“I lay 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 effects: the leaf markings are effectual 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. “Nor are we helped here by the assumption of _purely internal motive forces_, which Nägeli, Askenasy, and others have put forward as supplying a _mechanical_ force of evolution. It is impossible to regard the coincidence of an Indian butterfly with the leaf of a tree now growing in an Indian forest as fortuitous, as a _lusus naturæ_. Assuming this seemingly mechanical force, therefore, we should be led back inevitably to a teleological principle which produces adaptive characters and which must have deposited the directive principle in the very first germ of terrestrial organisms, so that after untold ages at a definite time and place the illusive leaf markings should be developed. The assumption of preëstablished harmony between the evolution of the ancestral line of the tree with its prefigurative leaf, and that of the butterfly with its imitating wing, is absolutely necessary here, as I pointed out many years ago, but as is constantly forgotten by the promulgators of the theory of internal evolutionary forces.”