mimicrv. 157 



the mid-rib of the leaf. This line cuts the * veins ' and ' cells ' of 

 the wing in the most disregardful fashion, here in acute, there in 

 obtuse angles, and in absolute independence of the regular system of 

 divisions of the wing, which should assuredly be the expression of the 

 ' formative law of the wing,' if that were the product of an internal 

 directive principle. But, leaving this last question aside, this much 

 is certain with regard to the markings, that they are dependent, not on 

 an internal, but on an e.vternal directive force." 



This is the first occasion in which Weismann states distinctly 

 that he is dealing with an " external directive force." Had he been as 

 explicit throughout his paper, there would have been no fault to find. 

 There will be found few, at any rate, to disagree with this, we 

 venture to think. It appears to us that the " directive force " must 

 be an external one. 



Weismann then continues : — " Should anyone be still unconvinced 

 by the evidence we have adduced, let him give the leaf-markings a 

 closer inspection. He will find that the mid-rib is composed of two 

 pieces, of which the one belongs to the hind-wing and the other to 

 the fore-wing, and that the two fit each other exactly when the 

 butterfly is in the attitude of repose, but not otherwise. Now, these 

 two pieces of the leaf-rib do not begin on corresponding spots of the 

 two wings, but on absolutely non-identical spots. And the same is 

 also true of the lines which represent the lateral ribs of the leaf. 

 These lines proceed in acute angles from the rib ; to the right and to 

 the left in the same angle, those of the same side parallel Avith each 

 other. Here, too, no relation is noticeable between the parts of the 

 wing over which the lines pass. The venation of the wing is utterly 

 ignored by the leaf-markings, and its surface is treated as a tabula 

 rasa, upon which anything conceivable can be drawn. In other 

 words, we are presented here with a hilateralbj si/unnetriral figure, 

 engraved on a surface which is essentially radially symmetrical in its 

 divisions." 



Weismann says that he lays unusual stress upon this point, 

 because it shows that we are dealing here " with one of those cases 

 which cannot be explained by mechanical, that is, by natural means, 

 unless natural selection actually exists, and is actually competent to 

 create new properties ; for the Lamarckian principle is excluded here 

 ab initio, seeing that we are dealing with a formation which is only 

 passive in its efiects. The leaf-markings are eftectual, simply by 

 their existence, and not by any function which they perform. They 

 are present in flight as well as at rest, during the absence of a danger, 

 as well as during the approach of an enemy." 



It appears to us, after careful consideration, that this passage is 

 considerably overdrawn. Are the leaf-markings simply " eltectual by 

 their existence, and not by any function which they perform ?" Does 

 this quite state the case ? Is not their very mode of development for 

 a particular purpose a suggestion of function, and do they not perform 

 the very necessary function of making the butterfly resemble a leaf ? 

 True it is that their function necessitates their remaining passive to 

 be of service to them, and that movement would destroy the functional 

 value of the specially-developed scales which form the leaf-markings. 

 Neither do we, except in a limited sense, accept the view that natural 

 selection has created new properties. It has distinctly produced a 

 new pattern, but only by the modification of a previous one. 



