according to Kû¥ster35 various forms of annular, spiral and reticulated thickenings in the vascular tissue of plants may be closely imitated; and he and certain other writers have of late been inclined to carry the same chemico-physical phenomenon a very long way, in the explanation of various banded, striped, and other rhythmically successional types of structure or pigmentation. For example, the striped pigmentation of the leaves in many plants (such as Eulalia japonica), the striped or clouded colouring of many feathers or of a catãs skin, the patterns of many fishes, such for instance as the brightly coloured tropical Chaetodonts and the like, are all regarded by him as so many instances of ãdiffusion-figuresã closely related to the typical Liesegang phenomenon. Gebhardt has made a particular study of the same subject in the case of insects36. He declares, for instance, that the banded wings of Papilio podalirius are precisely imitated in Liesegangãs experiments; that the finer markings on the wings of the Goatmoth (Cossus ligniperda) shew the double arrangement of larger and of {431} smaller intermediate rhythms, likewise manifested in certain cases of the same kind; that the alternate banding of the antennae (for instance in Sesia spheciformis), a pigmentation not concurrent with the segmented structure of the antenna, is explicable in the same way; and that the ãocelli,ã for instance of the Emperor moth, are typical illustrations of the common concentric type. Darwinãs well-known disquisition37 on the ocellar pattern of the feathers of the Argus Pheasant, as a result of sexual selection, will occur to the readerãs mind, in striking contrast to this or to any other direct physical explanation38. To turn from the distribution of pigment to more deeply seated structural characters, Leduc has shewn how, for instance, the laminar structure of the cornea or the lens is again, apparently, a similar phenomenon. In the lens of the fishãs eye, we have a very curious appearance, the consecutive lamellae being roughened or notched by close-set, interlocking sinuosities; and precisely the same appearance, save that it is not quite so regular, is presented in one of Kû¥sterãs figures as the effect of precipitating a little sodium phosphate in a gelatinous medium. Biedermann has studied, from the same point of view, the structure and development of the molluscan shell, the problem which Rainey had first attacked more than fifty years before39; and Liesegang himself has applied his results to the formation of pearls, and to the development of bone40. {432}
Among all the many cases where