A current of evolutionary thought can be traced through the literature dealing with organic nature from ancient times. It began as a small rill among the Greek philosophers and dwindles to a mere thread in the Middle Ages, sometimes almost disappearing, but is never completely broken off. Near the close of the eighteenth century it suddenly expands, and becomes a broad and prevailing influence in the nineteenth century. Osborn, in his book, _From the Greeks to Darwin_, traces the continuity of evolutionary thought from the time of the Greek philosophers to Darwin. The ancient phase, although interesting, was vague and general, and may be dismissed without much consideration. After the Renaissance naturalists were occupied with other aspects of nature-study. They were at first attempting to get a knowledge of animals and plants as a whole, and later of their structure, their developments, and their physiology, before questions of their origin were brought under consideration. Opinion before Lamarck.--The period just prior to Lamarck is of particular interest. Since Lamarck was the first to give a comprehensive and consistent theory of evolution, it will be interesting to determine what was the state of opinion just prior to the appearance of his writings. Studies of nature were in such shape at that time that the question of the origin of species arose, and thereafter it would not recede. This was owing mainly to the fact that Ray and Linnæus by defining a species had fixed the attention of naturalists upon the distinguishing features of the particular kinds of animals and plants. Are species realities in nature? The consideration of this apparently simple question soon led to divergent views, and then to warm controversies that extended over several decades of time. The view first adopted without much thought and as a matter of course was that species are fixed and constant; _i.e._, that the existing forms of animals and plants are the descendants of entirely similar parents that were originally created in pairs. This idea of the fixity of species was elevated to the position of a dogma in science as well as in theology. The opposing view, that species are changeable, arose in the minds of a few independent observers and thinkers, and, as has already been pointed out, the discussion of this question resulted ultimately in a complete change of view regarding nature and man's relation to it. When the conception of evolution came upon the scene, it was violently combated. It came into conflict with the theory designated special creation.