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But we have also many important grounds in comparative anatomy and ontogeny for assuming a common origin for all the Vertebrates. If the general theory of evolution is correct, all the Vertebrates, including man, come from a single common ancestor, a long-extinct "Primitive Vertebrate." Hence the genealogical tree of the Vertebrates is at the same time that of the human race.
In the brain of man and many of the lower vertebrates, hanging by two peduncles, or strands of nerve fibre, from the thalami, or beds of the optic nerve, is a small rounded or heart-shaped body of about the size of a pea, known as the pineal gland.
Man and the vertebrates have disappeared, and the highest wonder of animal life is represented by giant crustaceans, which in turn give way to a lower form.
A large proportion of them were no doubt Ganoids; for they had the same peculiar articulation of the vertebrae, the flexibility of the neck, and the hard scales so characteristic of our Gar-Pikes. There is another type of these ancient Vertebrates, which has also its representatives among our modern fishes.
If the testes were developed from the beginning in a different part of the abdomen, there might be some reason in calling the change a mutation. Moreover, if it is a mutation, why has it never occurred in any other class of Vertebrates except Mammals? In 1903 Dr. Zool. Dr. Woodland correlated the dislocation of the testes with the special mechanical features of the mode of locomotion in Mammalia.
Now the legs and arms, or fore- and hind-legs, of higher vertebrates and the corresponding paired fins of fish develop in the embryo as portions of a long ridge extending from front to rear of the side of the body.
The gorilla at least seems to have been sociable in olden times, if the apes mentioned in the Periplus really were gorillas. We thus see, even from the above brief review, that life in societies is no exception in the animal world; it is the rule, the law of Nature, and it reaches its fullest development with the higher vertebrates.
And here I find myself in a difficulty. We have been but too learned already, and now we run the risk of becoming still more so, if we commence an attack on the three primary divisions which follow the vertebrates. We shall have to encounter terrible names and tedious details, besides having to take into account a thousand things of which we have not yet spoken.
Was he, then, a zoologist in the entire acceptation of the word, a kind of Cuvier of the New World, decomposing an animal by analysis, or putting it together again by synthesis, one of those profound connoisseurs, versed in the study of the four types to which modern science refers all animal existence, vertebrates, mollusks, articulates, and radiates?
So, while nature has frankly evolved in the direction of instinct in the arthropods, we observe in almost all the vertebrates the striving after rather than the expansion of intelligence. It is instinct still which forms the basis of their psychical activity; but intelligence is there, and would fain supersede it.
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