Mamivle and Size of Litter. 



29 



tions every endeavor has been made to get as near the actual fact 

 as possible. When no information could be found regarding the 

 point at issue in a particular species it was assumed that the mid- 

 point of the range, as given by Bellingeri, could be taken as the 

 centering point, with sufficient accuracy. It is reasonable to 

 suppose that any errors introduced by making this assumption 

 will not be biased, but will as often be in one direction from the 

 truth as in the other. 



The interracial correlation table derived from Bellingeri's 

 data is shown in Table I. 



TABLE I. 



Showing the Interracial Correlation Surface for the Variables (a) Number 

 of Mamm/E, (6) Number of Young at a Birth. 

















Size 



of Litter. 

















Mammje of Dam. 



on 



V 



On 

 J 



on 



i 



on 



i 



On 

 vt 

 I 



10 



On 



NO 



ON 



On 



GO 



1 



CO 



On 

 f 



ON 



Q\ 

 O 



A 



ON 

 1 



ON 



N 



h 



0\ 



ro 

 1 



CO 



0 



V 



On 

 1 



Total 



2- 3-9 



26 



4 



2 



I 



I 







I 

















35 



4- 5-9 



11 





3 





2 









I 







i 









18 



6- 7.9 





2 



3 



2 



2 





I 





I 















11 



8- 9-9 





1 



1 



2 



3 





2 



I 

















10 



10-11.9 







2 



2 



2 



2 







2 













I 



11 



12-13.9 





1 







I 



2 



















I 



5 



Totals 



37 



8 



11 



7 



II 



4 



3 



2 



4 



0 



0 



1 



0 



0 



2 



90 





















TABLE II. 



Constants of Variation Deduced from Table I. 



Character. 



Mean. 



Standard Deviation. 



Coefficient of 

 Variation. 



Number of mammae 



6.09 ±.23 

 3.92 ±.22 



3.25^16 

 3-05 *-IS 



53-45 ^3-36 

 77-80 ±S-8l 



Number of young at birth .... 



Coefficient of correlation r = .594 ±.046. 



From this table the following points are to be noted : 



1. Interracially the mean size of litter is approximately two 

 individuals below the mean number of mammae possessed by the 

 mother. This may be taken as a rough measure of the evolu- 

 tionary "factor of safety" in regard to these characters. 



2. There is relatively (cf. coefficients of variation) somewhat 

 more variation exhibited in size of litter than in number of mammae. 



