[11] On the same day on which the present address was delivered at the International Congress of Zooelogists in Leyden, and on the same occasion, Dr. W. B. Scott, Professor of Geology in Princeton College, New Jersey, read a very interesting paper on the tertiary mammalian fauna of North America, in which, without a knowledge of my paper, he took his stand precisely on this argument and arrived at the opinion that it could not possibly be the ordinary individual variations which accomplished phyletic evolution, but that it was necessary to assume in addition phyletic variations. I believe our views are not as widely remote as might be supposed. Of course, I see no reason for assuming two kinds of hereditary variations, different _in origin_. Still it is likely that only a relatively small portion of the numberless individual variations lie on the path of phyletic advancement and so under the _guidance_ of germinal selection mark out the way of further development; and hence it would be quite possible in this sense to distinguish continuous, _definitely directed_ individual variations from such as fluctuate hither and thither with no uniformity in the course of generations. The root of the two is of course the same, and they admit of being distinguished from each other only by their success, phyletic modification, or by their failure. [12] H. F. Osborn, "The Hereditary Mechanism and the Search for the Unknown Factors of Evolution," in _Biological Lectures delivered at the Marine Biolog. Lab. at Wood's Holl in the Summer Session of 1894_. Boston, 1895. [13] In 1886. See my paper on "Retrogression in Nature," published in English in Nos. 105, 107, 108, and 109 of _The Open Court_, and also in my essays on _Heredity_, Jena, 1892. [14] _Neue Gedanken zur Vererbungsfrage_, Jena, 1895.