United States or Dominican Republic ? Vote for the TOP Country of the Week !
It was not every one who could dare to range so far and wide as Huxley did from the original line of investigation he had taken up. Friends warned him against what appeared to be a scattering of his energies. If he devoted himself to that morphology of the Invertebrates in which his new and illuminating conceptions had promptly earned the Royal Medal, he would easily be the first in his field.
Many lower and higher forms of animals and plants appear in the geological strata, so far as they have been explored, in a wholly independent way. We have mentioned, in the foregoing section, that the main types of the invertebrates appear somewhat contemporaneously and without any traceable intermediate form.
The octopus of our own time has advanced still further, and become the most powerful of the invertebrates. The ink-bags of the Belemnite also are sometimes preserved, and we see how it could balk a pursuer by darkening the waters. It was a compensating advantage for the loss of the shell. In all the other classes of aquatic animals we find corresponding advances.
How high, for instance, stand the vertebrates above the invertebrates! Yet how could the first deviation from the ganglionic system of the nerves of the invertebrates to the cerebro-spinal system of the vertebrates have occurred? and, especially, how could the first deposit of the vertebral column have procured any benefit to the individual in the struggle for existence?
This period is called the age of invertebrates because, although there is an enormous wealth of animal and plant life in the Silurian, there are no backboned animals except the lowest kinds of fishes. It was supposed for a long time that even fishes were absent. Now we know they existed, but they were small and inconspicuous.
He looks over the whole course of the zoölogical system and of palæontological discoveries, and searches for the points where the branches and twigs of the animal pedigree of man must have diverged. To begin with the lowest branches, he thinks the most important divergence took place where the series of vertebrates may have been developed out of the invertebrates.
The discovery was confirmed by other zoologists, and there can no longer be any doubt that of all the classes of invertebrates that of the Tunicates is most closely related to the vertebrates, and of the Tunicates the nearest are the Ascidiae.
In the following Silurian period we find corals, radiata, worms, mollusca, and crustacea, in great number, also all the main-types of the invertebrates; and in the highest Silurian strata there are also to be found representatives of the lowest class of vertebrates, of fish, but still of very low organization and little differentiated.
Both are at the border between the two chief divisions of the animal kingdom the vertebrates and invertebrates. Following the example of Lamarck, it is usual to put all the other animals together under the head of invertebrates. But, as I have often mentioned already, the group is composed of a number of very different stems.
Here we are in much the same position as we were in regard to the origin of the higher Invertebrates. Once the fish plainly appears upon the scene it is found to be undergoing a process of evolution like all other animals. In the teeth and tails, also, we find a gradual evolution toward the higher types.
Word Of The Day