THE COMMON FLEA 265 



imagine larger animals, such as the vertebrates, similarly 

 constituted and endowed with powers enabling them in 

 all cases to perform gigantic leaps to similar propor- 

 tionate heights; on such a showing, thjs flea's leap 

 certainly would be a marvel of muscular effort, for it 

 would be as if an ordinary-sized man, supposed to be 

 constituted like a flea, were able to take a vertical bound 

 into the air which would carry him to a greater height 

 than that of the top of St. Paul's. 



A popular writer, taking a more moderate estimate 

 of a flea's capabilities, says, "Perhaps we have not 

 reflected that the average jump of a flea is about thirty 

 times its own height, and that, supposing a man of six feet 

 in stature were to perform the same leap, he would jump 

 as high as the gallery of the Monument." Now, such an 

 assumption is altogether erroneous, and a leap of that 

 height would not by any means represent a similar 

 muscular efficiency, but, on the other hand, a far greater 

 one in other words, it would not be " the same leap," 

 but one immensely in excess. The problem is, in fact, 

 not quite so simple as it seems at first sight. Suppose 

 we consider the work done. The flea raises its own mass 

 against its own weight through the height of a foot 

 (taking our former estimate of the maximum leap). If 

 a man leapt up, say, only through the same height, he 

 also would raise his mass against his weight through the 

 height of a foot, and therefore the work done in the two 

 instances would be proportional to the weights of the 

 two animals. Next arises the question of the energy 

 available for doing this work ; this, on the supposition 

 of the two animals being similarly constituted, and 

 having muscles of a similar character, will be pro- 

 portional to the volumes of the muscles, and therefore 

 to their weights, which, again, in animals similarly 



