628 THE POPULAR SCIENCE MONTHLY. 



even in the best cases the very origin of each variation would still 

 remain to be found. Darwin fully understood this necessity ; and 

 the physiological and mechanical origin of variations is what so 

 many biologists are now working at. Several such investigations 

 are already well known to English readers through the works of 

 Cope, Semper, Lloyd Morgan, J. T. Cunningham, and P. Geddes. 

 Many others ought to be analyzed and discussed ; but for the time 

 "being I can only mention a few recent works relative to the origin 

 of animal colors. 



Wherever we go we see animals colored in accordance with 

 their surroundings. White and light gray colors predominate in 

 the arctic regions ; tawny and yellow colors in the deserts ; gor- 

 geous colors in tropical lands. The striped tiger in the jungle is 

 hardly recognizable among the shadows of the tall grasses. Insects 

 resemble the flowers which they usually visit; caterpillars have 

 the colors and often the forms of the twigs and the leaves they 

 feed upon. Dusty-colored nocturnal insects; moths which take 

 autumnal tints if they begin life in autumn ; dark squirrels in the 

 dark larch forests, and red squirrels in the Scotch-fir groves; 

 animals changing their color with the season — all these are famil- 

 iar instances. But are they all due to natural selection alone ? 

 Does not environment take some part in itself producing these 

 colors ? 



In a very suggestive work*, Alfred Tylor has shown in how 

 far the different markings and the diversified coloration of animals 

 follow the chief lines of structure ; and A. R. Wallace has readily 

 admitted that, while the fundamental or ground colors of animals 

 are due to natural selection, the markings are probably due to 

 internal physiological causes. \ Coloration responds to function ; 

 and there is a law in the distribution of colors and the develop- 

 ment of the markings, while there ought to be none under the 

 hypothesis of selected accidental variations. Wallace goes even a 

 step further, and shows that those birds possess the most brilliant 

 colors which have developed frills, chests, and elongated tails, or 

 immense tail-coverts, or immensely expanded wing feathers, all 

 appearing near to where the activities of the most powerful 

 muscle of the body would be at a maximum. He considers " a 

 surplus of vital energy/' increased at certain periods, as a vera 

 causa for the origin of ornamental appendages of birds and other 

 animals. And it is difficult to examine these and like facts with- 

 out coming to the same conclusion. 



But if partial vigorous coloration is so much dependent upon 

 vital energy, is it not possible to suppose that the decoloration of 

 animals with the approach of the winter is in some way connected 



* Coloration in Animals and Plants. London, 1886. f Darwinism, p. 288 et seq. 



