GROWTH AND MORPHOLOGICAL CHARACTERS. 



63 



multiple factors for size with incomplete dominance. For example, 

 the I wild males were as variable as the | wild males. Had the parent 

 races and the Fi hybrids shown a comparatively small degree of varia- 

 bility and the F2 hybrids a decided increase in variability, then we 

 might have concluded that there were indications of a recombination 

 of factors for size. The results by no means disprove that the size- 

 difference between the guinea-pig and the wild species may not be 

 due to a difference in size factors, but the various crosses actually 

 made failed to give evidence to that effect. One could conclude more 

 logically that (1) the guinea-pig was dominant, or very nearly so, to 

 the wild species in respect to size; (2) the immediate hybrids, the 

 I wild, were very vigorous because of heterozygosis; and (3) therefore, 

 repeated crossing back to the dominant form would not increase the 

 variability. 



In deciding what the normal growth curve of any individual is, in 

 order to obtain the smoothed curves and calculate the averages and 

 coefficients of variability, errors in judgment may occur. In this 

 particular case the number of individuals was small and experimental 

 errors may have been large; hence no probable errors were calculated 

 for the average weights or coejEcients of variability. The adult 

 skeletal dimensions offered material with less objections than did the 

 growth curves. The results of both can be compared. 



In passing, it may be pointed out that all classes of individuals in 

 both sexes appeared to be less variable as they grew older. 



14. SKELETAL DIMENSIONS. 

 THE DATA ON SKELETAL DIMENSIONS. 



It was shown that the adult weight of C. rufescens was much less 

 than that of C. porcellus. The bones of the wild are likewise shorter 

 and more slender than those of the tame guinea-pig. In order to 

 make a more extended study of the size relation between the two 

 parent species and their hybrids, measurements of bones were taken 

 from prepared adult skeletons. The materials available were as 

 shown in the accompanying table. 



