ii2 THE MAIN CURRENTS OF ZOOLOGY 



may be considered as offshoots of general physiology. 

 In recent years, these subjects have become much 

 cultivated fields by zoologists and psychologists. 

 The results of these studies throw light on some of 

 the basal problems of the development of animal 

 intelligence and afford means of estimating the men- 

 tal equipment of different animals. A few suggestive 

 titles, taken at random, supply illustrations The 

 Mental Powers of Spiders, by the Peckhams, a widely- 

 known study in comparative psychology; The Be- 

 havior of Lower Organisms, by H. S. Jennings; Ro- 

 mane's Starfish, Sea-urchins and Echinoderms, a 

 study of physiological response; Chapters on the 

 reactions of the Protozoa in the classical General 

 Physiology of Max Verworn. 



The mental evolution of animals is extremely 

 interesting as affording illustrations, on another side, 

 of that oneness of nature, the recognition of which as 

 we have seen is one of the results of zoological study. 

 Just as there are gradations of structure connecting 

 the lower and the higher animals (the specialized 

 arising by modifications from simpler ones) so, in the 

 rise of animal intelligence, there is a graded series of 

 states keeping pace with the structural differentiation 

 of the nervous system. 



