Januaby 22, 1904.] 



SCIENCE. 



133 



review. I have, therefore, chosen to re- 

 strict my discussion chiefly to problems of 

 coloration among lower invertebrates, in- 

 cluding incidental references to correlated 

 subjects, and the probable limitations of 

 color as a factor in organic adaptation. 



Interesting as it might be to glance at 

 the earlier views of a sub,iect, the nature 

 of which from earliest times must have 

 been a source of keen interest to mankind 

 in general, and which must have appealed 

 to the esthetic and rational nature, inspir- 

 ing not only poetic imagery, but admiring 

 awe and a devout fervor akin to reverence, 

 it must suffice in the present discussion to 

 hold attention well within the period of 

 thought immediately concerned, which, as 

 already indicated in the opening para- 

 graph, was brought into prominence by the 

 'Origin of Species.' 



As is perfectly well known, color in na- 

 ture is due to one of two causes, or to a 

 combination of both, namely: (1) What 

 has been termed optical or structural con- 

 ditions, such as ditfraction, interference 

 or unequal reflection of light, examples of 

 which are familiar in the splendid hues of 

 the rainbow, the iridescent sheen and me- 

 tallic colors of the feathers of many birds, 

 wings of insects, etc. (2) What are known 

 as pigmentary colors, due to certain mate- 

 rial substances lodged within the tissues of 

 animals or plants which have the prop- 

 erty of absorbing certain elements of light 

 and of reflecting others, and thereby pro- 

 ducing the sensation of color. While the two 

 are physically quite distinct it is not unus- 

 ual to find them associated in producing 

 some of the most exquisite color effects of 

 which Ave have knowledge. In a general 

 way one may usually distinguish between 

 these two sorts of color by noting that 

 those which are purely optical in their 

 character produce a constantly changing 

 impression as the relative position of ob- 

 ject or observer may happen to vary with 



reference to the angle and direction of 

 light; while, upon the other hand, colors 

 which are due to pigments show this prop- 

 erty very slightly or not at all, and that, 

 moreover, pigment colors are usually more 

 or less soluble in various reagents, such as 

 alcohol, ether, acids, alkalies, etc., and that 

 they often fade rapidly under the in- 

 fluence of strong light or in its absence, or 

 upon the death of the organism. 



The presence of many and various colors 

 in inorganic nature, the large majority of 

 which are due to purely physical causes, 

 such as the colors of the ocean, the sky, 

 the clouds, the mineral or gem, while ap- 

 pealing to our sense of beauty elicit no 

 special inquiry as to their significance or 

 purpose. It suffices to know that they are 

 constitutional or structural, inseparable 

 from the physical conditions in which they 

 have their place. 



It is different, however, with much of 

 the color found in the organic world. 

 While such colors as those of the grass or 

 leaf might seem to have hardly any differ- 

 ent significance or to call for special ex- 

 planation different from the preceding, as 

 Wallace has pointed out, on the other 

 hand, as he has also forcefully expressed 

 it: "It is the wonderful individuality of 

 the colors of animals and plants that at- 

 tracts our attention— the fact that colors 

 are localized in definite patterns, some- 

 times in accordance Avith structural char- 

 acters, sometimes altogether independent 

 of them ; while often differing in most 

 striking and fantastic manner in allied 

 species. We are, therefore, compelled to 

 look upon color not merely as a physical 

 but also as a biological characteristic, 

 Avhich has been differentiated and special- 

 ized by natural selection, and must, there- 

 fore, find its explanation in the principle 

 of adaptation or iitility." 



It is under the stimulus of this concep- 

 tion that the significance of color has come 



