GROWTH AND MORPHOLOGICAL CHARACTERS. 61 



curve rises above the smaller race of guinea-pigs at this date, and this 

 is not due to an abrupt change in their curve, as was the case with 

 their brothers. Like their brothers, they averaged less than their 

 5 wild parent at all ages and the difference is also well defined. 



Summarizing, we may say that the j wild of both sexes lacked the 

 vigor of the § wdld, although the ^ wild females were used as one parent. 

 The i wild males were in general smaller than the guinea-pig parent; 

 but the I wild females did not agree perfectly with their brothers, for 

 they did not average less than the guinea-pig as constantly. 



(4) The I wild showed a complete return to the parental guinea-pig 

 average and any possible indication of the loss of vigor shown by the 

 I wild parent was absent. The | wild males have a composite growth 

 curve which is actually higher than the larger guinea-pig race after 

 the 140th day. The | wild females agree closely with the larger guinea- 

 pig race. It is possible that the composite curve of this hybrid class 

 of males is higher than it should be, for on account of sterility they 

 were unmated and often kept alone to prevent fighting. On the whole, 

 we may consider the | wild of both sexes the equal of the larger race 

 of guinea-pigs. The | wild males averaged larger than the J wild 

 males throughout their whole life. Their sisters were larger than the 

 I wild females up to the age of 340 days, or, in other words, until 

 about that time when the adult size was reached. The | wild, however, 

 did not equal the vigor of the ^ wild. The data on skeletal dimensions 

 will corroborate all these facts in a general way. 



(5) Two composite curves are given for each sex in the case of the 

 guinea-pig. One curve represents the average growth curve of a 

 healthy, vigorous strain of guinea-pigs. The other curve is taken 

 from the records of a closely inbred strain which was not so vigorous; 

 hence the latter lies below the former at all points. The stock used 

 as the guinea-pig parent in these experiments corresponded closely to 

 the larger strain. The difference between the two curves shows the 

 possibilities with the species C. porcellus itself. 



(6) The average weights of the females, and hence their composite 

 gro^i^h curves, were below those of the males at all ages. This was 

 true of both parent species and the three classes of hybrids given. It 

 was equally true of the other classes of hybrids subsequently produced. 



Summarizing the general results obtained as shown by all the dif- 

 ferent averages of weights, it was obvious that (1) the | wild were more 

 vigorous than either parent species; (2) the i wild lacked this vigor; 

 (3) the I wild regained the size of the original larger parent species by 

 the continued crossing back to this species. These facts will be consid- 

 ered later in connection with the discussion of the averages of the skeletal 

 dimensions. 



