204 ADAPTATION 



causes all accessible places to become inhabited. 

 Food itself is dependent on other food, and this 

 ultimately on light, heat, depth, nature of bot- 

 tom, current, and other elements of the physical 

 environment. The habitat once selected, the 

 effect of the changed physical environment vs^ill 

 cause the changes already discussed, and the 

 changed biological environment vdll cause an 

 animal to adopt a changed mode of existence. 



It is again possible either that innate charac- 

 ters in certain individuals of a crovpded com- 

 munity cause them to migrate in certain direc- 

 tions, or that chance individuals migrate, and that 

 intrinsic or accidental extrinsic causes then start 

 nev7 activities. It is certain that new^ activities 

 once adopted the result is individual modifica- 

 tions.^ It has long been claimed and as vigor- 

 ously denied that these adaptive individual devia- 

 tions are transmitted. 



The factors of both Buffon and Lamarck 

 hinge on the possibihty that somatic modifica- 

 tions are transmissible to the reproductive cells. 

 We have not been able to imagine hov7 somatic 

 changes could so influence the reproductive cells 

 that they could, in their turn, produce individuals 



'■ We can imagine that this process of overproduction and con- 

 sequent adoption of different areas may take place in a small 

 basin, but certainly the larger the basin the greater the diversity 

 of conditions, the greater chance of comparative isolation in dif- 

 ferent sorts of environments, and the greater the number of 

 species. 



No small stream long isolated contains many species of a given 

 genus. Notable exceptions are Orestias in Lake Titicaca, and 

 Chirostoma in the Lerma. What applies to the species of a genus 

 applies with equal force to the genera of a family. 



