ORGANIC EVOLUTION — DAVENPORT 



421 



In considering the fixity of some species it must first of all be 

 recognized tliat a species is a complex of morphological and physio- 

 logical characters that can not exist alone but is absolutely dependent 

 upon the external world for its existence. The organism must live in 

 a medium of such and such physical qualities, at such a temperature, 

 in the midst of such radiant energy, with access to such and such 

 foodstull's which it is capable, of taking in and utilizing for its 

 metabolism. Every organism is extraordinarily closely fitted to its 

 environment. And that enviromnent may be very complex. 



I will illustrate this principle by reference to the almost micro- 

 scopic Collembola that live on the beach at Cold Spring Harbor. 



40'5S 



. 40 52 



FlliLKE 1. 



-Map of CuW Spring' Harbor, showing spit 

 (Cold Spring Beach) 



(Fig. 1.) They live in an area of apparently washed sand and peb- 

 bles in a region that is covered twice a day several feet deep by sea 

 water and then exposed to the air ; in a region swept by strong winds, 

 overlaid by ice in winter, and exposed to the hot sun's rays in 

 summer. (PI. 1.) A region where the sandy substratum is caused 

 to shift by the action of waves, and its pebbles to roll. The region 

 looks unpropitious for any organism, yet of one species of Col- 

 lembola in an area a kilometer long and 5 to 8 meters broad there 

 are probably in the middle of summer a hundred million individuals. 

 And they are meeting successfully the difficult and complex condi- 

 tions imposed by that particular habitat. If the habitat be compared 



1.'8095— 31 lis 



