Sir Arthur Thomson 265 



walls of the gill chamber the crab has produced delicate 

 projections which contain blood and are able to absorb 

 dry air. 



There are other kinds of land crab found in the West 

 Indies and elsewhere, and these, like the robber crab, 

 have to go back to the sea to produce their young. They 

 are examples of a change which is comparatively recent. 

 Sir Arthur Thomson gives an example of another change 

 as strongly established, yet much older. 



If you break off a piece of bark from a decaying log you 

 will almost certainly find beneath it one or more of those 

 odd little many-legged, armoured creatures called wood 

 lice. Count the legs and you find that the creature has 

 nineteen pairs. This means a great deal, for almost all 

 lobsters, shrimps, and prawns also have nineteen pairs. 

 This is not a mere coincidence, but proof that the wood 

 louse, a land creature, sprang originally from the marine 

 sea-slaters or isopods, which are often found between 

 high- and low-tide marks "beginning the exploration 

 which the wood lice have finished/ ' 



Likewise earthworms, which drown in a puddle, un- 

 doubtedly sprang from water-worms. A proof of this is 

 that there are several varieties of earthworm which still 

 have gill-like outgrowths near the head end. 



The invasion of the land by the worms was of great 

 importance to man, for it is worms more than anything 

 else that have made fertile soil fit for plants. This inva- 

 sion was followed by what Sir Arthur calls the " centipede- 

 millipede-insect-spider invasion,' ' which was also of great 

 importance, because it linked the flowers and flower- 

 visiting insects. Third was the great amphibian invasion, 

 starting probably with certain bold fresh-water fishes. 

 In India to-day there is a kind of small fish which crawls 

 out of the water and clings to the bank high and dry. 

 Millions of years ago the same sort of thing happened and 

 fish became amphibious (able to live on land or in water). 



