THE FACTORS OF EVOLUTION 163 



deeper regions of the ocean are hidden in eternal night. It is, 

 therefore, remarkable that in spite of this fact the greatest depths, 

 abysses of 7,000 to 8,000 metres, are inhabited by fishes, cephalo- 

 pods, crustaceans, etc., which have well-developed, highly organized 

 eyes. This seems to contradict all previous experience, which 

 shows that organs which have ceased to fulfil their functions 

 become degenerate. The Proteus is blind, the mole has diminu- 

 tive eyes. How is it possible that in the absolute darkness of the 

 ocean-bed there exist animals with well-developed eyes ? The 

 reason is, however, simple : though it is true that deep down in 

 the ocean there was originally darkness, the denizens of the deep 

 sea have entirely changed these uncomfortable conditions by 

 creating an artificial illumination, for the large majority of the 

 animals which are known to us as inhabiting the ocean-bed are 

 furnished with luminous organs. 



The ability to produce light is widely distributed among 

 organisms, but we are still in the dark concerning its biological 

 importance. Among the vegetable organisms it is in particular 

 certain bacteria and fungi which show this remarkable property. 

 Among animals we know phosphorescent species among fishes, 

 tunicates, insects, crustaceans, vermes, molluscs, echinoderms, 

 coelenterates and protozoans. 



Among the terrestrial arthropods it is above all the glow~ 

 worm, Lampyris noctiluca, which attracts our attention. It is 

 distributed throughout the whole of Europe. During warm June 

 nights we may frequently observe the males in swarms of hun- 

 dreds flying about like floating drops of fire, searching for the 

 wingless females who sit on the ground in the grass of meadows 

 or under shrubs, pointing the way by means of their natural 

 lanterns. One may keep glow-worms easily in a terrarium or 

 large preserve-glasses planted with grass, and observe night after 

 night their scintillating movements, provided that they are always 

 well supplied with moisture. But these ' beauties of the night ' 

 look most disappointing when seen in the daylight. The female 

 is an ugly ' worm/ 2 centimetres long ; nor can the male, which 

 is about half as long, lay claim to good looks. The luminous 

 organs of these insects lie on the ventral surface of two of the last 

 abdominal segments, which even during the day may be distin- 

 guished by their yellow colour. The larger American relatives of 



