THE BASIS OF ADAPTATION 263 



which is possible within certain hereditary limits of variation, 

 and so furnishes a basis for the modification of the organism. 

 Differences of heredity make it necessary for organisms to seek 

 different conditions of environment. Since the individuals making 

 up a species are not all the same, changing environment may 

 favor some and destroy others, so that the limits of variation 

 of the species may be narrowed. Response of organisms to 

 changing conditions may be accomplished in various waj^s, but 

 one important result is interchange between various hal)itats and 

 regions. The result is a constant dispersal of organisms from 

 their centers of development. This dispersal is hindered and 

 aided by various factors, and in turn causes changes which may 

 further influence the behaviour of other living things. Dispersal 

 has brought about a definite geographic distribution of organisms 

 and has been accompanied by adaptive radiation or branching 

 within limited groups. The complexity of these interactions is 

 the immediate explanation of adaptation. The transfer of changes 

 from the individual to the species as a permanent component of 

 the heritage is yet to be explained. 



REFERENCES 



Lydekker, R., a Geographical History of Mammals, 1896. 



Pagenstecher, a., Die Geographische Verbrcitung der Schmetterlinge, 1909. 



Clarke, W. E., Studies in Bird Migration, 1912. 



Meek, A., The Migrations of Fish, 1916. 



Lull, R. S., Organic Evolution, 1917. 



Dahl, Fr., Okologische Tier geographic, 1921. 



CoMSTOCK, J. H., Introduction to Entomology, 1924. 



Holdhaus, K., Schroder's Handbuch der Enlomologie, II (7), 1927. 



