the poet Goethe (1749-1832) dwelt upon the evidences for evolution in his essay on "The Metamorphoses of Plants" and in his various writings on comparative anatomy. Even in his eighty-first year he followed with keen interest the debates over evolution in the French Academy which were carried on by Cuvier and St. Hilaire. The conception of unity of type which persists throughout a group of organisms appealed strongly to Goethe's imagination, and he explained this unity as due to a common inheritance, the various modifications of the type being the result of adaptations to the conditions of the outer world.

Perhaps the most prominent of the evolutionists before the time of Charles Darwin was the French naturalist Lamarck (1744-1829) whose chief work on evolution, La Philosophie Zoologique, is devoted to a presentation of the evidences of descent, and a consideration of the influence of environment and activity in the production of evolutionary changes. Lamarck had little influence upon his contemporaries. Evolutionary speculations were attacked and ridiculed by Cuvier (1769–1832) who was then the dominant figure in the zoological world. Nevertheless, evolution was coming to be looked upon with favor by an ever increasing number of naturalists.

Herbert Spencer deserves a prominent place among the preDarwinian evolutionists, although most of his works were written after the appearance of Darwin's Origin of Species. In 1852, Spencer advocated the theory of evolution in a vigorous and closely reasoned essay entitled "The Development Hypothesis," and in 1855 appeared his highly original Principles of Psychology, a distinctive feature of which is the treatment of mind from the evolutionary point of view. Spencer's great life work, the Synthetic Philosophy, deals with biology, psychology, sociology, and ethics from the standpoint of the law of evolution which he attempted to deduce, in his introductory volume on First Principles, from the general laws of matter and motion. Spencer ranks as, par excellence, the philosopher of evolution, and his writings exerted a profound influence upon the thinking of his day and generation.

The most memorable date in the history of the theory of evolution is marked by the appearance of Darwin's Origin of Species in 1859. Darwin's great work supported the theory with a great wealth of facts drawn from a variety of fields and marshalled with rare skill and judgment. It also supplied, in the doctrine of natural selection, a good working hypothesis as to how evolution might have been brought about. Few books have aroused such immediate and wide

spread attention. It was the

subject of numerous attacks

from pulpit and press, and while it was received favorably by a

few eminent biologists such as Huxley, Hooker, and Asa Gray, it met with no little opposition on the part of scientific men. The eminent geologist Sir Charles Lyell, who had hitherto opposed the doctrine of the transmutation of species, soon declared himself an adherent of Darwin's views. In Germany, evolution found a vigorous and able champion in Professor Ernst Haeckel, whose more popular works on The Natural History of Creation and The Evolution of Man exerted a wide influence in The years following the publica

FIG. 183-Thomas H. Huxley. favor of the new doctrine. tion of The Origin of Species constituted a controversial pe

riod in which a great, new, and revolutionary conception was gradually winning its way to general acceptance in the world of scholars. Scientific men are now practically unanimous in adopting some form of the doctrine of organic evolution. They differ as to the cause of evolution, and they hold all sorts of opinions in regard to Darwin's doctrine of natural selection. But evolution, in some form, has now come to be as much a part of the prevailing scientific conception of nature as the Copernican system of astronomy, or Newton's laws of motion.

That the theory of evolution came to be established as late as the latter half of the nineteenth century may seem at first a strange and anomalous fact. The reasons for the tardy acceptance of the theory are to be found partly in the influence of current theological ideas, and partly in the late development of several branches of science. Not many decades ago life was regarded as having existed on the globe for only a few thousand years. A widely accepted date of creation, which was deduced from the Old Testament narrative, places this event in the year 4004 B.C. It is not surprising that the teachings of geology, which reveal the gradual development of the earth extending throughout many millions of years, met with long and bitter opposition. The science of geology had made but little progress until the latter part of the eighteenth century, and during the first half of the nineteenth century its devotees were denounced for the heretical character of their teachings. Although the nature and significance of fossils had been correctly interpreted by many writers, it was mainly during the first part of the nineteenth century that it came to be generally recognized that previous geological periods were characterized by faunas and floras very unlike those existing today, and that there is a succession of forms of life of gradually advancing organization as we pass from the older to the more recent geological strata. Naturally, so long as it was held that the earth was a creation of only a few thousand years ago, no theory of evolution could gain much headway. Then the subject of comparative morphology, which furnishes so many convincing evidences of descent,

was little studied before the nineteenth century. It is true that much had been learned about the structure of both animals and plants, but it was only when comparative studies were made that a need for a rational interpretation of homologies, unity of type, and the existence of rudimentary organs came to be felt. The science of embryology also made relatively little advance before the nineteenth century, and the cell theory, which figures so prominently in the interpretation of embryonic development, was placed on a firm foundation only in 1838-39.

A sound basis, therefore, was not laid for the theory of evolution until a relatively late period in the history of science. The writings of the earlier evolutionists made little impression upon the thought of their time. Nevertheless in the period immediately preceding the announcement of Darwin's theory, scientific men were growing more and more dissatisfied with the doctrine of special creation, and many were casting about for an interpretation of the origin of living forms which was more satisfying to the scientific understanding. The Origin of Species came at an opportune time, and, despite the uproar which it created, its author lived to see the general doctrine of evolution accepted by almost all noteworthy men in the field of biology.

B. GENERAL EVIDENCES FOR EVOLUTION

Evidences for evolution are derived from several sources. There is, of course, a general presumption in favor of the theory of evolution arising from the fact that our experience with the universe teaches us that things happen in accordance with natural law. We usually believe without question that inorganic objects have come to be what they are through a series of natural events. If we find a rock near the margin of a stream we should not think of appealing to a miracle in order to account for its presence. Were I to come home and find a strange cat in the house I should not think of concluding (nor would the most ardent creationist conclude) that the cat had been created during my absence. So far as our experience goes, cats arise by a process of generation from other cats. Even had I felt perfectly

sure that no cats were in the house when I left, and that they could not have entered from the outside, I should hesitate long— and I think most other people would also-before concluding that the cat in question was the product of a miraculous creation.

Now if I shift my inquiry from the origin of this particular cat to the origin of the kind of cats to which she belongs, what conclusion should be drawn? Direct experience here comes to our aid since it discloses many cases of organisms coming from other organisms of a slightly different kind. Cats differing from other members of their species in possessing six instead of five digits, or in showing distinctive peculiarities of color, have been known to arise occasionally as variations, or mutants. Consequently, in my inquiry, I should naturally look for related varieties or species in the hope of finding one from which our variety might probably have been derived. I might not be able to prove that any one of the numerous species of cats supplied the ancestor sought for, but my search, if sufficiently extended, would reveal many facts of great significance in regard to the probable derivation of cats in general. It would show that, although domesticated cats differ considerably in different countries, they often strikingly resemble the indigenous wild species. And my search would also reveal numerous gradations in the degree of divergence between differences which are barely discernible and those which characterize very distinct types.

One who believes that species were miraculously created would find it very puzzling to decide as to what forms arose from others by the natural process of generation, and what forms were the product of a special creation. He would have to admit that the naturally generated and the miraculously created forms strikingly resemble each other in having much the same catty aspect and much the same catty ways. And he would find himself in a very difficult position if he attempted to show that while natural causes could account for the origin of the cats of the group A, they could not account for the origin of the slightly more divergent cats of the group B.

The thesis defended by Mr. Darwin in his epoch-making