296 Vniversity of California Publications in Zoology [Vol. 11 



a living organism can within certain limits adjust itself to the 

 varying factors in its life-complex is its own peculiar charac- 

 teristic. Yet even here there is a limit to the element of plas- 

 ticity ; perhaps the moth, unless it changes its physical make-up, 

 will ever be doomed to meet death about an arc-light or a trap 

 lantern. Not all behavior is adaptive in character, and partic- 

 ularly is this true when the animal is placed amid the factors 

 of a new environment. In the words of Wasmann: "Both 

 elements, automatism and plasticity, are found in different pro- 

 portions with all animals from the highest to the lowest. ' ' Modi- 

 fiability in the behavior of animals, generally, falls into two 

 categories : (1) modifications of reflex re.sponses without the inter- 

 vention of intelligence, and (2) the intelligent modifications, or 

 those effected through the power of association formation. In 

 the study on leeches presented in this paper, only the first of 

 these types of modifiability has been considered. 



TJie random movements of the leech afford an excellent illus- 

 tration of the function of modifiability in a lower organism. 

 Illumination of a fair intensity seems to interfere with the 

 normal processes of the leech. When it is subjected to light of 

 100-watt intensity random movements are the response immedi- 

 ately given by the organism. Through these the leech is enabled 

 to locate finall}' a region of lower light intensity in another part 

 of the dish. Through the same agency the leech succeeds, after 

 numerous trials, in locating its food, as well as in performing 

 numerous other features of its daily life. In the lower organisms 

 modifiability plays the role largely assumed by intelligent action 

 in the higher animals. 



The analysis of this modifiability is very largely facilitated 

 through the principles already demonstrated in the higher ani- 

 mals by such able workers as Sherrington (1911), Lee (1907, 

 1910), and numerous others. Whatever merit this paper on 

 leeches may have is due, in the opinion of the writer, to the fact 

 that its chief attempt has been to determine to what extent these 

 principles are manifested in the behavior of the leech. While 

 no one can be more fully aware than the writer of the imper- 

 fection of the methods employed in many of the experiments and 

 the inadequacy of the data presented in support of certain con- 



