No. 598] SHORTER ARTICLES AND DISCUSSION 



027 



work became indispensable. It is just here, too, that the purely 

 descriptive significance of fertility gives way to genetic, eco- 

 iioii c and' sociological sources of interest. 



As yet information on these subjects is all too meager. Lloyd 1 

 has emphasized slight differences in fertility in species format ion 

 in the rodents. Donaldson- lias brought together the available 

 data for fertility in the rat. For swim-. Rommel-' and Hitting' 

 have given extensive data for different breeds and periods, fur- 

 ther records are available for swine from the studies of Wc.it- 

 worth and Aubel to be discussed below. Equations for theo- 



sheep by Rietz and Roberts. Taken altogether, only a beginning 

 has been made in a field that has not merely great biological in- 

 terest, but in certain of its bearings is of material economic im- 

 portance. 



The most extensive and exact work on differences in fertility 

 has been done on man. but a discussion of this subject falls out- 

 side the scope of the present review. 



Since data for the solution of the problems of group (a) are as 

 yet inadequate, it is idle to attempt any detailed discussion of 

 those of group (b) and (c). Data for such purposes are, how- 

 ever, now becoming available much more rapidly than heretofore. 



Birth Order and Litter Size 

 Fairly large series of records showing the relationship between 



