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 



