56 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



The two parent species, C. porcellus and C. rufescens, and their 

 hybrids of various blood dilutions, which formed the material for Part I, 

 are also used as the basis for Part II. Each parent species is of very- 

 distinct and specific size, such that environment does not obliterate the 

 difference. Unfortunately, the cross involved sterility and necessitated 

 crossing back to males of the parent species. A careful examination 

 of growth curves and skeletal dimensions was made to study size- 

 inheritance in such a mammalian species cross and to compare it with 

 the work already cited. 



13. GROWTH. 



THE DATA. 



Cavia rufescens is a smaller species than Cavia porcellus. The 

 average healthy adult weighed about 425 grams; the females were a 

 trifle lighter, or about 420 grams. One male (cfl) alone reached the 

 500-gram mark in any of his weights; but he was fat, and his weight 

 was above normal. His son, though shghtly larger in skeletal dimen- 

 sions and in good condition, was about the average weight. The 

 average weight of guinea-pigs is twice that of the wild; and since the 

 average is so much larger, it follows that many guinea-pigs are more 

 than twice as heavy. I have never seen a guinea-pig of either sex, 

 with a normal healthy growth curve, maintain such a low weight as 

 the wild. This statement is made on the basis of an intimate acquaint- 

 ance with the growth curves of several hundred guinea-pigs. In order 

 to study the growth curves of the parent species and hybrids, the weights 

 were taken about once a week until the curve was well established. 

 After that, observations were made at less frequent intervals. The 

 weights of pregnant females were taken dm-ing the period of gestation, 

 but not used on account of the varying number of fetuses. Having 

 obtained the weights, the growth ciurve of each animal was plotted on 

 coordinate charts by placing the days on the abscissas and the grams 

 on the ordinates. 



Any individual curve would naturally show a depression when 

 external conditions were poor and an elevation when conditions were 

 conducive to fatness. Since no growth curve is in itself an infallible 

 expression of the general growth tendencies of an animal, a second set 

 of curves was drawn, in which the irregularities were smoothed to 

 show as nearly as possible the normal growth of each individual. This 

 may seem arbitrary; but in reaUty it does not signify any bias, for 

 in all cases the smoothed curve was determined by the majority of 

 points in the actual curve. Minot (1891) has shown "that any irregu- 

 larity in the growth of an individual tends to be followed by an opposite, 

 compensating irregularity; and that variability decreases with age." 

 To be concrete, all animals in these experiments showed a decrease 



