United States or Tuvalu ? Vote for the TOP Country of the Week !
Can we fail to see the significance of the order in which life has appeared upon the globe the ascending series from the simple to the more and more complex? Can we doubt that each series is the outcome of the one below it that there is a logical sequence from the protozoa up through the invertebrates, the vertebrates, to man? Is it not like all that we know of the method of nature?
INVERTEBRATES. Among the echinoderms, crinoids are now exceedingly abundant, sea urchins are more plentiful, and sea cucumbers are found now for the first time. Trilobites are rapidly declining, and pass away forever with the close of the period. Eurypterids are common; stinging scorpions are abundant; and here occur the first-known spiders.
In the sea, whales, sharks, and teleost fishes of modern types rule in the stead of huge swimming reptiles. The lower vertebrates, the invertebrates of land and sea, and the plants of field and forest take on a modern aspect, and differ little more from those of to-day than the plants and animals of different countries now differ from one another.
But the arrangement of these muscles and their relation to the solid skeleton are different in the Vertebrates from the Invertebrates. The human skeleton. From the right. The human skeleton. In most of the lower animals, especially the Platodes and Vermalia, we find that the muscles form a simple, thin layer of flesh immediately underneath the skin.
The lower animals devoid of backbones, and therefore called invertebrates, are not so well-known except to the student of comparative anatomy, because they are not so often met with, and because they are usually very small or microscopic; but in many respects their importance to the evolutionist surpasses that of the vertebrates.
The important fact is that the second major period, called the Palæozoic, or "age of ancient animals," saw the evolution of the lowest members of the series, the invertebrates, and the most primitive of the backboned animals, like fishes and amphibia. The rocks of this long age include about 106,000 feet of strata, demanding some 21,000,000 or 22,000,000 years for their deposition.
When I made the first attempt in my General Morphology to work out the theory and apply it to classification, I found no problem of phylogeny that gave me so much trouble as the linking of the vertebrates with the invertebrates. But just at this time the true link was discovered, and at a point where it was least expected.
All vertebrates breathe by gills or lungs, and these are modified portions of the digestive system, of the walls of the oesophagus, from which even the lung is an embryonic outgrowth. Now practically all invertebrates breathe through modified portions of the integument or outer surface of the body, and their gills are merely expansions of this.
We have, in the first place, bridged the wide gulf that has existed up to the present between the Vertebrates and Invertebrates; and, in the second place, we have discovered in the embryology of the Amphioxus a number of ancient evolutionary stages that have long since disappeared from human embryology, and have been lost, in virtue of the law of curtailed heredity.
But the fact remains that Aristotle's grand divisions correspond to the grand divisions of the Lamarckian system vertebrates and invertebrates which every one now accepts. Aristotle, as we have said, based his classification upon observation of the blood; Lamarck was guided by a study of the skeleton.