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JOURNAL OF VARIATION. 



Vol. VIII. No. 8. October 1st, 1896. 



MIMICRY. 



V. — The origin of leaf-markings as a mimetic pattern (contd.). 

 By J. W. TUTT , F.E.S. 



There can be no doubt, Weismann considers, that the leaf -markings 

 readily admit of production by natural selection in the manner 

 suggested by Darwin and Wallace, and that, by the method they have 

 explained, the markings would be developed " with a gradual but 

 constant increase of fidelity," provided a single condition is fulfilled, 

 /.('., " the occurrence of the right variations at the right place." But 

 he suggests that some persons may consider this condition to be an 

 insurmountable barrier to the explanatory power of this principle, and 

 such may ask, " What is to be our guarantee that dark scales shall 

 appear at the exact spots to which the midrib shall be prolonged ? 

 That still later such dark scales shall appear at the places whence the 

 lateral ribs start ? That here, also, a definite acute angle shall be 

 accurately preserved, and the mutual distances of the lateral ribs be 

 alike, and their courses parallel ? That the prolongation of the 

 median rib from the hind-wing to the fore-wing shall bo extended 

 exactly to that spot where the fore-wing is not covered by the hind- 

 wing in the attitude of repose ? " 



It will be observed that we have now left the " why " of the 

 markings, and reached the "how." Before entering into the detail 

 necessary for the explanation of how it happens that the required 

 variations are always to be found, Weismann deals with some generalisa- 

 tions. He considers that the markings have not risen suddenly, but 

 have been perfected very gradually ; that, in one species, they probably 

 " began on the fore- wing, and in another on the hind- wing ; and 

 that, m many, they never, until recently, proceeded beyond one wing; 

 in other species they went only a little way, and in only a few did they 

 spread over the entire surface of both wings." He then continues : — 

 "That these markings advanced slowly and gradually, but with mar- 

 vellous accuracy, is no mere conjecture. But it follows that the right 

 variations at the right places must never have been wanting, or, as I 

 expressed it before, the useful variations were always present." 



He then raises the possible objections against the theory of selection. 

 He asks how is it possible that these " useful variations are always 

 present in such long extensive lines of dissimilar variations as have 

 gradually come to constitute markings of the complexity here pre- 

 sented ? Suppose that the useful colours had not appeared at all, 



