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Then, as the coralline could not have fixed itself, if the Crania had been covered up with chalk mud, and could not have lived had itself been so covered, it follows, that an inch of chalk mud could not have accumulated within the time between the death and decay of the soft parts of the sea-urchin and the growth of the coralline to the full size which it has attained.
Cases of this kind are admirably described by Sir Charles Lyell. He speaks of the frequency with which geologists find in the chalk a fossilized sea-urchin to which is attached the lower valve of a Crania. This is a kind of shell-fish, with a shell composed of two pieces, of which, as in the oyster, one is fixed and the other free.
We shall see later that in certain instances where the conditions of preservation were good we can sometimes trace a perfectly gradual advance from species to species. Several shellfish have been traced in this way, and a sea-urchin in the chalk has been followed, quite gradually, from one end of a genus to the other. It is significant that the advance of research is multiplying these cases.
Leaving the urchin with us, the porpoises went off once more to hunt up a starfish. They were not long getting one, for they were quite common in those parts. Then, using the sea-urchin as an interpreter, they questioned the starfish. He was a rather stupid sort of creature; but he tried his best to be helpful.
Suppose that the valve of the Crania upon which a coralline has fixed itself in the way just described, is so attached to the sea-urchin that no part of it is more than an inch above the face upon which the sea-urchin rests.
Anyhow, the loss of an arm or two matters little, for others grow in their place. Another cousin of the Starfish is the Sea-urchin, a round prickly creature rather like the burr of the sweet-chestnut tree. This mass of prickles is not a vegetable; he is very much alive. Nature has given many plants and animals these prickles, like fixed bayonets, for a defence against their enemies.
Suppose that the valve of the Crania upon which a coralline has fixed itself in the way just described is so attached to the sea-urchin that no part of it is more than an inch above the face upon which the sea-urchin rests.
Beyond that he may do a little with seine and dredge, murder a few million herrings a year as they come in to spawn, butcher his fellow air-breather, the whale, or haul now and then an unlucky king-crab or strange sea-urchin out of the deep water, in the name of science; but the life of the sea as a whole knows him not, plays out its slow drama of change and development unheeding him, and may in the end, in mere idle sport, throw up some new terrestrial denizens, some new competitor for space to live in and food to live upon, that will sweep him and all his little contrivances out of existence, as certainly and inevitably as he has swept away auk, bison, and dodo during the last two hundred years.
"The upper valve is almost invariably wanting, though occasionally found in a perfect state of preservation in the white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away.
The result of life, however, is to fill the world more and more with things displaying organic unity. By this is meant any arrangement of which one part helps to keep the other parts in existence. Some organic unities are material, a sea-urchin, for example, a department store, a civil service, or an ecclesiastical organization.
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