212 GENERAL EVOLUTION. 



avoid one or another color as to result finally in a loss of the power 

 to produce it. 



Thus, it appears to be a fact that not only are species of fishes 

 which dwell in the mud of darker hues than those that inhabit 

 clear water, but that individuals of the same species differ in a simi- 

 lar manner in relation to their habitats ; those that live in impure 

 or muddy waters having darker tints than those of clear streams. 



Land animals present equally abundant and remarkable imita- 

 tions of the objects or substances on which they live. This is 

 well known in insects and spiders, which look like sticks or 

 leaves, or the flowers on which they feed. It is seen in reptiles, 

 which in very many cases can voluntarily assume the hue of leaf, 

 stone, or bark, or have constantly the gray color of their native 

 desert sands. 



These cases are largely selective or optional in their origin, for 

 though metachrosis is also induced by some external stimulus, as 

 an enemy or a food animal, yet other means of escaj)ing the one 

 and procuring the other are generally open. 



These facts pave the way for a consideration of the phenome- 

 non of mimetic analogy, which, though well known to naturalists, 

 may be illustrated by the following new facts : 



On the plains of Kansas there is a species of Mutilla whose 

 abdomen and thorax are colored ochraceous, or brown-yellow, 

 above. A spider of the genus SaUicus is equally abundant, and 

 is almost j)recisely similar in the color of the upper surfaces, so 

 much so as to deceive any but a most careful observer. The 

 Mutilla being a well-armed insect, and a severe stinger, there can 

 be no doubt that the SaUicus derives considerable immunity from 

 enemies from its resemblance. 



On the same plains, the Crotalus conjiuentus, or prairie rattle- 

 snake, abounds. It is an olive gray, with a series of transverse 

 brown dorsal spots, and two rows of smaller lateral ones. The 

 head exhibits a number of brown and white bands. The prairie 

 Heteroclon {H. nasicus) possesses not only the same tints but the 

 same pattern of coloration, and at a short distance can not be dis- 

 tinguished from it. 



In consequence, as one may justly say, this species is, with the 

 rattlesnake, the most common serpent of the plains, as it shares, 

 no doubt, in the protection which the armature of the Caudisona 

 gives its possessor. This is in accordance with the views of Wal- 

 lace and Bates. 



