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Show 158 MR. R. MELDOLA ON VARIABLE PROTECTIVE [Feb. 4, present class ; and this definition will be best found by comparing the present class with the above-given classes, and noting in what points they differ. Now, of the four classes already named, Class II. makes the nearest approach to the class now under consideration. The cases of Class II. are indeed, to a certain extent, cases of "variable protective colouring;" it becomes necessary, therefore, to draw a sharp line of demarcation between these two classes ; and for this purpose I will take a known instance belonging to Class II. and trace it through the hypothetical conditions necessary to make it a member of Class V. The example given is from a paper " O n the Adaptive Colouring of the Mollusca," by Mr. Edward S. Morse*. The shell of a common coast species of Littorina presents two varieties, one of an olive-brown and the other of a yellowish colour, these two forms corresponding in colour to the bulbous portion of the bladder-weed on which the shell swarms, and which is olive-brown or yellow according to age. Now if we imagine that, instead of the brown and yellow bladders being borne on the same plant, they were borne on different plants, and that the two forms of the shell were always limited to the plants of their own colour, we should then refer the case to the present class. Thus in Class II. the conditions to be met are the same, or very nearly the same, for each individual of the species ; whereas in Class V. the object imitated may differ in character for each individual of the species; and this can obviously only be met by a power of adaptability on the part of each individual. In other words, in Class II. it is the aggregate of individuals that is adapted to the surrounding conditions, while in the present class each particular individual is capable of being adapted to the characters of its environment. It is obviously quite unnecessary to draw any distinction between the cases of Class I. and the cases of the present class, since there is no possibility of a confusion arising between these two groups. On referring to the examples above given in illustration of Classes III. and IV. it will be seen that the characters of the imitated object change once in the course of time in Class III., and periodically in Class IV. The imitated object, as it exists in space, may in these two classes be either constant or variable. In Class V., on the contrary, the characters of the imitated object, while either constant or variable in the order of time, may vary in space irregularly and in a different manner for each individual of the protected species. Expressing, as before, the characters of the class in terms of the object imitated, the present group may be defined as follows:- V. Cases in which the imitated object is constant or variable in time, but variable in space, for each individual of the species, this variability being met by a power of adaptation on the part of each individual of the disguised species. Theory of variable protective colouring. The cases of ordinary protective resemblance, as contained in the * Proc. Boston Soc. of Nat. Hist. vol. xiv. April 5th, 1871. |