158 DEVELOPMENT AND HEREDITY. 



interesting to note, in this connection, that different 

 groups of fishes, also whales, sea-cows, seals, and 

 porpoises, all preserve the same general proportion 

 between the size of the body, and the size of the fin 

 or flipper. 



If we admit, as I think we must, that one of the 

 most conspicuous properties of living matter is its 

 power to restore losses caused by waste and wear, 

 and to increase the restoration beyond the loss so 

 that the result is growth, then we have one of the 

 keys to unlock the mysteries of development. We 

 have already seen one application of this principle 

 in the previous paragraphs. We have next to con- 

 sider its application to the bones, the hard parts 

 which determine the shape of the body. According 

 to Professor Martin, " the experience of physicians 

 shows that any continued pressure, such as that of 

 a tumor, will cause the absorption and disappear- 

 ance of bone almost quicker than that of any other 

 tissue ; and the same is true of any other continued 

 pressure." ^ The bones, though apparently so un- 

 yielding, are among the most easily modified parts 

 of the body. Remembering that, as heretofore, we 

 have in mind a succession of generations acted upon 

 by the environment as though it were a single indi- 

 vidual, I think we can explain the shape of the bones 



^ Martin's Human Body. 



