260 [Dee 15; 
Cope.] 
tion of the tail, appears to me to be the result of choice, and not of com- 
pulsion. For the cobra, of India, for the same purpose, expands the an- 
terior ribs, forming a hood, which is a very different habit. Here are two 
alternatives, from which choice might be made; and violent hissing is a 
third, which the species of the colubrine genus Pityophis, haveadopted to 
some purpose. As to the benefit of the rattle, it no doubt protects the 
animal from all foes other than man; but is rather a disadvantage as re- 
gards the latter, being by a beautiful turn of events a protection to the 
higher animal. 
On the principal of natural selection it might be supposed that the 
harmless snakes which imitate the Crotalus for the sake of defence were _ 
preserved ; but if the above explanation of the origin of the habit in the 
latter be true, the second explanation is not valid. 
"The power of metachrosis, or of changing the color at will, by the ex- 
pansion under nerve influence of special pigment cells, exists in most 
Reptilia, Batrachia and fishes. It is then easy to believe that free choice 
should, under certain circumstances, so habitually 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 similar 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 imitations of 
the objects or substances on which they live. This is well known in in- 
sects 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 escaping the one and procuring the other, 
are generally open. 
These facts pave the way for a consideration of the phenomenon of 
mimetic analogy which, though well known to naturalists, may be illus- 
trated by the following new facts : 
On the plains of Kansas, there isa species of Mutilla whose abdomen 
and thorax are colored ochraceous or brown-yellow, above. A spider of 
the genus Saitéews is equally abundant, and is almost precisely similar in 
the color of the upper surfaces, so much so as to deceive any but a most 
careful observer. The Mutilia being a well armed insect, and a severe 
stinger, there can be no doubt that the Saltiews derives considerable im- 
munity from enemies from its resemblance. 
On the same plains, the Caudisona confluenta, or prairie rattlesnake 
abounds. It is an olive grey, with a series of transverse brown dorsal 
spots, and two rows of smaller lateral ones. The head exhibits a num- 
