COLOUR AND PATTERN IN ANIMALS 67 



not only a pigment, but one that is soluble in soft water, and is 

 washed out in a heavy shower of rain. A less well known case is 

 that of the black colour of the Malay tapir. If the hand be rubbed 

 over the dark portion of the body a black, greasy stain comes off, 

 whilst the grey part of the body is devoid of this secretion. In 

 some of these cases, perhaps in most of them, the pigment has a 

 direct physiological importance, as, for instance, the red colour of 

 blood, due to the presence of haemoglobin, the substance which 

 carries oxygen to the tissues ; or some of the greens and yellows, 

 which are products of the chemical changes of the body, and are 

 waste matters on the way to be removed ; or blacks, which also 

 not infrequently are products of excretion. I have already said 

 that the blood was red long before the colour became a visible 

 ornament of the body. So also the black lining of the body-cavity 

 in many reptiles, the brilliant greens and golden yellows of the 

 gall-bladder, the vivid green of the bones of fishes like the South 

 American lung-fish, are clear instances of strongly marked colour, 

 for which, were they visible externally, we should attempt to 

 find an adaptive explanation, to interpret in the light of suitability 

 to the surrounding conditions. Precisely as in the case of pattern, 

 we must not be too certain that colour has a direct purpose. Colours 

 may be useful, and often are turned to use, but their utility may 

 only be secondary, a laying hold of something that was already 

 there. All warm-blooded animals radiate out heat, varying in 

 amount with the physical activities of their bodies, with the 

 structure and disposition of their protective coverings and so forth, 

 and if we possessed organs as sensitive to heat as our eyes are sensi- 

 tive to colour and hght, we should learn to recognise the presence 

 and perhaps the nature of animals near us by means of the messages 

 that such a heat-sense would convey to the brain, and the heat 

 diffusion of the animals themselves might have been turned to 

 account for their own purposes. When small birds are roosting 

 in the open air on a cold night they fluff out their feathers until 

 the bodies become almost globular, and by so doing they retain 

 more of their internal heat. In such a condition they would be 

 unnoticed by a heat-sense at a much greater distance. They are thus 

 accidentally protected against a danger that does not exist. And 

 so it may be with some of the colours of animals. 



Finally, the presence of colour, whether it be due to structure or 

 to pigment, makes pattern more conspicuous, while the existence 

 of pattern calls attention to differences of colour. When micro- 



