410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



fraction of the time during which living organisms have been in 

 existence. The development of the power of man to modify his 

 surroundings as a result of the development of his mental faculties 

 is a still more rapid and recent growth and is to be measured in 

 centuries. Indeed, it may be claimed that man has expanded his 

 powers in this direction more during the last century than during 

 the whole of his previous history. 



We have in man, then, the most perfect adaptation to environ- 

 ment shown by any form of life. So great is his power of modify- 

 ing his surroundings, and so rapidly is this power increasing, that 

 it would seem that further adaptation of his physical structures has 

 become unnecessary. It has even been suggested that his increasing 

 use of artificial mechanisms may bring about a degeneration of some 

 of his bodily powers, and that any further evolutionary develop- 

 ment in man may be restricted to the growth of his mental facul- 

 ties. The past history of the evolutionary adaptation of living or- 

 ganisms to their environment would, however, lead us to expect that 

 any changes which may take place in the organism of man will not 

 be such as would adversel}'^ afiect the conditions of life of the es- 

 sential units of his structure. In so far as the changes which have 

 taken place in his habits of life are really adaptations to his en- 

 vironment, we may expect that their effect will be to establish more 

 securely the primitive conditions of his cells. 



SUMMARY 



A characteristic feature of living organisms is the possession of 

 mechanisms which protect them against the effects of changes of 

 their environment. 



These mechanisms in the earlier forms exert their action by re- 

 stricting the interchange which they allow between the organism and 

 its surroundings. As the}^ develop in efficiency, they become more 

 selective in action, and are able to preserve the essential characters 

 of the organism while allowing a free interchange with its environ- 

 ment. They have preserved, even in the higher organisms, some 

 of the conditions of cell life which probably existed at very early 

 stages of their evolution. 



When sufficiently broad distinctions of form are considered they 

 are found to possess equall}^ distinct chemical features, for example, 

 in the proportions of some of the elements which they contain. 



As the complexity of organisms has increased, they have rendered 

 themselves more independent of their external environment by pro- 

 viding their cells with an immediate environment of their own. By 

 this means external changes are only allowed to reach the cells in a 

 modified form. The possession of this internal environment enables 



