ORIGIN OF VARIATIONS. 151 



muscles used in running are not called into play 

 at all, while those of the shoulder chiefly used in 

 swimming are employed almost constantly. The 

 muscles of the lower part of the legs not being 

 used, and the blood supply there being hindered 

 by the pressure of the water, it follows that growth 

 in those parts will be hindered, and the legs tend 

 to become shorter. Again, the legs no longer 

 support the weight of the body, and a different 

 stress and tension is therefore brought to bear on 

 the young developing bones, thus introducing a 

 new element of change. Further, the buoyancy 

 of the legs will tend to float them upward and 

 outward, causing eventually a change of position. 

 Again, we must take account of the new co-ordi- 

 nations and new training of the nervous system 

 under this new system of stimuli. The land 

 motions of the animal — walking, running, leaping — 

 will never have been properly learned, perhaps 

 the two latter not learned at all, through want of 

 practice, while through practice it grows more 

 and more perfect in the art of swimming. A young 

 animal growing to maturity under such circum- 

 stances as we have pictured would, without a 

 doubt, show in its locomotive mechanism and ner- 

 vous co-ordinations a noticeable difference from 

 another of its species which might have lived the 



