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T h e production of definite modifications in the structure, color or size of animals or plants by artificial changes in the conditions of life has been successfully accomplished over and over again by a large number of investigators. I need only allude to such classical instances as the experiments of Dorfmeister, Weismann and others on butterflies, Schmankewitsch on Artemia, Cunningham on flounders, Naegeli and Bonnier on Alpine plants, or to the more recent work of Tower' upon beetles and Beebea upon birds. T h e fact that such considerable modifications may be produced within the lifetime of an individual by physical or chemical means is in itself interesting if regarded simply as an illustration of the plasticity displayed by many organisms. But when we push our inquiry beyond the merely descriptive plane, we are brought face to face with some of the most fundamental problems of biology. Are these modifications adaptive in their character ? Do they in any instance correspond to the features which distinguish one natural species or geographical variety from one another ? And finally, do such artificially produced modifications reappear in offspring which have not themselves been subjected to the conditions of the experiment? These are some of the questions which demand an answer from the investigator. They are not so simple and easy of solution as may appear on the surface: the first 1 An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa. Carnegie *Geographic Variations in Birds, with Special Reference to the Effects of Humidity. Zoologica: Institution, 1906, pp. 320, pl. 30.

Summary Fishes and, in lesser degree, Amphibia respond to backgrounds in such a manner that their shade, and to a certain extent their colour, tend to conform to that of the substratum on which they lie, or over which they swim. The integrity of the eyes and of major portions of the nervous system is essefitial to these phenomena. The immediate, transitory or “physiological” dour changes are due to the rearrangement of pigment particles already present. When the effective stimuli are continued for some days or weeks, changes become evident both in the number of chromatophores and in the pigment contents of each (quantitative or “morphological” colour changes). All three of the types of chromatophores (melanophores, guanophores, lipo‐phores) are affected by these changes. Dark backgrounds favour the production of melanin and inhibit the production of guanin. Pale backgrounds have a reverse effect. In fishes at least, production (or retention) of the yellow pigment xantho‐phyll is favoured by black backgrounds and retarded by white ones, agreeing thus with melanin. To what extent there is any specific effect of coloured backgrounds (sensu stricto) upon the quantity of xanthophyll is not clear at present. Intensity of illumination, above a rather low level, has very little effect upon pigment formation in fishes. There is some evidence, however, of a slight degree of positive correlation between light intensity and melanin formation. Total darkness leads to pigment reduction both in fishes and Amphibia. Blinding of both eyes, in both of these groups, results in a marked increase of melanin, but only in animals which are kept in the light. Experiments involving illumination from below are known to have resulted in considerable increases in pigmentation of the ventral surface, both in fishes and Amphibia. It is not certain in these cases whether optic stimuli have been concerned, or whether the effects have been due to direct illumination of the skin. The response of a fish to its background is primarily a response to albedo, this being defined as the proportion of incident light which is reflected or dispersed from a given surface. On the basis of considerable evidence, a rule has been formulated which has been found to hold approximately, at least for certain fishes. This rule is that, when the animals are subjected to a variety of backgrounds, under uniform illumination, the amount of melanin (or the number of melanophores) produced varies inversely as the logarithm of the albedo of the background. The close analogy between these pigmentary responses of fishes, and the phenomena of sense perception in man for which the “Weber‐Fechner Law” was formulated was pointed out. The question of how a fish recognizes, and responds to, a given albedo, regardless of the absolute degree of illumination present, resolves itself into the question as to how the animal perceives the ratio between the source of light and the light reflected from the bottom and surrounding objects. This last does not seem to be so difficult an achiev...