60 THE FOUNDATIONS OF ZOOLOGY 



wonderfully fitted for the normal life of its species. What interests 

 us is not that the hen's egg becomes a chick while the duck's egg 

 becomes a duckling ; but that one grows into exquisite adjustment 

 to the life of fowls, while the other becomes as admirably fitted for 

 the life of ducks. 



In truth, the assertion that the future chick is latent in the egg 

 seems to be no more than a generalized statement of observed 

 facts, and of our confidence that they may be repeated ; although it 

 by no means follows that the words, inherent potency, are useless ; 

 for they serve a useful purpose if they fix attention on the fact that, 

 while that which was an egg may under certain conditions become a 

 chick adapted for the life of fowls, knowledge of these conditions 

 fails to show us why it should. 



Here the stimulus comes from the external world, but the case 

 is just the same when it is internal. The well-known results of 

 castration prove that the normal development of many male mam- 

 mals and birds depends upon some constitutional stimulus which 

 comes from the reproductive organs to the parts of the growing 

 body; but who can believe this an adequate explanation of the 

 short, sharp horns, the thick neck, and the ferocity of the bull, or of 

 the bright colors, the sharp spurs, and the high courage of the cock ? 



Have we any reason for a different opinion when the result 

 varies with the stimulus ? Under one internal stimulus a bud 

 becomes a jelly-fish, while, under others, it may become a hydranth 

 or a machopolyp or a blastostyle, but the real problem, in this 

 case as in the others, is the production of a beautifully coordinated 

 organism, with the distinctive characteristics of its species, and 

 with exquisite fitness for a life like that of its ancestors. 



I showed, some years ago, that a small crustacean, Alpheus 

 heterochelis, develops according to one plan at Beaufort in North 

 Carolina, according to a second at Key West in Florida, while 

 it has still a third life history at Nassau in the Bahama Islands; 

 but no one can believe that the influences which cause this diver- 

 sity in the metamorphosis of Alpheus have anything to do with 

 the final outcome, which is the same in all three places. The 

 case is exactly the same when a cell which would normally give 

 rise to a half or a quarter of the body gives rise to the whole 

 under a different stimulus. 



