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Thus, for instance, in the development of the human embryo, the transition from the invertebrate to the vertebrate may be represented in the reality by the isolated amphioxus, which remains stationary where vertebrate man begins, and can make no step forward, while the human embryo advances farther and farther till it reaches its highest limit.
Every one who admits the principle of evolution, and yet feels great difficulty in admitting that female mammals, birds, reptiles and fish, could have acquired the high taste implied by the beauty of the males, and which generally coincides with our own standard, should reflect that the nerve-cells of the brain in the highest as well as in the lowest members of the Vertebrate series, are derived from those of the common progenitor of the whole group.
In the vast lakes which still remained on either side of the Rocky range, tertiary strata were slowly formed to the ultimate depth of two or three miles, enclosing here and there those vertebrate remains which were to be exposed again to view by denudation when the land rose still higher, and then, in our own time, to tell so wonderful a story to the paleontologist.
I believe in the existence of a mammal with a powerful constitution, belonging to the vertebrate branch like baleen whales, sperm whales, or dolphins, and armed with a tusk made of horn that has tremendous penetrating power." "Humph!" the harpooner put in, shaking his head with the attitude of a man who doesn't want to be convinced. "And why this powerful constitution?" Ned asked.
This author, Mr. Haeckel, has "lines of descent" which involves the idea of a plurality of beginnings in the history of organic being; that is, Mr. Haeckel claims a vertebrate series with a vertebrate lying at the base of the series, and an articulate series with an articulate lying at its base.
The formation of the corpora cavernosa, which cause the stiffness of the phallus and its capability of penetrating the vagina, by certain special structures of their spongy vascular spaces, also shows a good deal of variety within the vertebrate stem.
The last of these is amply represented by the four zoophyte groups, three classes of articulates, five classes of mollusks, and three vertebrate classes: mammals, reptiles, and those countless legions of fish, an infinite order of animals totaling more than 13,000 species, of which only one-tenth belong to fresh water. The sea is a vast pool of nature.
On the one hand, the repeated discovery of vertebrate remains in strata previously supposed to contain none, of reptiles where only fish were thought to exist, of mammals where it was believed there were no creatures higher than reptiles, renders it daily more manifest how small is the value of negative evidence.
Both, in fact, are early arrested stages in the development of mammals from the old common vertebrate ancestor; and they could only have struggled on to our own day in a continent free from the severe competition of the higher types which have since been evolved in Europe and Asia.
"It is of itself an extraordinary fact without reference to other considerations, that the order adopted by Cuvier in his "Animal Kingdom," as that in which the four great classes of vertebrate animals, when marshalled according to their rank and standing, naturally range, should be also that in which they occur in order of time.
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