182 The Science of Life. 



influence of their peculiar environment on their ordinary 

 functions, and on their growth and reproduction? 

 There is little but analogy to suggest an answer, and it 

 does not follow that what is deduced from experiments 

 (by no means numerous) will hold true of organisms 

 which have been habituated to their environment for 

 many millennia. 



There is a marked resemblance between the Arctic 

 and Antarctic abyssal fauna Is this resemblance 

 thorough- going, is it primary? or is it the secondary 

 result of migration to and fro along the bottom ? 



How far are the observations numerous enough to 

 warrant conclusiveness of statement as to the uni- 

 formity which some speak of, and the localized distri- 

 bution which the Challenger statistics tend to prove? 



What of the origin of deep-sea animals? Was there 

 any truth in Sir Wyville Thomson's theory of the exist- 

 ence of an abyssal fauna from Palaeozoic times? or is 

 Sir John Murray altogether right in his view, that the 

 deep-sea fauna has been the result of migrations from 

 the region of the mud-line in relatively recent times? 



We may, perhaps, use the term fluvial to include 

 fresh -water areas, whether they be lakes or ponds, 

 Fluvi i r i vers or streams; and although observa- 

 tions on their tenants are as old as the 

 naturalist, we must again record that the systematic 

 study of fresh-water faunas and floras is of very recent 

 date. The biological station over which Prof. Zacharias 

 presides at the great lake of Plon has been an example 

 to the biological world, and the hint has been taken in 

 America and elsewhere, though Britain lags discredi- 

 tably behind. There are interesting practical problems 

 in connection, for instance, with fishes and water- 

 supply; and there are yet more interesting theoretical 

 problems in connection with adaptation, migration, and 

 origin. 



Some clearness has been introduced by distinguish- 

 ing, as may be done in regard to other life-areas, four 

 sets of tenants : (a) the recent immigrants, e.g. the 

 bivalve Dreissenia from the sea; (b) the relics which 

 have been left behind as survivors of the inhabitants of 



