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 "vari- 

 able 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 " On 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 indivi- 

 dual 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 envi- 

 ronment. 



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 con- 

 trary, 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. 



