86 MEMOIRS NATIONAL ACADEMY OF SCIENCES. ivol.xii,no.8. 



And finally to the late lamented Prof. C. O. Whitman, head of the department of zoology 

 of the University of Chicago, and to the university itself, for the facilities available for my 

 purpose. The work begun with Dr. Davenport was continued with Dr. Whitman. To Dr. 

 Whitman personally, for his high ideals, character, and broad sympathies I am indebted to an 

 unusual degree. His comprehensive breadth of view, his thoroughness and insight into biolog- 

 ical problems, made it possible, in the department of which he was head, to pursue lines of 

 ecological investigation which had not been permitted or encouraged in many institutions. 



Note. — This paper waa completed April, 1912, and since then Ortmann's valuable paper: " The Alleghenian 

 Divide, and its Influence upon the Freshwater Fauna" (Proc. Amer. Phil. Soc, vol. 52, pp. 287-390, 1913), has ap- 

 peared, but it has not been possible to incorporate his results in this paper. Attention should also be called to the 

 discussion of local variations and allied problems in Bateson's "Problems of Genetics," 1913; and to comments on 

 the crosses of snails in Przibram'e " Experimentale-Zoologie. 3. Phylogenese." 1910, pp. 63-68. 



