Page:The Zoologist, 4th series, vol 3 (1899).djvu/316

290 natural conditions; whereas we should think not of years but of geological epochs, for time is only an imaginary quantity, alike useful to the mathematician and historian, a result of expressing the term of our short lives. Thus we may seek to multiply the years of our fugitive existence into a product which shall represent the limits of an unknown past, whilst we can only imagine space by the equivalent of time.

We have already ventured some suggestions on the subject of assimilative colouration, and we now approach a different class of phenomena, where the resemblance is not of colour alone, but also frequently of structure, by which animals exhibit a close resemblance to some inanimate object, and to which the term "Protective Imitation of Particular Objects" has been aptly proposed by Mr. Wallace. One of the most striking examples is found in the Orthopterous family Phasmidæ, and in what are generally known as the "Walking-stick insects." To use the graphic and accurate description of Mr. Wallace:—"Some of these are a foot long, and as thick as one's finger, and their whole colouring, form, rugosity, and the arrangement of the head, legs, and antennæ, are such as to render them absolutely identical in appearance with dead sticks. They hang loosely about shrubs in the forest, and have the extraordinary habit of stretching out their legs unsymmetrically, so as to render the deception more