PHYSIOLOGICAL 419 



We need not continue our illustrations of the uses of colour 

 among living creatures, but we venture to reiterate the two points, 

 that the first inquiry should refer to the primary significance of 

 the pigment or of the architecture on which the coloration depends, 

 and that the secondary significance of the coloration, if it has any, 

 should be, as far as possible, demonstrated. In many cases, we 

 venture to think, the colour has no use at all, but is purely inci- 

 dental, as in withering leaves. Yet what a delight to human eyes! 

 The Earthworm's Colour. — As an instance of the familiar 

 discovery that things are seldom so simple as they seem, we may 

 refer to the redness of earthworms. What makes an earthworm 

 red? Is it always blushing? Or is the skin so thin that the red blood 

 shines through? And is there any use in its being red? It seems to 

 have been securely established that the earthworm has in its blood 

 the same red pigment, haemoglobin, as we and all backboned animals 

 have. But the red blood-pigment in the earthworm is in the fluid 

 of the blood, whereas in backboned animals it is in the red blood 

 corpuscles. This is an unimportant difference, however; for the 

 earthworm and for man the physiological significance of the haemo- 

 globin is the same, it captures oxygen from the outer world — on 

 the earthworm's skin, on the lining of our lungs — and surrenders . 

 it again to the tissues, where it is required to sustain the vital 

 combustion that living implies. 



But the redness of the earthworm's skin is not directly due to 

 haemoglobin; it is due to another pigment called porphyrin. And 

 this requires just a word of explanation. Haemoglobin is a com- 

 bination of an iron-containing brownish pigment called hcBmatin 

 and a white-of-egg-like or protein substance called globin. But if 

 the haematin in the blood be treated with strong sulphuric acid, 

 the iron is filched away to make ferrous sulphate, and a pigment 

 called haematoporphyrin is left. This pigment is also formed in the 

 course of the everyday chemical routine of many animals, and it 

 is not very unfamiliar, because there are traces of it in normal 

 urine and quantities of it in some kinds of abnormal urine. Well, 

 to come to the point, the pigment in an earthworm's skin is a 

 porphyrin, but Kobayashi has recently shown that it is different 

 from haematoporphyrin. The fact is that it is nearer to a porphyrin 

 which can be derived from chlorophyll, the green pigment of plants. 

 It is possible, then, that the redness of the earthworm's skin comes, 

 not from the haemoglobin of the blood, but from the chlorophyll 

 in the vegetable remains on which the earthworm feeds. A porphyrin 

 of this origin may be absorbed from the earthworm's food-canal 

 by the blood, and then deposited in the skin. This may seem much 

 ado about nothing, but it is a fresh illustration of the danger of 

 thinking of things too simply. As to the use of the porphyrin in 

 the earthworm race', it probably protects the earthworm from the 



