EXPENDITURE AND GENESIS. 473 



to breed. These two factors compounded, result in a dif- 

 ference of fertility far greater than can be ascribed to un- 

 likeness of the two creatures in size. 



Perhaps the most striking piece of evidence which Mam- 

 mals furnish, is the extreme infertility of our common Bat. 

 The Cheiroptera and the Rodentia are not very dissimilar in 

 their internal structures. Diversity of constitution, therefore, 

 cannot vitiate the comparison between Bats and Mice, which 

 are about the same in size. Though their diets differ, the 

 difference is in favour of the Bat : its food being exclusively 

 animal while that of the Mouse is mainly vegetal. What 

 now are their respective rates of genesis? The Mouse has 

 several litters in a year of 5 to 7 in each; while the Bat pro- 

 duces only one at a time. Whether the Bat repeats its one 

 more frequently than the Mouse repeats its 7 is not stated; 

 but it is quite certain that even if it does so (an absurd 

 supposition), the more frequent repetition cannot be such 

 as to raise its fertility to anything like that of the Mouse. 

 And this relatively-low rate of multiplication we may fairly 

 ascribe to its relatively-high rate of expenditure. 



Here let us note, in passing, an interesting example of the 

 way in which a species which has no specially-great power of 

 self-preservation, while its power of multiplication is ex- 

 tremely small, nevertheless avoids extinction because it has 

 to meet an unusually-small total of race-destroying forces. 

 Leaving out parasites, the only enemy of the Bat is the Owl ; 

 and the Owl is sparingly distributed. 



§ 351. These general evidences may be enforced by some 

 special evidences. We have few opportunities of observing 

 how, within the same species, variations of expenditure are 

 related to variations of fertility. But a fact or two showing 

 the connexion may be named. 



Doctor Duncan quotes a statement to the point respecting 

 the breeding of dogs. Already in § 341 1 have extracted a part 

 of this statement, to the effect that before her growth is com- 



