674 REPORT— 1897. 



taken at the surface, and is always found to contain a numter of Leptocepliali. 

 When a Leptocephalus has completed its first stage of growth, it ceases to feed, 

 loses bulk, and develops pigment on the surface of the body. At the same time 

 the larval teeth are cast, and the larval skeleton is replaced. Then the fish begins 

 to feed again, comes to the surface, enters the mouth of a river, and, if caught, is 

 immediately recognised as an Elver or yoimg Eel. It is now a year old, and about 

 two inches long. 



This history suggests a question. Are the depths of the sea free from severe 

 competition ? The darkness, which must be nearly or altogether complete, 

 excludes more than the bare possibility of vegetation. A scanty subsistence for 

 animals is provided by the slowly decomposing remains of surface-life. When the 

 dredge is sunk so low, which does not often happen, it may bring up now and 

 then a peculiar and specially modified inhabitant of the dark and silent abyss. 

 There cannot, we should think, be more than the feeblest competition where living 

 things are so few, and the mode of life so restricted. Going a step further, we 

 might predict that deep-sea animals would lay few eggs at a time, and that these 

 would develop directly — i.e. without transformation. The risk of general reason- 

 ing about the afi'airs of living things is so great that we shall hold our conjectures 

 cheap unless they are confirmed by positive evidence. Happily this can be sup- 

 plied. The voyage of the ' Challenger ' has yielded proof that the number of 

 species diminishes with increasing depth, and that below .300 fathoms living things 

 are few indeed.^ Dr. John Murray gives us the result of careful elaboration of all 

 the facts now accessible, and tells us that the majority of the abyssal species 

 develop directly .- 



We seem therefore to have some ground for believing that the depths of the 

 sea resemble the fresh waters in being comparatively free from enemies dangerous 

 to larvse. The Eel finds a safe nursery in the depths, and visits them for the same 

 reason that leads some other fishes to enter rivers. It may be that the depths of 

 the sea are safer than rivers, in something like the same degree and for the same 

 reasons that rivers are safer than shallow seas. But we must be careful not to go 

 too fast. It may turn out that deep recesses in the shallower seas — holes of 

 limited extent in the sea-bottom — enjoy an immunity from dangerous enemies not 

 ehared by the great and continuous ocean-floor.^ 



After this short review of the facts I come to the conclusion that the general 

 rule which connects the presence or absence of transformation with habitat is well- 

 founded, but that it is apt to be modified and even reversed by highly special 

 circumstances. The efiect of habitat may for instance be overruled by parasitism, 

 parental care, a high degree of organisation, or even by a particular trick in egg- 

 laying. The direct action of the medium is probably of little consequence. Thus 

 the difference between fresh and salt water is chiefly important because it prevents 

 most species from passing suddenly from one to the other. But the abyssal and 

 the flu\'iatile faunas have much in common, as also have the littoral and the 

 pelagic faunas. Relative density and continuity of population seem to be of vital 

 importance, and it is chiefly these that act upon the life-history. 



In Zoology, as in History, Biography, and many other studies, the most inter- 

 esting part of the work is only to be enjoyed by those who look into the details. 

 To learn merely from text -books is notoriously dull. The text-book has its uses, 

 but, like other digests and abridgments, it can never inspire enthusiasm. It is 

 the same with most lectures. Suppose that the subject is that well-worn topic, 

 the Alternation of Generations. The name recalls to many of us some class-room 

 of our youth, the Cfudely coloured pictures of unlikely animals which hung on 

 the walls, and the dispirited class, trying to write down from the lecture the irre- 

 ducible minimum which passes a candidate. The lecturer defines his terms and 



' Challenger Reports. Summary of Scientific Results (1895), pp. 1430-6. 



=> Nature, March 25, 1897. 



» I am aware that other things affect the interests of animals, and indirectly 

 determine their structure, besides danger from living enemies. So complicated a 

 subject can only be discussed in a short space if large omissions are tolerated. 



