i:;2 



THE AMERICAN NATURALIST 



[Vol. XLII 



these substances are intimately concerned with heredity. 

 This discussion makes clear how extremely important 

 certain materials are to the body and yet how impossible 

 it is at this stage of scientific progress to frame any clear 

 and consistent conception of the method by which these 

 materials exert their influences. 



If such relatively simple physiological questions as 

 this concerning the material basis of heredity meet with 

 difficulties such as I have pointed out, how vastly more 

 intricate and perplexing must be the problem of the rela- 

 tion of the living materials of animals' bodies to their 

 nervous and mental states. That such a relation exists is 

 well recognized, but what this relation is will probably 

 require many years even to outline. 



It is in the direction of comparative physiology that 

 the more important new advances in zoology are to be 

 made. In my opinion zoology will meet with an expan- 

 sion in this century quite as much as the study of elec- 

 tricity has in the last hundred years. When Franklin 

 tried his hazardous experiment with lightning, no one 

 suspected that he was dealing with a factor that could 

 come to be of such paramount importance in every-day 

 affairs as electricity has become, and it seems to me 

 probable that the zoologist of to-day is working obscurely 

 with problems that will eventually lead to revolutionary 

 results. I have already pointed out the importance of 

 certain minute quantities of material in inheritance, and 

 the significance of this in animal breeding and in social 

 problems must be evident. But in a thousand other ways 

 the doings of animals are worthy of closest attention. 

 Many of the most difficult problems that the human race 

 has attacked have already found their solution among 

 the lower animals. Secure aerial transportation, which 

 is almost a dream with us, is an accomplished fact among 

 many animals. Our own efforts are not so safe if they 

 are more extensive than those of a flying fish. They are,, 

 however, by no means equal to those of a bat or an insect, 

 and they are immensely inferior to those of a bird. Our 



