14 Experimental Zoology 



At alow temperature, io° C, Anolis changes from green to brown, 

 irrespective of illumination. At a high temperature, 40° to 

 45° C, it turns from brown to green, irrespective of illumination. 

 Thus heat is the controlling factor at these extremes. Between 

 these extreme temperatures there is a range from 25° to 35° 

 through which light is the controlling factor, although heat is 

 not without its influence, as shown by the rate of the change. 

 Parker and Starratt have discovered the astounding fact that the 

 eflfects of the illumination may be produced when an area of 

 the skin no larger than a square miUimeter is exposed, the rest 

 of the animal being in the dark. 



The changes that have just been described, except perhaps the 

 last ones, seem to be of benefit to the animal, either in directly 

 protecting it from the agent that brings about the result, as in 

 the effects of pressure, cold, sunlight, etc., or in more effectually 

 concealing the animal from its enemies. These responses are 

 said to be adaptive, but it is remarkable how rare are adaptive 

 structural responses, when we recall the fact that adaptation of 

 the organism to its surroundings is one of its most characteristic 

 properties. The poverty of adaptive structural response does 

 not encourage one to look to external agents as having brought 

 about directly the structural adaptation of organisms to exter- 

 nal conditions, even if it could be shown that such influences 

 are inherited. 



There are, on the other hand, many cases of physiological 

 responses that are adaptive; in fact, nearly all functional 

 changes are directly beneficial to the organism. Animals re- 

 spond in a most remarkable way to poisons. If certain alka- 

 loids are injected in ever increasing doses, the animal becomes 

 immune to a dose which if given in the first instance would 

 have been fatal. In the case of the poisonous ptomaines pro- 

 duced by bacteria, the animal produces a counter poison, an 

 antitoxin, that nullifies the effects of the ptomaine. In a num- 

 ber of bacterial diseases the animal becomes more or less im- 

 mune after the first attack. 



Equally striking are the adaptive responses shown by animals 



