324 LIFE BY THE SEASHORE. 



breathing organs, and so asphyxiating the animals. But the 

 danger is so common that many animals notably crabs 

 have special means of filtering the water before it finds 

 access to the gills. In crabs the filtering arrangement is 

 obtained by spines and notches on carapace and claws, or 

 by hairs," all structures subject to variation. In the Firth of 

 Forth the increasing impurity of the water is certainly 

 eliminating certain animals, as it is probably contributing 

 to the increase of other mud-loving forms. In the case of 

 crabs, for instance, there must be, as it were, a premium on 

 the forms best adapted for filtering the water used in 

 respiration, for these only can thrive and multiply. The 

 result must be to produce relatively rapid variation, for the 

 progeny of parents which had both an elaborate filtering 

 apparatus will have a better chance of success than the 

 progeny of less specialised forms, or of a mixed union. 

 Similar variations of physical environment take place every- 

 where on the shore area ; as the conditions change and new 

 combinations occur, new places in nature are left vacant for 

 progressive forms, with the result that the shore area is one 

 where life is fast, and evolution rapid it is not the place 

 for decadents or survivals. It is probable that this rapid 

 evolution has always occurred in the littoral zone, so we 

 should expect to find that the genera and species now living 

 in the area are modern in type, and may reasonably be 

 regarded as having arisen within the area. But where did 

 their progenitors come from? Has there always been an 

 abundant fauna, or can we go back to a period when the 

 shore waters were comparatively empty *? What relation has 

 the littoral fauna to the two other great faunas the pelagic 

 and the abyssal ? 



The answers to these questions are difficult and debated, 

 but it may be worth while to look for a little at the matter, 

 even if we cannot hope to reach a definite conclusion. In 

 the first place we may clear the way a little by excluding 

 the abyssal fauna from consideration. Its members are 

 strangely modified animals, which, there is reason to believe, 

 have been derived at very different periods from littoral or 

 pelagic forms. Apart even from the fact that these deep-sea 

 animals display many peculiarities of structure, the physical 

 conditions which prevail in the great depths the darkness, 



