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We shall find them developing with great richness in the following period, but, imperfect as the record is, we may venture to say that they were checked in the Cretaceous. There were good conditions for preserving them, but few are preserved. And of the other groups of invertebrates we need only say that they show a steady advance toward modern types.
During the entire Quaternary, invertebrates and plants suffered little change in species, so slowly are these ancient and comparatively simple organisms modified. The Mammalia, on the other hand, have changed much since the beginning of Quaternary time: the various species of the present have been evolved, and some lines have become extinct.
"This wonderful creature dates back like the kangaroo and many other Australian hydrocephalous invertebrates, to an age long anterior to the advent of man upon the earth; they date back, indeed, to a time when a causeway hundreds of miles wide, and thousands of miles long, joined Australia to Africa, and the animals of the two countries were alike, and all belonged to that remote geological epoch known to science as the Old Red Grindstone Post-Pleosaurian.
For our present knowledge of the power of intracellular digestion possessed by the endoderm cells of the lower invertebrates removes all difficulties both as to the mode of entrance of the algae, and its fate when dead. In short, we have here the relation of the animal and the vegetable world reduced to the simplest and closest conceivable form.
It may be noted, in general terms, that they shared the opulence of the mid-Tertiary period, produced some gigantic specimens of their respective families, and evolved into the genera, and often the species, which we find living to-day. A few illustrations will suffice to give some idea of the later development of the lower invertebrates and vertebrates.
It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all the other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates.
In the Silurian ages, invertebrates brachiopods and crinoids and cephalopods were the dominant types. But very early no one knows just when there came fishes of many strange forms, some of the early ones enclosed in turtle-like shells.
The breaks which formerly separated our groups from one another, as animals from plants, vertebrates from invertebrates, cryptogams from phanerogams, have either been filled up, or shown to have no theoretical significance.
Scorpions are found in this period both in Europe and in America. The limestone-making seas of the Silurian swarmed with corals, crinoids, and brachiopods. With the end of the Silurian period the AGE OF INVERTEBRATES comes to a close, giving place to the Devonian, the AGE OF FISHES.
Bergson thinks that in the vertebrates intelligence has been developed at the expense of instinct, and that in the invertebrates instinct has been perfected at the expense of intelligence.
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