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Much less can one picture to one's self what his ancestor was like in the age of the invertebrates, amid the trilobites, etc., of the earlier Palaeozoic seas. But we must go back even earlier than that, back to unicellular life and to original protoplasm, and finally back to fiery nebulous matter.
Of these twenty to thirty stages, ten to twelve belong to the older group of the Invertebrates and eighteen to twenty to the younger division of the Vertebrates.
Greenland had the temperature of Cuba and southern Florida, and the time was yet far distant when it was to be wrapped in glacier ice. INVERTEBRATES. During the long succession of the ages of the Mesozoic, with their vast geographical changes, there were many and great changes in organisms. Species were replaced again and again by others better fitted to the changing environment.
Besides, we have with us yet the invertebrates that have not yet become vertebrates; marine animals that have not become amphibians; amphibians that have not become reptiles; reptiles that have become neither mammals nor birds, and a multitude of simians that have not become human, and are not moving toward man either in bodily form or intelligence or spirituality.
But the integument of the whole dorsal surface was, as is not uncommon in invertebrates, hardening by the deposition of carbonate of lime in the integument. And this in time increased to such an extent as to replace the primitive, probably horny, shell. Into the anatomy of this animal or of its descendants we have no time to enter, for here we must be very brief.
It is not found in the Acrania and Cyclostomes, or any of the Invertebrates. It has been transmitted from the earliest fishes to all the Craniotes. The heart also, the central organ of circulation in all the Craniotes, shows an advance in structure in the Cyclostomes.
In the lower animals, and as illustrating the embryology of the marine invertebrates, they were especially valuable.
Were we to study in detail the psychology of adult human beings, we would find only more truly that instinct and intelligence play a large part in our everyday mental life, and more certainly that even the highest reasoning powers we possess are only more complex in nature than the nervous processes of lower mammals and invertebrates.
These physical forces have worked as slowly and silently in sculpturing the landscapes as the biological laws have worked in evolving man from the lower animals, or the vertebrates from the invertebrates. The rains, the dews, the snows, the winds how could these soft, gently careering agents have demolished these rocks and dug these valleys?
Then come the mollusca, which include the oysters, clams and other shell-fish; snails, cuttle-fish, sea-squirts, etc. All of the above families of animal-forms are what are known as "invertebrates," that is, without a backbone. Then we come to the "vertebrates," or animals having a backbone. First we see the fish family with its thousands of forms.
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