EVOLUTION 997 



laboratory it begins to develop spots, and in a week or two it is 

 dark-coloured like a common newt. And if a larval Proteus is reared 

 under red light, its eyes grow larger, reach the skin, and become 

 seeing eyes. The reason for red light is that white light would 

 evoke a dark skin, thus preventing the light from reaching the 

 developing eyes. 



An experiment in the opposite direction illustrates what may be 

 called a minus modification. A goldfish was kept for three years in 

 complete darkness, care being taken to secure abundant food and 

 freshness. The result was complete blindness, which was indicated 

 in the structure of the eye by the degeneration of the rods and 

 cones — the percipient elements of the retina. This proved that the 

 normal stimulus of the light is required in this animal to keep the 

 structure of the eye up to the mark. The experiment would have had 

 even greater interest if it had led to some knowledge of the state of 

 the eyes in the blind goldfish's offspring, but there are not as yet 

 any data on this subject. 



Cave Animals. — Many of the animals, e.g. fishes, crustaceans, 

 and insects, that live permanently in caves are more or less blind, 

 depending mainly on their other senses, especially touch and smell. 

 There are many different gradations in the state of the eye in different 

 cavernicolous types, but some species are entirely blind. To some 

 naturalists it seems justifiable to say that the blindness illustrates 

 the hereditary accumulation of the results of disuse and darkness. 

 But it is necessary to proceed cautiously. As we have already ex- 

 plained, part of the blindness of Proteus is the outcome of an influ- 

 ence operating on each successive crop of individuals, for under the 

 red light in the laboratory the usually arrested eyes may become 

 seeing eyes. Therefore the sense of sight is not actually lost to the 

 race. Perhaps the same is true of other blind cave-animals. They are 

 certainly walkers in darkness, but it is possible that they may 

 recover their sight. No one knows, for no one has tried with sufficient 

 care, what may follow restoration to the light. 



But there is another point to be considered. Eyes are very variable 

 structures, meaning by variable that they may be better or worse 

 from birth onwards, apart from the direct influence of illumination 

 or of darkness. A variation is an inborn peculiarity whose origin is 

 still obscure ; it is the outcome of some germinal change and is quite 

 different from a bodily modification. It is increasingly explained (so 

 far) as due to some disturbance or rearrangement, some strengthen- 

 ing or weakening, of the hereditary items or "factors" that make up 

 the inheritance carried by the egg-cell and the sperm-cell. Albinism, 

 the absence of pigment, may be mentioned as an example of a minus 

 variation; the appearance of long Angora hair in various types 

 of mammal is a plus variation. In regard to many variations 

 we know that they are readily entailed by heredity on the off- 



