Surely it is not unreasonable to class this instance with the others before given, in which a common modification of form or colour coexists with a certain geographical distribution quite independently of the destructive agencies of animals. If physical causes connected with locality can abbreviate or annihilate the tails of certain butterflies, why may not similar causes produce an elbow-like prominence on the wings of other butterflies? There are many such instances of simultaneous modification. Mr. Darwin himself[67] quotes Mr. Gould as believing that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast. Mr. Darwin also informs us that Wollaston is convinced that residence near the sea affects the colour of insects; and finally, that Moquin-Tandon gives a list of plants which, when growing near the sea-shore, have their leaves in some degree fleshy, though not so elsewhere. In his work on "Animals and Plants under Domestication,"[68] Mr. Darwin refers to M. Costa as having (in _Bull. de la Soc. Imp. d'Acclimat_. tome viii. p. 351) stated "that young shells taken from the shores of England and placed in the Mediterranean at once altered their manner of growth, and formed prominent diverging rays _like those on the shells of the proper Mediterranean oyster_;" also to Mr. Meehan, as stating (_Proc. Acad. Nat. Sc. of Philadelphia_, Jan. 28, 1862) "that twenty-nine kinds of American trees all differ from their nearest European allies in _a similar manner_, leaves less toothed, buds and seeds smaller, fewer branchlets," &c. These are striking examples indeed! But cases of simultaneous and similar modifications abound on all sides. Even as regards our own species there is a very generally admitted opinion that a new type has been developed in the United States, and this in about a couple of centuries only, and in a vast multitude of individuals of {89} diverse ancestry. The instances here given, however, must suffice, though more could easily be added. [Illustration: THE GREAT SHIELDED GRASSHOPPER.] It may be well now to turn to groups presenting similar variations, not through, but independently of, geographical distribution, and, as far as we know, independently of conditions other than some peculiar nature and tendency (as yet unexplained) common to members of such groups, which nature and tendency seem to induce them to vary in certain definite lines or directions which are different in different groups. Thus with regard to the group of insects, of which the walking leaf is a member, Mr. Wallace observes:[69] "The _whole family_[70] of the Phasmidae, or spectres, to which this insect belongs, is more or less imitative, and a great number of the species are called 'walking-stick insects,' from their singular {90} resemblance to twigs and branches." [Illustration: THE SIX-SHAFTED BIRD OF PARADISE.]