ADAPTATION 197 



arisen by mutation. Their degeneration is due 

 to orthogenesis or to use-transmission. 



F. Environmental Adaptation May Be 

 Intrinsic or Extrinsic. 1. Geographical va- 

 riation or divergence. The facts of geographical 

 distribution make it certain that adaptations 

 have not arisen through intrinsic causes only.^ 

 In fact, they make us doubt at times whether 

 intrinsic causes have had anything whatever to 

 do with the origin of adaptations. If all forms 

 were the result of mutation, due to intrinsic 

 causes, there is no reason why a large river such 

 as the Rio San Francisco should not contain all 

 the modifications possible to the genera inhabit- 

 ing it, for ShuU has shown that new forms may 

 arise in a restricted area. But it does not. Of 

 equal sized streams belonging to different sized 

 river systems the one belonging to the larger sys- 

 tem harbors a larger number of species of any 

 genus. ^ And other things equal, the wider the 

 distribution of any genus the more species com- 



' Tower; Evolution in Chrysomelid Beetles, p. 314, says: ". . . All 

 evidence showing them (mutants) to be most rigorously extermi- 

 nated by natural selection. On the other hand, the study of geo- 

 graphical distribution and variation gives the strongest of 

 circumstantial evidences for direct and rapid transformation in 

 response to environmental stimuli as to the result of dispersion 

 . . . according to the method of trial and error, with 

 natural selection acting as the conservator of the race by limiting 

 the variation to a narrow range of possibilities." 



' Bean Blossom Creek of Monroe County, Indiana, draining an 

 area of about 250 square miles, is known to harbor in two miles 

 of its course 44 species of fishes. The Colorado, draining an area 

 nearly 1,000 times as large, contains but 33 species of fishes. But 

 Bean Blossom is part of the Mississippi basin that far exceeds 

 the Colorado basin in size and harbors at least 300 species. The 

 still larger Amazon basin harbors at least 700 species. 



