Variation and Natural Selection, 



the same way. The mean length is stated; and the 

 amount by which a bone in any bat exceeds or falls short 

 of the mean can be seen and readily estimated by means 

 of the horizontal lines of the table. Any one can reconvert 

 the tables into figures representing our actual measurements. 



Now, it may be said that, since some bats run larger 

 than others, such variation is only to be expected. That is 

 true. But if the bones of the wing all varied equally, all the 

 curves would he similar. That is clearly not the case. The 

 second metacarpal is the same length in 5 and 6. But the 

 third metacarpal is two millimetres shorter in 6 than in 5. 

 In 10 the radius and ulna are longer than in 11 ; but the 

 second metacarpal is shorter in 10 than in 11. A simple 

 inspection of the table as a whole will show that there is a 

 good deal of independent variation among the bones. 



The amount of variation is itself variable, and in some 

 cases is not inconsiderable. In the long-eared bats 4 and 

 5 in Fig. 14, the phalanges of the third digit measured 26*5 

 millimetres in 4, and 34 millimetres in 5 — a difference of 

 more than 28 per cent. This is unusually large, and it is 

 possible that there may have been some slight error in the 

 measurements.* A difference of 10 or 12 per cent, is, 

 however, not uncommon. 



In any case, the observations here tabulated show (1) 

 that variations of not inconsiderable amount occur among 

 the related bones of the bat's wing ; and (2) that these varia- 

 tions are to a considerable extent independent of each other. 



So far we have compared a series of individuals of the 

 same species of bat, each table in Figs. 13-16 dealing 

 with a distinct species. Let us now compare the different 

 species with each other. To effect such a comparison, we 

 must take some one bone as our standard, and we must 

 level up our bats for the purposes of tabulation. I have 

 selected the radius and ulna as the standard. In both the 



* In nearly all cases the measurements were checked by comparing th© 

 two wings. In one or two instances there were diflferences of as much as two 

 or three millimetres between the boues of the two sides of the body, but In 

 most cases they exactly corresponded. 



