26 THE POPULAR SCIENCE MONTHLY. 



merits they will fail to bear more violent movements. Tims, say- 

 ing nothing of the required changes in the pelvis as well as in the 

 nerves and blood-vessels, there are, counting bones, muscles, ten- 

 dons, ligaments, at least fifty different parts in each hind leg 

 which have to be enlarged. Moreover, they have to be enlarged 

 in unlike degrees. The muscles and tendons of the outer toes, for 

 example, need not be added to so much as those of the median 

 toes. Now, throughout their successive stages of growth, all 

 these parts have to be kept fairly well balanced ; as any one may 

 infer on remembering sundry of the accidents he has known. 

 Among my own friends I could name one who, when playing 

 lawn-tennis, snapped the Achilles tendon; another who, while 

 swinging his children, tore some of the muscular fibers in the calf 

 of his leg ; another who, in getting over a fence, tore a ligament 

 of one knee. Such facts, joined with every one's experience of 

 sprains, show that during the extreme exertions to which limbs 

 are now and then subject, there is a giving way of parts not quite 

 up to the required level of strength. How, then, is this balance 

 to be maintained ? Suppose the extensor muscles have all varied 

 appropriately ; their variations are useless unless the other co- 

 operative parts have also varied appropriately. Worse than this. 

 Saying nothing of the disadvantage caused by extra weight and 

 cost of nutrition, they will be causes of mischief causes of de- 

 rangement to the rest by contracting with undue force. And 

 then, how long will it take for the rest to be brought into adjust- 

 ment ? As Mr. Darwin says concerning domestic. animals: "Any 

 particular variation would generally be lost by crossing, rever- 

 sions etc., . . . unless carefully preserved by man." In a state 

 of nature, then, favorable variations of these muscles would dis- 

 appear again long before one or a few of the co-operative parts 

 could be appropriately varied, much more before all of them 

 could. 



With this insurmountable difficulty goes a difficulty still more 

 insurmountable if the expression may be allowed. It is not a 

 question of increased sizes of parts only, but of altered shapes of 

 parts, too. A glance at the skeletons of mammals shows how un- 

 like are the forms of the corresponding bones of their limbs ; and 

 shows that they have been severally remolded in each species to 

 the different requirements entailed by its different habits. The 

 change from the structures of hind limbs fitted only for walking 

 and trotting to hind limbs fitted also for leaping, implies, there- 

 fore, that along with strengthenings of bones there must go alter- 

 ations in their forms. Now the spontaneous alterations of form 

 which may take place in any bone are countless. How long, then, 

 will it be before there takes place that particular alteration which 

 will make the bone fitter for its new action ? And what is the 



