64 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



It was found that the skeletons had completed growth at the end 

 of 15 months. Osseous nodules and ridges, to be sure, are laid down 

 at a later date; but they do not influence the measurements considered. 

 Care was taken to see that sutures between the epiphyses and diaphyses 

 were closed. Furthermore, the suture between the basioccipital bone 

 and the basisphenoid bone is one of the last to fuse in mammalian 

 skulls, and this was completely fused at the age of 15 months. The 

 bone measurements were, therefore, taken from fully adult animals 

 whose bones had reached their maximum size. 



In preparing the skeletons all individuals were boiled separately in 

 soap and water. The flesh was brushed away and the bones were dried, 

 properly labeled, and filed in separate boxes. Errors were thus avoided. 

 In all cases the skull, lower jaw, scapula, right front leg, and right hind 

 leg were saved. Whenever possible the entire skeleton of the wild and 

 early hybrids was saved. Unfortunately, a number of adult wild and 

 adult I wild were discarded by a laboratory helper when they died. 



Sixteen different measurements were taken on all skeletons. In 

 addition to these, 13 more measurements were taken in the case of 

 the wild, the ^ wild, and the i wild. The results, given in tables 

 63 to 66, were calculated from these measurements. In deciding upon 

 the different possible measurements to be used, those actually used 

 were chosen for the following reasons: (1) They could be taken accu- 

 rately without any slipping of the calipers; (2) they were the largest 

 measurements, thus diminishing the effect of any experimental errors; 

 (3) they took into account those dimensions in which the wild and 

 tame parents differed in the most marked degree. All the dimensions 

 were taken with sliding vernier calipers and recorded in terms of 0.1 mm. 

 The averages, however, are given in millimeters. For example, the 

 average skull length of 78 male guinea-pigs was 68.48 mm. 



The use of skeletal dimensions in a study of size-inheritance has 

 advantages which the weights lack. In the case of the growth curves, 

 two observers might arrive at different conclusions with regard to an 

 adult weight; or even the same observer has slightly different views 

 at different times. The measurements of the adult skeleton, however, 

 were so exact that a remeasurement gave the same result at all times 

 within =t: 0.2 mm. In repeating many bone measurements, it was 

 found that the second observation tallied completely with the first in 

 almost all cases. When a difference did occur, it was so small as to be 

 negligible. Furthermore, the adult skeletal dimensions were far less 

 variable than the adult weights, meaning that the environment prob- 

 ably affects the weights more. Of course, no claim is made that the 

 adult skeletal dimensions represent the precise genetic possibility of 

 an animal, but under normal conditions they probably approximate 

 it more closely than do the weights. All of these considerations made 

 the skeletal dimensions a better basis for study than weights. 



