452 LAWS OF MULTIPLICATION. 



Perhaps the most striking piece of evidence which Mam- 

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

 The Cheiroptera and the Rodentia are very similar 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 pro- 

 duces many young at a time, reaching even 10 or 12 ; while 

 the Bat produces only one at a time. Whether the Bat 

 repeats its one more frequently than the Mouse repeats its 

 ten is not stated ; but it is quite certain that even if it does 

 so, 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 that has no specially- great power of 

 self-preservation, while its power of multiplication is extremely 

 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 tho 

 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 I have extracted a part 

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

 plete, a bitch bears at a birth fewer puppies than when she 

 becomes full-grown. An accompanying allegation is, that 

 her declining vigour is shown by a decrease in the number of 



