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Updated: June 22, 2025
The lamprey, as I have already told you, ranks almost lowest among fishes, and consequently among vertebrate animals, of which fishes form the rear-guard. If you ever had leeches on, you will remember the sharp sting you felt when the little beasts bit you. Well, the lamprey feeds herself just in the same way as the leech does.
That of the insect is almost purely locomotive, but external, that of the vertebrate purely locomotive and internal. It does not lie outside even of the nervous system, although this system especially required, and was worthy of, protection. It does not protect even the brain; the skull of vertebrates is an after-thought. It is almost the deepest seated of all organs.
In order now to infer from these and similar facts that man at one time really existed in this scarcely vertebrate condition of the amphioxus,—a conclusion which, strictly understood, yields no meaning,—we can make the case much more easily conceivable if we represent the thinking, or invention of the world, as an ascending scale, in which even the least chromatic tone must have a place without a break, while the principal tones do not become clear and full until the requisite number of vibrations is attained.
Another thing that indicated that this sexual generation must take place outside the body of the vertebrate host was the fact that the investigators found that the parasites in certain of the cells did not sporulate as did the others.
All physiologists admit that the swim-bladder is homologous or "ideally similar" in position and structure with the lungs of the higher vertebrate animals; hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swim-bladder into a lung, or organ used exclusively for respiration. p. 191.
Those which settled and tried to compete in this sort of life perished. We may have to except the ascidia, but they paid for their success by the loss of nearly all their vertebrate characteristics. The future progress of vertebrates depended upon their continual activity in the swimming life. And they were forced by their environment to maintain this.
I can indeed hardly doubt that all vertebrate animals having true lungs have descended by ordinary generation from the ancient prototype, of which we know nothing, furnished with a floating apparatus or swim-bladder p. 191. We must be cautious
So the vertebrate must forever remain a vertebrate, and the articulate forever an articulate. Were they both evolved from the same unit? We are anxious to know, how from a pulpy mass of flesh, from a moneron, a creature of one substance, vertebrates were evolved.
Science can fairly well trace the successive steps in the development of the central nervous mechanism, from the amoeba to the highest type of vertebrate. "Nerve channels" are worn by the repeated transmission of impulses over the same tracts. Coördinations become in successive generations more complex and more perfect.
We may thus account even for the distinctness of whole classes from each other for instance, of birds from all other vertebrate animals by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other vertebrate classes.
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