332 VICTOR E. SHELFORD AND EDWIN B. POWERS. 



from the tank the preference for the higher oxygen content was 

 decided (graph 12). 



V. Summary and Discussion of Conclusions. 



In these brief experiments we have only outhned the possi- 

 bilities of much more extensive work along similar lines. Such 

 experimental study alone can of course not solve the problems 

 of migration but the extreme sensitiveness of the fishes studied, 

 as shown by their detection of slight deviations from neutrality, 

 temperature differences as small as 0.2 of a centigrade degree, of 

 small fractions of a cubic centimeter per liter of hydrogen 

 sulfide, etc., makes it very clear that there is no difficulty in 

 fishes determining the direction to large rivers from hundreds 

 of miles out at sea or of finding their way into any bay or harbor 

 or river or other arm of the sea which their particular physio- 

 logical condition at a given time demands. It is not necessary 

 to appeal to "instinct" to explain the return of certain salmon 

 to certain rivers, or the running of herring in certain localities. 

 The mere fact of their origin in the region, the probable limited 

 tendency to leave it (Johnstone, '08), coupled with their ability 

 to detect and follow slight differences in water is a sufficient 

 explanation of all their peculiar migrations. The close way in 

 which animals stay about certain localities from generation to 

 generation is hardly appreciated. Thus as Johnstone points 

 out, the herring of the east coast of Britain are largely local, 

 having formerly been assumed to belong to shoals that came 

 from distant points. 



The experimental method cannot of course determine the 

 cause for the absence of fishes from any given point but must be 

 accompanied by hydrographic studies. Such combined efforts 

 must give very trustworthy results; hydrographic studies alone 

 may lead to entirely erroneous assumptions because of the lack 

 of knowledge of the sensibilities of the fishes concerned and the 

 selection of some insignificant factor correlated with their 

 absence or presence, as "an explanation. Such correlates, offered 

 as explanations, become the basis of erroneous remedial measures. 



Noting the remarkable discriminations of fishes for differences 

 in alkalinity, acidity and neutrality, a note of warning may be 

 sounded in regard to the relation of pollution to the run of her- 



