IV ABSENCE OF COLOUR IN CAVE ANIMALS 91 



To tliis cause I also ascribe the first commencement of the 

 vivid colours in the skin of many animals, e.(j. of reptiles, 

 which, clearly in consequence of the increased circulation of 

 blood, appear under a warm sun, and disappear under a low 

 temperature, and which with the aid of selection have 

 developed into the permanent brilliant colours in the skin of 

 southern animals. All the animals that pass their lives in 

 darkness must have become colourless in consequence of the 

 absence of light, since their nearest relatives which live in the 

 daylight are coloured. Absence of pigment is indisputably 

 an acquired and inherited character. No one will seriously 

 maintain that male and female individuals which chanced to 

 be slightly pigmented have everywhere crept into caves, and 

 there, in consequence of sexual mixture and selection, have 

 given rise to colourless races. 



Weismann advocates the view that the absence of colour 

 in cave animals is due to pammixis, to indiscriminate sexual 

 mixture consequent upon the cessation of selection. But 

 how can selection have contributed to the evolution of the 

 oriojinal dark colourins; — what use can this have had ? Sexual 

 selection on the ground of beauty is out of the question in the 

 gloom of the caves. And the principal question is. Why is a 

 colourless race evolved everywhere in darkness by pammixis, 

 and not one of some other colour, green, yellow, blue, or red ? 

 Since we know that light is favourable to pigmentation as it 

 is to the development of chlorophyll, since we know that 

 pigmentation like chlorophyll universally vanishes in the 

 absence of light, we need, it seems to me, no other 

 explanation than that which lirst offers for the absence of 

 pigment in cave animals. Animals are in general more or 

 less colourless in the early stages of development, and ^•e^y 

 many are born light-coloured or colourless, and only sub- 

 sequently develop a dark colouring. This circumstance 

 evidently facilitates the loss of colour in cave animals : since 



