90 



LIFE : OUTLINES OF GENERAL BIOLOGY 



caves. In the famous Mammoth Cave of Kcntiirky, which has over 

 IOC miles of passages, with streams, pools, and dry ground, there 

 are over 40 different species of animals. The temperature is very 

 equable, varying little more than a degree throughout the year; 

 it is, of course, dark; and there are no plants other than a few 

 fungi. Thus the conditions present some analogy with those of the 

 deep sea. The fauna is of much interest to evolutionists, for we 

 wonder how far the peculiarities of the cave-animals, e.g. absence 

 of coloration and frequent blindness, are due to the cumulative 

 effect of the environment and of disuse, or how far they represent 

 the survival of germinal variations, and the result of the cessation 

 of natural selection along certain lines. Have the seeing animals 



I'IG. 2.J. 



Thrrr Closely Related Newts, living in different conditions of life: I, in full 

 ilhimination ; III, in darkness, where the eye is practically absent; 

 and II. Ill intermediate conditions, where the eye is small. ' 



found their way out, leaving only the blind sports, which crop up 

 even in daylight? Or is the lo.ss of eyes the result of disuse and 

 absence of stimulus.-* Or again, if it be granted that pigment is an 

 organic constitutional necessity, e.g. a waste-product, while colora- 

 tion is explicable as an adaptation wrought out in the course of 

 natural elimination, then the question arises, whether the cessation 

 of natural selection — a condition awkwardly called "panmixia" — 

 which might accoimt for the di.sap|x^arance of the coloralion when 

 there is no premium .set upon it, can also account for the loss of 

 pif^pfirnt — that is. of a character which was not acquired in the 

 course of natural selection. (Sec Heddard's Anifftal Coloration.) 

 Our only answer at present is that there is need for experiment. 



(f) Parasitic fauna. — It seems legitimate to rank together those 

 animals whose habitat is in or on other organisms, from which they 

 derive subsistence, without in most cases killing them quickly, if at 



