152 THE PROBLEMS OF EVOLUTION 



mals must be very different; in one case speed is 

 valuable but in the other the ability to stalk 

 cleverly and to spring quickly must be much more 

 useful. Prey which depends on speed for safety 

 cannot readily be stalked, and prey which is never 

 far from an open burrow can scarcely be lured into 

 a race for its life. 



Obviously, if new conditions descend upon the 

 home of a species as a result of physical conditions, 

 or if some individuals migrate permanently to a 

 region where a different environment prevails, any 

 adjustment which the individual may make to the 

 environment will be made by all individuals, gen- 

 eration after generation, and will appear as surely 

 as any character in the hereditary complex. As 

 long as the condition prevails, the adjustment is 

 present and the visible results are characteristic of 

 the species. To what extent are the minor taxo- 

 nomic units of the present due to such responses.^ 

 It has been suggested that geographic races de- 

 velop in this way,^ and it is certain that many of 

 them may do so. Jordan's observations on the 

 Loch Leven trout support this conclusion per- 

 fectly.^ Loch Leven trout were planted in the 

 waters of Alder Creek in Yosemite National Park 

 in 1896. In 1905 Captain Benson, superintendent 

 of the park, took **some fifty -four fishes" from this 



^ Rensch, B., Das Prinzip geographisoher Rassenkreise und das Problem 

 der Artbildung, Berlin, 1929. 

 6 Science N. S., Vol. XXII, pp. 714-715, 1905. 



