14 BULLETIX 1090, XJ. S. DEPAETIMEXT OF AGEICULTURE. 



1 



All the factors which have been considered — constitutional vigor, 

 heredity, season, age of dam, and interval since preceding litter — ^if 

 combined, determine the size of litter onl}- to a small extent, prob- 

 ably less than one-tenth. The determination of the size of any par- 

 ticular Utter must be due largely to rather temporary conditions. 

 The immediate direction of change in the condition of the dam at a 

 critical period, for example, may be the important factor. From 

 the standpoint of the condition of the female, during an appreciable 

 period of time, it appears that variations in size of Utter are largely 

 a matter of chance. The most vigorous female may have a Utter of 

 1 under what seems the best of conditions, and a litter of 4 may be 

 bom when everything seems opposed. 



MORTALITY AMONG THE YOUNG. 



Guinea pigs may be born dead for a variety of reasons. A large 

 percentage of those classified as born dead are born prematurely, 

 and the average weight is much less than that of those born aUve. 

 There are, however, not infrequent cases of animals which are unu- 

 suaUy large at birth, but are found dead apparently because of dif- 

 ficulties in parturition. Many of those classified as born dead un- 

 doubtedly were born aUve but died before being recorded. In gen- 

 eral the percentage born aUve obviously depends largely on the 

 health of the dam. Unfavorable environmental conditions act in 

 the main indirectly on the young. The inherent vigor of the young, 

 however, plays some part, as is shown by the improvement in the 

 percentage born aUve when inbred females are mated with um-e- 

 lated males, instead of with their brothers, a point which will be 

 discussed in a bulletin to follow. 



The percentage of the young which is raised among those bom 

 alive also depends much on the health of the dam, but to a less ex- 

 tent than the percentage born aUve. There have been cases in 

 which the young reached 33 days largely through their own efforts, 

 the mother having died a few days after their birth. Environmental 

 influences act on the young directly as well as indirectly in this case. 

 The inherent vigor of the young counts for much. Thus we find 

 that the percentage raised of those born alive shows a more marked 

 improvement when inbred females are crossed with unrelated males 

 instead of with brothers than is the case with the percMitage bom 

 aUve. 



Both of these percentages reflect closely the changes in environ- 

 mental conditions. Inadequate or inferior green feed causes a large 

 number of stillbirths and an increased mortaUty among the Uving 

 young. A change from alfalfa to timothy hay has been observed 

 to have the same effects. The records are usuaUy much better in 

 both respects in summer than in winter. In years in which one of 



