GROWTH AND MORPHOLOGICAL CHARACTERS. 57 



in weight about the end of April, when the quahty of beets and turnips 

 was poor and the supply was low; but toward the middle of May 

 the}^ recovered completely through copious feedings with fresh green 

 grass and winter rj^e. The irregularities in growth were caused by 

 external conditions, and observations of these conditions were made. 

 It requires no stretch of the imagination or undue speculation to smooth 

 such irregularities and thus procure a curve which more truly shows 

 the general growth tendency in an animal. Smoothed curves were 

 made for parent and hybrid individuals as follows: 



C. rufescens 9 



* wild 15 



i wild 37 



I wild 138 



C. porcellus (small inbred strain) 45 



(normal strain) 109 



Total 353 



All sick animals and those whose curves were not wpII established 

 becauseof early death or present immaturity, were neglected. Diseased 

 animals show an irregular curve with a large final loss in weight and 

 w^pre therefore neglected. Many other guinea-pigs and hybrids have 

 been studied, and can be added, when their growth is sufficiently com- 

 plete. It is quite significant that a duplicate set of smoothed curves 

 was made for about 75 animals. This set did not vary much from 

 the first set. We thus have a check on errors in judgment, for the 

 duplicate set was made a number of months after the first set and with- 

 out any reference to the same. I am, therefore, led to believe that the 

 average of the smoothed curves is correct within ±25 grams. 



Having once obtained the smoothed curves, composite curves for 

 the males and females of different classes were calculated. The method 

 is simply to find the arithmetic average of the weights in the smoothed 

 curves at 15 different intervals (tables 59 and 60). For example, if we 

 average the weights of all | wild hybrid males at the age of 100 days, 

 as given in their smoothed curves, we obtain an average of 555 grams. 

 This gives one point from which to plot the average of the curves of 

 animals in that group. The other points were similarly calculated, 

 and a composite curve or average of the smoothed curves was plotted. 

 The composite curves of the wild and tame species and three classes 

 of hybrids are shown in text-figures 1 and 2. 



Needless to say, all animals were kept in healthful quarters, with 

 an abundant supply of food and water. The food at all times con- 

 sisted of oats. In the winter this was supplemented by daily feedings 

 of beets or turnips; in the summer, by fresh green grass and clover. 



