524 ADAPTATIONS TO PHOTIC QUALITY 



Italy, there are both green and brown varieties of the praying mantis. 

 In 1904, di Cesnola tethered 20 green mantids in green grass and 20 

 brown ones on some brown, withered grass. Seventeen days later, all 

 were still alive. When he tethered 25 green mantids on brown grass, all 

 had been eaten, by birds etc., eleven days later. Of 45 brown insects 

 placed in green grass, 35 were dead in seventeen days. Similar experi- 

 ments, with similar results, were made years ago by Poulton, Sanders, 

 Crampton, Bumpus, Davenport and Weldon, and more recently by 

 Carrick, Young, Gerould, and Isely. 



Very recently, the protective value of changeable coloration has been 

 shown experimentally by Sumner. When his fishes (Gambttsia) were 

 allowed to adapt to the shade of their background they were far less 

 often caught by penguins, herons, and predaceous fishes than other in- 

 dividuals placed in tanks which they did not match. Certainly, adaptive- 

 ness of an animal's coloration is the more likely, the more that coloration 

 is altered by the animal. If the alteration is adaptive, we must suppose 

 that the pre-change pattern had been adaptive, and has ceased to be so 

 under the conditions which produce the change. Sometimes — as in most 

 lizards — the change has nothing whatever to do with making the animal 

 less conspicuous. In such cases, we have a right to look for other ways in 

 which the change may yet be interpreted as adaptive to some end or other. 



Modes of Color Change — The warm-blooded animals are under strict 

 limitations as to the changes they can possibly make. Their colorations 

 reside in lifeless hairs and feathers. They can sometimes be altered 

 quickly — locally — by skin muscles, as when a pronghorn displays his 

 white rump-patches, or when a running antelope-jackrabbit turns white 

 in its flight by revolving the belly skin up onto the side toward the pur- 

 suer and laying back his ears. But when a weasel or a willow ptarmigan 

 prepares for winter by turning white practically all over, it is by the ardu- 

 ous growth of new, white hairs or feathers and the shedding of the old. 

 One can cram a canary with foods rich in carotene, but the resulting 

 golden-yellow color will appear in the plumage only after the next sea- 

 sonal moult. Similar passive changes can be forced even upon man by 

 manipulation of his diet, or by exposing him to the sun until, in self- 

 defense against ultraviolet light, he becomes tanned by increased melan- 

 ization. 



The fishes, most amphibians, and many reptiles expose to view living 

 pigmented tissues over the whole surface of the body. For some of these 

 animals, color changes may be only seasonal, as in the adoption of a 



