this way. We can not, however, as yet assert that such dark colors are not also advantageous for concealment or some other purpose.
The white appearance of arctic birds and mammals must be advantageous for concealment in a region so largely covered with snow, but it is very probable that advantage is also secured by checking the loss of heat through radiation.
Thus Lord Walsingham's experiments and conclusions seem to prove that colors are sometimes of direct physiological value to animals, although a great deal more work must be done before we can safely estimate the proportion which this advantage bears to others also conferred by the same colors.
By far the most wide-spread use of color is to assist an animal in escaping from its enemies or in capturing its prey; the former is protective, the latter aggressive. It is probable that these were the first uses to which non-significant colors were put. The resemblances are of various kinds; the commonest cases are those of simple concealment. The animal passes undetected by resembling some common object which is of no interest to its enemies or prey respectively, or by harmonizing with the general effect of its surroundings; the former is special, the latter general resemblance, and both may be protective or aggressive. Among the most interesting special aggressive resemblances are the cases of alluring coloring, in which the animal, or some part of it, resembles an object which is attractive to its prey.
Mimicry is in reality a very important section of special resemblance. The animal gains advantage by a superficial resemblance to some other, and generally very different, species which is well known and dreaded because of some unpleasant quality, such as a sting or an offensive taste or smell, etc., or it may even be protected from the animal it resembles: this is protective mimicry. When, however, the animal resembles another so as to be able to injure the latter or some other form which accompanies it or is not afraid of it, the mimicry is aggressive....
When an animal possesses an unpleasant attribute, it is often to its advantage to advertise the fact as publicly as possible. In this way it escapes a great deal of experimental "tasting." The conspicuous patterns and strongly contrasted colors which serve as the signal of danger or inedibility are known as warning colors. In other cases such colors or markings enable individuals of the same species easily to follow those in front to a place of safety, or assist them in keeping together when safety depends upon numbers. It is these warning colors which are nearly always the objects of protective mimicry.
Finally, in the highest animals, the vertebrata and many of the most specialized invertebrate groups, we have some evidence for the existence of an æsthetic sense. Darwin believed that this
