278 Proceedings of tJie Royal Society of Victoria. 



More accurate results may be had by taking the families 

 separately and adopting for each its own value of n, these values 

 being nearly, but not quite, equal. In the same way by putting 

 ;/ ^ 55 we find that the Suidce and Hippopotafiiidce make a 

 consistent group, though not running uniformly with the rest of 

 the Artiodactyls. 



The period of gestation among these animals is generally only 

 iKUghly determined by observing in what months the sexes come 

 together, and then observing in wliat month the young are born. 

 How fallacious this may be has been shown by BischofF in the 

 case of the Roebuck, the female of which does not produce her 

 young until more than nine months after the rutting season. 

 But it is now known that this is not the period of gestation, for 

 the spermatozoa lie for four months in the uterus without fertil- 

 ising the ovum, so that the real period is only some tive months. 

 The same phenomenon is observed with bats and other mammals. 

 Selenka has shown that with the Virginian Opossum the time 

 from copulation to birth is 13 days while the actual time of 

 gestation is only 1~ days. So in the U.S. Fisheries' Report of 

 1884, the statement is made that while the males of Evibiotocidce 

 impregnate the females in autumn the young are born alive in 

 the following summer. Hence the spermatozoa must lie inactive 

 for many months. 



It is quite probable, as the foregoing list suggests, that beside 

 the Roebuck, there are other species of deer in which the same 

 peculiarity occurs to a less extent. Perhaps the same thing 

 occurs in the case of the Beaver which is a very aberrant species, 

 as will be seen from the list given in the appendix of all the 

 Rodentia for which information is available. The Perissodactyla 

 make another consistent group. All the species for which informa- 

 tion is availaljle are given in the appendix. 



In dealing with the mammals we have found it necessary to 

 give different values to the constant. There are two biological 

 reasons for this. The first is that some animals are carried by 

 their mothers till fairly well able to take care of themselves. A 

 calf, or a foal, or a young deer is sufficiently matured to trot after 

 its mother in a few hours after birth ; while a kitten, or a 

 puppy, or a tiger cub is for a long time helpless. One animal 

 therefore remains in its mother's womb until tolerably complete 



