Vcuriation and Natural Selection. 



65 



to support the membrane which stretches from the ankle 

 to the tip (or near the tip) of the tail. 



Thus the wing of the bat consists of a membrane 

 stretched on the expanded or spread fingers of the hand, and 

 sweeping from the point of the little finger to the ankle. 

 Behind the ankle there is a membrane reaching to the tip 

 of the tail. This forms a sort of net in which some bats, at 

 any rate, as I have myself observed, can catch insects. 



I have selected the wing of the bat to exemplify varia. 

 tion, (1) because the bones are readily measured even in 

 dried specimens ; (2) because they form the mutually related 

 parts of a single organ ; and (3) because they offer facilities 

 for the comparison of variations, not only among the 

 individuals of a single species, but also among several 

 distinct species. 



The method employed has been as follows : The several 

 bones have been carefully measured in millimetres,* and 

 all the bones tabulated for each species. Such tables of 

 figures are here given in a condensed form for three species 

 of bats. 



Bat-Measurements (in Millimetres). 



It would be troublesome to the reader to pick out the 



* A millimetre is about ^^ of an inch, or more exactlj? "OSOB? inch. 



