INTRODUCTION 3 



of animal camouflage illustrated in the colors and forms 

 of a great variety of organisms. From this standpoint 

 the importance of the eyes in color responses became at 

 once apparent, for an animal evidently must see that 

 which it tends to resemble before it can assume the like- 

 ness. Thus color reactions became incorporated among 

 the reflex activities of animals, and a wide and novel 

 field for investigation was thrown open. 



In 1858 the celebrated British physician Joseph Lister, 

 then a student of medicine some thirty years of age, 

 published in the Transactions of the Royal Society of 

 London a scholarly paper on the color changes of the 

 common frog. Here he summarized up to his own time 

 the important general conclusions in this field of re- 

 search. According to him the eyes of any animal that 

 possessed the property of changing its tint were the only 

 channels, to use his expression, through which the rays 

 of light could gain access to the nervous system so as 

 to induce changes of color in the skin. He declared 

 further that the cerebro-spinal axis was chiefly, if not 

 exclusively, concerned in regulating the functions of the 

 pigment-cells. These brief statements give the physio- 

 logical foundations upon which has been based the ex- 

 perimental work on animal coloration during the last 

 half of the nineteenth century. 



Incidentally these early investigations afforded a gen- 

 eral survey of the animal kingdom so far as color changes 

 were concerned. As an outcome of such an inspection 

 it was found that these changes are limited in the main 

 to comparatively few representatives of five important 

 groups of the higher animals. These are the cephalo- 

 pods such as the octopus, the cuttlefish, and the squid; 

 the crustaceans, especially the shrimps and prawns; and, 

 among the vertebrates, the fishes, the amphibians, and 

 the lizards. The highest vertebrates, the birds and 



