SPOTTED AND STRIPED MAMMALS 5 



not, become reflected electrically on the skin, whether during 

 the embryonic stage or afterwards, and cause aggregation or 

 dissociation or other changes in pigment cells. 



Evolutionary biologists and probably there are at present 

 few or no biologists who have not accepted the doctrine of 

 Evolution seem inclined to consider that these markings in 

 animals are the result of natural selection, acting cumulatively 

 on some fortuitous variations that may have occurred, and do 

 occur, in an infinity of ways. By natural selection is meant 

 the weeding-out, generation after generation, of all those varia- 

 tions which are insufficiently protected by their surroundings 

 for either offence or defence, or both, and by keeping alive those 

 which are most fit. Reproduction and heredity then maintain 

 and improve this selection. 



Dr. Alfred Russel Wallace says l that ' Professor William 

 H. Bremer of Yale College has shown that the white marks or 

 the spots of domesticated animals are rarely symmetrical, but 

 have a tendency to appear more frequently on the left side. 

 This is the case with Horses, Cattle, Dogs, and Swine. . . . 

 Among wild animals, the Skunk varies considerably in the 

 amount of white on the body ; and this, too, was found to 

 be usually greatest on the left side. 2 A close examination of 

 numerous striped or spotted species, as Tigers, Jaguars, Zebras, 

 etc., showed that the bilateral symmetry was not exact, although 

 the general effect of the two sides was the same. This is pre- 

 cisely what we should expect if the symmetry is not the result 

 of a general law of the organism, but has been, in part at least, 

 produced and preserved for the useful purpose of recognition by 



1 Note to p. 217, Darwinism. 



2 I should say this is a sure indication that the difference does not depend on the 

 skin, but on the unequal action of the two halves of the nerve centre. 



