352 EVOLUTION, GENETICS, AND EUGENICS 



and others whose teeth are replaced by bony or chitinous plates thai 

 are used for crushing the hard shells of molluscs and crustaceans, may 

 not confidently be said to have developed these crushing appliances 

 and to have abandoned the use of teeth in adaptation to a habit of 

 feeding upon hard-shelled prey; but rather it seems more likely that the 

 loss of teeth and the development of crushers occurred through a 

 degenerative process incident to racial senescence and that the pos- 

 session of the crushing equipment enabled them to avail themselves of 

 a new type of food, formerly unavailable to them. 



The organic environment. — In his admirable chapter entitled 

 "The Web of Life," which we shall quote entire, Professor Thomson 

 has given us a vivid picture of vast systems of interdependencies that 

 exist throughout the organic world. No species, no creature, lives to 

 itself alone; it is intimately tied up with a host of other creatures with 

 interwoven destinies. Thus one species of animal is adapted to live 

 upon certain plants or other animals, which in turn may be dependent 

 upon still other animals or plants. The elimination of one species may 

 cause the elimination or the radical change of a dependent species. 

 We cannot afford ever to forget this great truth of the oneness of 

 nature. It is the keynote of life and of evolution. 



Adaptation due partly to functional activity. — It is a commonplace 

 which needs no special demonstration to say that organs improve 

 through use and deteriorate through disuse. Many organs, then, 

 which in the adult condition appear to us to be so admirably 

 adapted to perform certain duties, must be thought of as having been 

 gradually molded by functioning during the entire period of individual 

 development. If the motor nerve running to a limb bud of a growing 

 embryo be severed at an early stage and no secondary nerve connection 

 be established, the limb will continue to grow up to a certain point, but, 

 in its paralyzed condition, will be incapable of exercising its functions 

 and will cease to develop. A certain amount of development will 

 therefore be seen to be independent of functioning, but full develop- 

 ment of functional efficiency is obtained only through functioning. 



"The relation between structure and function in an organism," 

 says Professor Child, 1 "is similar in character to the relation between 

 the river as an energetic process and its banks and channel. From the 

 moment that the river began to produce structural configurations 

 in its environment, the products of its activity accumulated in certain 



1 C. M. Child, "Regulatory Processes in Organisms," Jour. Morf>h., Vol. 

 XXII (191 1). 



