He states that most existing types arose in early Paleozoic times when evolution was most rapid. Then came a period of slow changes, especially slow in all races which were in the acme or most flourishing period of their phylogeny. Finally, when retrogression sets in the pace of evolution again becomes more rapid, and startling new modifications are often introduced.
Hyatt acknowledged that natural selection was a factor which modified the course of evolution, but he believed that the history of a race was a predetermined thing and that natural selection played but a minor role in comparison with the effects of the environment. In this latter respect he was a neo-Lamarckian; a view which is now more popular than it was at the time of his death.
He believed that when a number of more or less distantly related or even unrelated forms live in the same environment they acquire a resemblance one to the other; the similarity of external conditions producing a "morphological equivalence" or parallelism.
These are the chief features of Hyatt's theories of evolution and heredity. To go deeper into the subject would, I fear, only introduce confusion into the mind of the reader.
The paleontologist possesses at least one advantage over the student of existing animals in that he may observe the changes that develop during thousands of generations, whereas the zoologist sees the present but dares not even vaguely guess upon the future of the race he studies. Accordingly, Hyatt made little impression upon zoologists, but some of the most brilliant paleontologists have applied his principles to the unraveling of the genealogy of fossil and living animals. We need only mention the classic studies of Professor Charles Emerson Beecher upon brachiopods and trilobites, or the many researches of Hyatt's former assistant. Professor Robert Tracy Jackson, at present one of the most progressive leaders of the Hyatt school, among whom are such active investigators as Bather, Buckman, J. M. Clarke, Cumings, Grabau, Ruedemann, Stanton, J. Perrin Smith, Burnet Smith, Schuchert and Van Ingen. In the untimely death of Professor Beecher in 1904, the school suffered a most serious and almost irreparable loss. The general attitude of zoologists toward Hyatt's theory of evolution is probably best expressed by quoting from two of the letters which Charles Darwin wrote to Professor Hyatt in 1872, in which Darwin says: