392 



SCIENCE 



[N. S. Vol. XXX. No. 769 



the surrounding medium. The coelomic 

 fluid is renewed and maintained uniform 

 in composition by the action of the organ- 

 ism itself, so that we may speak of it as an 

 environment created by the organism. 

 The formation of a body cavity filled with 

 salt solution at once increased the range of 

 adaptation of the animals endowed there- 

 with. Thus it enabled them to leave the 

 sea, because they carried with them the 

 watery environment which was essential 

 for the normal activity of their constit- 

 uent cell units. The assumption of a 

 terrestrial existence on most parts of the 

 earth's surface involved, however, the ex- 

 posure to greater ranges of temperature 

 than was the case in the sea, and indicated 

 the necessity for still further increase in 

 the range of adaptation. Every vital 

 process has its optimum temperature at 

 which it is carried out rapidly and effect- 

 ively. At or a little above freezing point 

 the chemical processes concerned in life are 

 suspended, so that over a wide range of the 

 animal kingdom there must be an almost 

 complete suspension of vital processes dur- 

 ing the winter months, and at all times of 

 the year a great dependence of the activity 

 of these processes on the surrounding tem- 

 perature. It is evident that a great ad- 

 vantage in the struggle for existence was 

 gained by the first animals which succeeded 

 in securing thermal as well as chemical 

 constancy of environment for their cells, 

 thus rendering them independent of 

 changes in the external medium. It is in- 

 teresting to note that the maintenance of 

 the temperature of warm-blooded animals 

 at a constant height is a function of the 

 higher parts of the central nervous system. 

 An animal with spinal cord alone reacts 

 to changes of external temperature exactly 

 like a cold-blooded animal, the activity of 

 its chemical changes rising and falling 

 with the temperature. In the intact mam- 



mal, by accurately balancing heat loss from 

 the surface against heat production in the 

 muscles, the central nervous system en- 

 sures that the body fiuid which is supplied 

 to all the active cells has a temperature 

 which is independent of that of the sur- 

 rounding medium. These are fundamental 

 examples of adaptation effected by crea- 

 tion of an environment peculiar to the 

 animal. Numberless others could be cited 

 which differ only in degree from the ac- 

 tivity of man himself. In some parts of 

 this country, for instance, the activity of 

 the beaver in creating an artificial envi- 

 ronment has until lately been more marked 

 than that of man himself. We are not 

 justified, then, in regarding mankind as 

 immune to the operation of natural forces 

 which have determined the sequence of 

 life on the surface of the globe. The same 

 laws which have determined his evolution 

 and his present position as the dominant 

 type on the earth's surface will determine 

 also his future destiny. 



We are not, however, dealing with or 

 interested in simple survival. Lower 

 forms of life are probably as abundant on 

 the surface of the globe as they were at 

 any time in its history. Survival, as Dar- 

 win pointed out, is a question of differen- 

 tiation. When in savage warfare a whole 

 tribe is taken captive by the victorious 

 enemy, the leaders and fighting men will 

 be destroyed, while the slaves will con- 

 tinue to exist as the property of the vic- 

 tors. Survival, then, may be determined 

 either by rise or by degradation of type. 

 Success involves the idea of dominance, 

 which can be secured only by that type 

 which is the better endowed with the 

 mechanisms of adaptation required in the 

 struggle against other organisms. 



Among the many forms of living matter 

 which may have come into being in the 

 earlier stages of the history of the earth, 



