Since the means were absolute, 

 standard deviations could not be used in making comparisons. 

 In ten fraternities the C. V. of the back cross animals are higher 

 than any of those of the first generation fraternities. In five fra- 

 ternities the coefficients of variability are lower than the highest 

 coefficients of the first generation. The weights show, then, that 

 greater variability is found in many of the back cross fraternities 

 than in any first generation fraternity. Whereas by themselves 



