134 ERNEST WAREEN. 



snow. In the case of the hydroicls the reaction may be 

 expected to be greater on tropical or sub-tropical coasts than 

 elsewhere. 



(2) Chloasma caloricum due to exposure to heat. In 

 small isolated rock-pools exposed to the blaze of a tropical 

 sun the water may become extremely warm and even fatally 

 so when the tide has receded, and the barrenness of some of 

 the pools may be partly attributed to this cause. 



(3) Chloasma traumaticum resulting from mechanical 

 irritation, paralleled by the agitation of the water beating the 

 hydroid against the rocks or other objects. 



(4) C h 1 o a s m a t o X i c u m resulting from irritating poisons. 

 Certain rock-pools near the high-water mark are not filled at 

 every high-tide. In these the water soon becomes stale and 

 will have a prejudicial influence on the majority of the inhabi- 

 tants, and, as a matter of fact, very few kinds of hydroids are 

 capable of living in such pools. 



These physical causes are not, however, the only ones which 

 lead to diseased pigmentation in the higher animals, since the 

 toxins in the blood resulting from syphilis, leprosy, tuber- 

 culosis, diabetes, etc., may induce hyperpigmentation in the 

 skin. Also in so-called "pigmentary fever," Avhich is supposed 

 to be due to insolation, a pigmentation of the face occurs, and 

 after the fever has left, it gradually fades away in the course 

 of several months. 



A closer study of the histology of the pigmented tissue 

 resulting from disease in the higher animals would be of the 

 greatest interest to compare with that of the blackened 

 hydroids, and there is no doubt that observations on the 

 pathology of simple organisms is capable of giving an 

 important insight into the fundamental causes and effects of 

 diseases as found in man and other mammals. 



