Page:The Zoologist, 4th series, vol 4 (1900).djvu/587

Rh and other di- or poly-morphism in the different individuals of the same species. A single instance will make this clear. There are certain genera of butterflies, such as Dismorphia (in the wide sense), Pseudacræa, and Hypolimnas (also in the wide sense), of which almost the whole of the numerous species are mimetic. Within the limits of each genus the most divergent models have been followed, so that utterly different colours and patterns have been produced in forms which are still closely related, and in other structural features exhibit no corresponding differences. In the most extreme case known to me, immense differences occur in the different races of a form which systematists consider as a single species, viz. Hypolimnas bolina. If we compare the Indian form of female with those of the Malayan region, Australia, and Polynesia, including Fiji (in which the local race itself contains the most widely divergent forms), and remember that no corresponding differences exist which would justify us in conferring specific rank in any of the cases, we are driven to the conclusion that colour and pattern are the most superficial of all specific characters,—of all the least likely to persist unchanged when the models upon which they were founded have long since disappeared.

In one special case which I have observed, there is evidence that changes in the nervous system have outlasted the markings which once gave a meaning to them. Some of the remarkable larvæ of the genus Ophideres have two eye-spots at the junction of the anterior and middle third of the body. They have the instinct of bending the anterior third so that it rests under the middle one, and thus the eye-spots are brought into an appropriate position apparently at the anterior end of a somewhat snake-like body. But a caterpillar of this genus which I found in Teneriffe assumed the attitude, on irritation, although the eye-spots were almost completely wanting.

11 It is worth considering whether the Müllerian principle may have been operative in this case.

12 Of course, no natural selectionist has ever been so unreasonable as to contend for absolute protection. In every species, whether defended by the most distasteful or dangerous qualities, or the most effective concealment, no more can be achieved than to keep up the average numbers under average conditions, and this means that an immense majority of individuals are doomed to failure. As regards concealment, success merely means that enemies have so far to work for their living that in the time at their disposal they cannot do more than reduce the number of individuals to the average. Warning colours and unpalatable or otherwise unpleasant qualities are more complex as a means of defence, depending as they do for their success upon the co-existence of other more desirable food. Their operation,