THE DEVELOPMENT OF THE ANIMAL WORLD 73 



fossils, of the Cambrian period, you find that the animal 

 world is already well developed. There are not only 

 corals and sponges, but Echinoderms, Molluscs, Worms 

 and Crustacea in an advanced stage. Shell-fish abounded 

 in the Cambrian sea, crinoids (sea-lilies) grew on their 

 long stalks, and trilobites (highly developed Crustacea 

 with compound eyes) and marine worms ploughed through 

 the mud at the bottom. It is quite hopeless to attempt to 

 trace the earlier evolution of these. There is a great gap 

 in the geological record, and such earlier strata as we 

 have, have been so charred by heat from below and 

 crushed by pressure and ground by folding that they can 

 tell us nothing. The land, it will be remembered, was 

 now emerging very considerably above the water, and 

 the struggle for life in the over-populated shallows must 

 have been terrific. Passage to the land was the natural 

 escape for those best fitted to effect it, and for ages 

 selection would be at work developing the land animal ; 

 though it is well known that in periods of change and 

 crisis evolution proceeds much more rapidly. As the 

 soft, swimming animals came to touch the bottom (and for 

 protection generally) they would find hard parts, external 

 and internal skeletons, a great advantage. So the 

 Molluscs get their shells, the Crustacea their coats of 

 armour, the Worms their ringed structure, and the 

 Echinoderms their hard coats ; but we must frankly 

 refrain from attempting to trace this evolution in detail. 

 Why do we speak so confidently of their evolution at 

 all, when the crinoid and the trilobite and the mollusc 

 come fully formed, as it were, on to the stage? We 

 have a very good and simple reason, besides the general 

 considerations we have seen above. From the moment 

 these animals do come on the stage to recent times (or 

 until they pass from it) they are in a continuous state of 

 evolution. The shell-fish world affords us some of the 



