270 Dll. C. CHILTON ON THE SUBTERRANEAN 



this action of Natural Selection which is of the most importance when we consider the 

 case of the blind inhabitants of caves, &c., as has been clearly pointed out by AVallace 

 [115, p. 413, &c.], Weismann [119, p. 90, &c.], &c. My former teacher, Professor 

 Hutton, put it very clearly and impressively in his lectures when he said that Natural 

 Selection consists not so much in the " Survival of the Fittest " as in the " Non-survival of 

 the Upfittest" ; and, as he proceeded to demonstrate, the difference between the two points 

 of view is a real one, and not a mere question of words. Thus, in the case of any animal 

 living in the full light, a certain degree of perfection of eyesight will be required by the 

 animal in order to enable it to escape its enemies, obtain food, &c., and all individuals 

 falling below this standard will perish ; so that by the action of Natural Selection the 

 eyesight of the animal will be kept in perfect adaptation to its environment. Now in the 

 case of animals that have taken to living in dark caverns, &c., the eyesight, being no 

 longer of use to the animal, will no longer be maintained in its state of perfection by 

 Natural Selection (although of course Natural Selection will still act on other organs 

 that are of use in the darkness) ; consequently all degrees of eyesight will stand an equal 

 chance of preservation, and by the intercrossing of individuals of varying degrees of 

 perfection there will result a degeneration of the eyesight — a " regression towards 

 mediocrity," as Galton has called it. The explanation of the gradual loss of the eyes in 

 cave animals is the one adopted by Wallace, in his ' Darwinism ' [115, p. 416], who also 

 adds that besides becoming useless, the eyes might also become injurious on account of 

 their delicacy of organization and liability to accidents and disease ; so that in addition to 

 the " regression towards mediocrity," owing to the withdrawal of the action of Natural 

 Selection in maintaining perfection. Natural Selection would also actively reduce and finally 

 abort them. It is important to observe that this " regression towards mediocrity " is a 

 general law of heredity, and produces its effect quite irrespective of any use or disuse of 

 the organ in question [Wallace, 115, p. 414]. A similar explanation has been given 

 by Weismann [119, pp. 90 and 292], who has introduced the term '■'Panmixia" for the 

 suspension of the preserving influence of Natural Selection, and the consequent inter- 

 crossing of animals of all standards of perfection. 



Of course, if we accept his dogma of the non-heredity of acquired characters, that 

 at once excludes the effects of disuse as an explanation of the blindness of cave animals ; 

 but even without going to this length the principle of panmixia, combined with the other 

 active effect of Natural Selection adduced by Wallace, will be sufficient to account for 

 much of the degeneration of eyesight, and to these must be added another equally 

 important consideration advanced by Lankester [70, p. 818-819]. After pointing out 

 that the eyesight of different individuals varies, owing to congenital fortuitous variations, 

 he remarks : — 



" Suppose that a number of some species of Arthropod or Fish be swept into a cavern 

 or be carried from less to greater depths in the sea, those individuals with perfect eyes 

 would follow the glimmer of light, and eventually escape to the outer air or the 

 shallower depths, leaving behind those with imperfect eyes to breed in the dark place. 

 A natural selection would thus be effected." 



This explanation is no doubt a true statement of fact, for caverns and underground 



