336 OUTLINES OF EVOLUTIONARY BIOLOGY 



to prevent the sponge from sinking into and being smothered by 

 the soft mud or ooze which covers the bottom of the ocean at 

 very great depths, and it is interesting to observe that species of 

 several distinct though related genera have adopted the same 

 device, thus affording a beautiful example of the phenomenon of 

 convergence. Other sessile deep sea animals have found different 

 means of overcoming the same difficulty, especially in many 

 cases by the development of long stalks. 



The absence of light at great ocean depths has led to the 

 acquisition on the part of many of the deep sea fishes of brilliant 

 phosphorescent organs, arranged like little lamps on various 

 parts of the body. In some cases at any rate these serve to 

 attract other animals upon which these fishes prey. Some of 

 them, again, develop long and delicate feelers by aid of which 

 they grope their way about in the dark. 



In the brilliantly illuminated surface waters of the ocean con- 

 ditions are very different, and here we find that the most favourite 

 device for preserving life amidst a host of enemies is transparency, 

 but we have already alluded to this in the preceding chapter and 

 need not dwell upon it further. It is a phenomenon which falls 

 under the head of protective colouration, of which we shall find 

 better instances elsewhere. 



The significance of the colouration of animals as a means of 

 adaptation to environment is a subject which has in recent years 

 developed into a special branch of biological science, and which 

 already has a copious literature of its own. Professor Poulton, 

 in his well known work on the Colours of Animals, 1 has suggested 

 an elaborate scheme of colour classification from this point of 

 view. He distinguishes, in the first place, between apatetic 

 (deceitful) colours, sematic (warning and signalling) colours, and 

 epigamic colours (displayed in courtship), all of which afford 

 marvellous instances of more or less highly specialized adaptation. 

 We have not space to follow out the details of this classification 

 but we shall presently refer to examples of all the more important 

 types of colouration included therein. 



Every observer of nature must have been struck with the 

 general harmony of colouration which exists between animals 

 and their surroundings. So complete is this harmony that our 

 sense of hearing is frequently a better guide to the whereabouts 

 of an insect, bird or mammal than our sense of sight. I 



1 International Scientific Series, Vol. LXV1II. 



