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Page:Popular Science Monthly Volume 32.djvu/121

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AMERICAN ZOÖLOGISTS AND EVOLUTION.

"On Critical Periods in the History of the Earth and their Relation to Evolution: and on the Quaternary as such a Period," may be found an excellent rejoinder of Professor Clarence King's lecture before the Sheffield Scientific School on the subject of "Catastrophism and Evolution."

Among the most interesting discoveries connected with these creatures is the determination by Professor Marsh[1] that these early mammals, birds, and reptiles had brains of diminutive proportions. He says in regard to the order Dinocerata, a group of gigantic mammals whose remains have been found in the tertiary deposits of the Rocky Mountain region, that they are the most remarkable of the many remarkable forms brought to light. The brain of these creatures was remarkable for its diminutive proportion. So small, indeed, was the brain of Dinoceras mirabile, that it could "apparently have been drawn through the neural canal of all the presacral vertebræ." In alluding to the successive disappearance of the large brutes, the cause is not difficult to find: "The small brain, highly specialized characters, and huge bulk, render them incapable of adapting themselves to new conditions, and a change of surroundings brought extinction. The existing proboscidians must soon disappear, for similar reasons. Smaller mammals, with larger brains, and more plastic structure, readily adapt themselves to their environment, and survive, or even send off new and vigorous lines. The Dinocerata, with their very diminutive brain, fixed characters, and massive frames, flourished as long as the conditions were especially favorable; but, with the first geological change, they perished, and left no descendants." Professor Marsh says that the brain of Dinoceras was in fact the most reptilian brain in any known mammal.

Professor Cope,[2] in describing the brain of Coryphodon from the deposits of New Mexico, says: "The large size of the middle brain and olfactory lobes gives the brain as much the appearance of that of a lizard as of a mammal." This is one of the lowest mammalian brains known. There are others from the Lower Eocene with equally low brains as Arctocyon of Gervais and Uintatherium of Marsh. Cope believes that the type of brain of these early creatures is so distinct as to necessitate the erection of a third sub-class of equal rank with the groups Gyrencephala and Lycencephala, which he would define as the Protencephala. He shows their approximation to reptiles.

Cope[3] refers to Gratiolet as showing that a great development of the olfactory is a character of an inferior type; in fact, the more we ascend into paleontological antiquity, the more we find that the olfactory lobes display a greater development in comparison with the cerebral hemispheres. Dr. B. G. Wilder[4] has shown that in the lamprey

  1. "American Journal of Science and Arts," vol. xxix, p. 173.
  2. "American Naturalist," vol. xv, p. 312.
  3. "National Academy of Sciences," 1876.
  4. "American Journal of Science and Arts."