ADAPTATION" TO ENVIRONMENT WARDLAW 409 



maintenance of a constant rate of metabolic activity under given 

 conditions, is certainly nullified to some extent by this adjustment. 

 The organism is outside of the range of perfect adaptation to the 

 temperature of its environment. But, on the other hand, such con- 

 ditions, even when the external temperature is extremely low, can be 

 survived indefinitely without apparent detriment to the organism. 

 On the whole, therefore, the organism can adapt itself to tempera- 

 tures below that of its body better than it can to higher temperatures. 



RATE OF ADAPTIVE CHANGE 



As the range and the scope of the mechanisms by which the organ- 

 ism is able to modify its environment increase, the necessity for 

 adaptive changes on the part of the organism itself must correspond- 

 ingly decrease. Thus we are led back to our original postulate that 

 the more effective any adaptive mechanism is, the better does it 

 enable the living organism to persist unchanged in a changing en- 

 vironment. We should, therefore, expect evolutionary changes of 

 structure and function to become progressivel}'- slower as we pass to 

 more and more complex organisms, and the mechanisms which were 

 at first developed to preserve the primitive characters of the cell 

 itself, eventually to become so effective as to be able to preserve the 

 characters of the whole organism. 



The organism which possesses, in an outstanding degree, the abil- 

 ity to modify its environment is, of course, man. He controls his 

 food supply by the hunting and rearing of food animals, by the 

 gathering and sowing of edible plants. He modifies certain of 

 his external conditions by the wearing of clothes and the erection 

 and use of houses. He adds to the effectiveness of his hands by the 

 use of tools. He increases the speed and the range of his movements 

 by traveling in vehicles. By means of various instruments he adds 

 to the acuity of his sense organs. All these external aids to his 

 natural powers may be classed as tools. It is his ability to devise 

 tools which has extended his ability to adapt his environment to his 

 needs so immeasurably beyond the similar power of any other animal. 



One of the most striking features of this type of adaptive mech- 

 anism is the extraordinary rapidity with which it has been developed, 

 as compared witli the evolutionary modifications of bodily structure. 



The conjectures which have been made as to the period Avhich has 

 elapsed since the appearance of living forms runs into hundreds of 

 millions of years. The period for w^hich records can be obtained 

 of the existence of man is measured, on the other hand, by hundreds 

 of thousands of years. However vague those estimates may be, there 

 seems to be little doubt that the enormous development of complexity 

 which some living: forms have undergone has occurred within a small 



