TIME OF BIRTH. 27 



less are in the desert region of the Great Basin or Arizona, or else 

 represent second or third litters. Dr. E. A. Mearns, United States 

 Army, who has examined many specimens in Arizona, states that it is 

 very common to find only 1 yonng- and that 2 is the usual number 

 in that region. Farther north, however, both in the case of the Prairie 

 Hare and the California Jack, 4 is probably not too high an average 

 for the first litter, but it is doubtless true that later iu the season the 

 litters are smaller. 



Time of hirtlt. — The evidence at hand not only fails to substantiate 

 the view that Jack rabbits breed every six weeks in the year, but there is 

 every reason to believe that each species has a regular breeding season 

 and a definite period of rest. Certainly no data have been found which 

 show that the young are born in the United States in October, Novem- 

 ber, or December. It is almost impossible to determine the exact dates 

 of birth unless the animals are kept in captivity, but the time can be 

 estimated approximately. As already stated, the period of gestation 

 is about thirty days, so that the specimens mentioned in the last table 

 can be utilized for this purpose by adding thirty days to the dates 

 given and the results will be within a month, and probably within two 

 or three weeks of the trv^e time. Furthermore, it may be assumed that 

 jack rabbits attain their full size (but not weight) in about two months, 

 and the size of the adults and of the young at birth being known, the 

 measurements of a young animal may be taken as a rough index of 

 its age. The following table is based on an examination of Mi young 

 rabbits selected for this purpose. Xo specimens were included which 

 seemed to be much more than half growu, and nearly all those given 

 may be assumed to be less than thirty days old and. hence the date of 

 birth less than a month earlier in each case. 



The collection contains several specimens which illustrate the size 

 and condition of the young at birth. Perhaps the most interesting are 

 4 fcetal Prairie Hares collected at Bridger Pass, Wyoming, May 5, 

 1890, evidently but a day or two before birth. The average measure- 

 ments of these specimens are: Total length, 140'"'"; hind foot, 3G""". 

 The animals are entirely covered with hair and the eyes are open. In 

 one, at least, the front teeth (incisors) are cut, and nearly all the molars 

 iu the upper jaw are just breaking through the gums. The specimens 

 having been i)reserved iu alcohol for four years are somewhat shrunken 

 and the total length is lu'obably about 'Jo'"'" too short, A specimen of 

 the Black-tailed Pabbit {Lcpun te.rianus) from Panamint Valley, Cali- 

 fornia, collected January 10, 1891 — evidently only a few days old — meas- 

 ures only 19l>""" in length, and hind foot 47""". Another of about the 

 same age from Santa Rosalia, Chihuahua, taken September 21, 1893, 

 measures 185'"'", hind foot, 43" "". Thus, the young at birth average a 

 little less than 200""" iu length; the hind foot about 40 or 45"'"^. The 

 dates of birth can be ap])roximated from the following table with suf- 

 ficient accuracy for present purposes by comparing the difference 



