THE CAUSE OF MIAIETIC RESEMBLANCE. 577 



of very different kinds combined in single individuals; but the 

 essentially composite nature of the effect easily yields to analysis, 

 fcsome of these complex combinations only require to be stated in 

 order to show the inadequacy of the theory which is most usually 

 substituted for natural selection, viz. the theory of external 

 causes. 



A mimetic appearance is commonly made up o£ (1) colour, 

 including («) structural as well as (b) pigment colours; (2) 

 pattern ; (3) form ; {4>) attitude ; (5) movement. 



It may be plausible to hold that direct local influences deter- 

 mine colour, but the case becomes much more difficult when 

 structural tints are included, as they frequently are. Thus ib 

 might well be held that the dark pigment of a female Si/po- 

 limnas and of the Eiiploea which it resembles are alike the 

 direct effect of the locality they both inhabit. But the most; 

 convinced advocate of direct local causes would probably hesitate 

 to explain, by the operation of the same forces, the structurally 

 caused blue sheen which overspreads the dark pigment in some of 

 these mimetic pairs. Similarly with pattern, it is much moie 

 difficult to understand the appropriate arrangement of the 

 colours by direct forces than th.e production of the tints them- 

 selves; still more difficult to understand how such forces could 

 modify shape, and again, more difficult to modify the nervous 

 and muscular systems so as to produce appropriate attitudes and 

 movements. Most difficult of all to understand, except on a 

 theory of selection, how the several elements in the complex set 

 of changes could be kept in their proper relationship and guided 

 to a dehnite end, viz., the production of a superficial resemblance 

 to anothtr species. 



The objection to the theory of internal causes is not that it is 

 inadequate to produce each of the effects, but that it is in the 

 highest degree improbable that so complete and harmonious an 

 effect could be frequently produced accidentally by the com- 

 bination of such diverse elements. 



When, therefore, it is argued that these resemblances are the 

 uniform result of uniform forces peculiar to the locality, investi- 

 gation proves that the results are very far from uniform. They 

 appear at almost any point in the structure of the body, super- 

 ficial or deep-seated, generally at many points in a single 

 individual both superficial aud deep-seated, and the only common 

 bond which can be established between the various elements 



