65n 



THE AMEBIC AX NATURALIST 



[Vol.XLVHI 



Jennings ('06) has- quoted various botanical workers' 

 observations on motile plants the behavior of which prob- 

 ably follows the general laws governing the behavior of 

 motile animals. As a result of the quick behavior re- 

 sponses of motile organisms, their distribution at any 

 given time is a better index of the conditions at that time 

 than the distribution of sessile organisms, because when 

 the conditions at a given point become unfavorable the 

 motile organisms usually move to another situation, 

 while the sessile forms remain and perhaps die. 



ii. Structural Responses. — Among motile animals, 

 structural and color changes occurring as a response to 

 environmental conditions (stimuli) are usually not of 

 importance to the organism concerned. The color differ- 

 ences induced in Lepidoptera by heat and cold (Stanfuss ; 

 Fischer) and the structural differences in Crustacea such 

 as were brought about in Cladocera by Woltereck, and 

 other modifications brought forward recently, are usually 

 of no known advantage or disadvantage to the animals 

 concerned (Bateson, '13, Oh. IX and X). Such re- 

 sponses in color and general form do not ordinarily take 

 place in adults subjected to such conditions. The strik- 

 ing structural responses of motile animals are often 

 responses to the organism's activity. The use and disuse 

 phenomena of the Lamarckians, the increase in size and 

 form of muscles, thickening of skin in man and mammals, 

 are well-known examples of a type of responses which 

 have influenced zoological speculation. Child ( '04) con- 

 trolled the form of Leptoplana by controlling activity. 

 Holmes ('07) found that the movements of pieces of 

 Loxophyllum have an important part in shaping the 

 general outline of the bodies of the resulting forms. The 

 general forms of motile animals are correlated with their 

 activities but whether form or structure correlated with 

 it appeared first in the course of evolution has been the 

 subject of considerable fruitless speculation. 



