WHAT ARE BATS? 



527 



and which, in the bat, by their contraction, strike the wings down- 

 ward in flight. 



Every one present must have observed, when carving a fowl, that 

 there is a ridge or keel to the breastbone, and that a voluminous mass 

 of muscle the breast of the fowl is situated on each side of such 



Fig. 2.s c, -wrist-bones ; p, bones of thumb ; ii- 4 , bones of middle part of hands. 



keel. Now, our bat has not got such a mass of muscle on each side 

 of the keel of its breastbone as has the bird, and for a very good 

 reason. In the bat, as in ourselves, the muscles which antagonize 

 those just noticed (and which draw the arms away from the breast) 

 are situated in the back ; but, in the bird, both the muscles which 

 strike the w T ings downward, and those which raise them upward, are 

 together placed upon the breast, and hence its much deeper and 

 more conspicuous keel. Still, though the muscular structure of the 

 breast of a bat is not so perfectly arranged for flight as is that of a 

 bird, it is an approximation to bird-structure, and one we can well 

 understand from the similarity of action. But it may puzzle some of 

 my hearers at first to think why the mole, of all creatures in the world, 

 should have a breastbone at all like that of a bird. But a moment's 

 reflection will make it obvious that the mole also requires most pow- 

 erful breast-muscles, in order that it may dig its way through the soil 

 with the wonderful speed with which it does dig through it. Similar 

 causes produce similar effects, and thus it is that the mole, like the bat 

 and the bird, comes to have a keeled breastbone. 



