CHAPTER XX 

 RUDIMENTARY EYES 



The adoption of peculiar habits by a species of animal frequently 

 stimulates the development of structural alterations suited to the unusual 

 environment ; in a previous chapter we have discussed the many striking 

 modifications adopted by the vertebrate eye to meet different conditions — 

 aquatic or aerial vision, for example. Changes in the opposite sense may also 

 occur when vision is no longer required, in which case the eyes may become 

 rudimentary or vestigial or even disappear. The adoption of a sessile or 

 sedentary habit involving sluggishness or quiescence so complete that light 

 stimuli are valueless may lead to the development of a state of quasi or 

 complete eyelessness in this way, but the more usual stimulus is a lightless 

 habitat as in abyssal depths of the sea, dark caves, muddy rivers, burrows 

 under the ground, or within the body of another animal. 



A sharp distinction should be noted here between the permanent adoption of an 

 environment wherein Hght is absent and the periodic adoption of nocturnal habits by 

 many species for purposes of concealment or hvmting — the daily use of caves by bats, 

 for example, as opposed to permanent residence in a cave by cavernicolous fishes, or 

 the use of a burrow as a home by the tuatara as opposed to the subterranean life 

 of the mole. As a rule these nocturnal animals show the opposite tendency ; their 

 eyes are elaborately developed to take every advantage of the dim illumination 

 available, being often provided with a large lens, a wide pupil and a rod-retina. 



This tendency for the structural recession and loss of function of an organ which 

 is no longer biologically useful is not, of course, confined to the eye: the fate of the 

 human appendix and coccyx are well-known examples of the regression of an organ, 

 while the loss of its alimentary canal by the tapeworm or the possible reduction of a 

 micro-organism to the bare bones of its nucleo-protein on the adoption of the habit 

 of intracellular parasitism as a virus may be cited as examples of the complete 

 disajjj^earance of unnecessary characters. The biological mechanism of the trans- 

 mission of such a disappearance, however, is not clear ; it is as if development has 

 become arrested from lack of use. It is generally accepted that biologically useful 

 characteristics tend to be I'etained in so far as they have survival value, but that 

 those which are no longer useful should be purposely discarded as excess 

 baggage is an expression of Lamarckian regression more positive than many would 

 accept. Regression, however, does not necessarily imply degeneration as the term is 

 generally understood. Darwin (1859) in his Origin of Species pointed out that both 

 the vise or disuse of an organ might equally lead to inherited changes both in plants 

 and animals, and that parasites and " degenerate " creatures are as inuch a product 

 of evolution as higher organisms ; they are as perfectly adapted to their restricted 

 environment.^ 



1 The opposing argument used by August Weismann in his Essay on Inheritance and Related 

 Biological Questions (1892) that successive generations of rats the tails of which had been cut off 

 persisted in breeding rats with normal tails is inapposite since an artificial mutilation bears no 

 biological relation to a purposive evolutionary regression. See Ray Lankester, Degeneration, 

 a Chapter in Darwinism (1895) ; Demoor and others, Evolution by Atrophy in Biology and 

 Sociology (1894) ; Vandervelde, Parasitism, Organic and Social (1895). 

 S.O.— VOL. I. 721 46 



