Those who enter on the discussion of the subject in this spirit have to trace, so far as the conditions permit, a continuous process of mental evolution, and have also to distinguish and name the successive stadia through which this evolution passes. By some emphasis will be laid on the continuity of the several stages; by others on their differentiation. And the incidence of this emphasis will be reflected in the use of technical terms. The germinal and embryonic stages of abstraction and generalisation, for example, may be reasonably inferred from the behaviour of animals low down in the scale of mental progress. It is not unnatural therefore that, where continuity of process is in the focus of thought, the terms "generalisation" and "abstraction" should be employed with the widest possible range of significance so as to comprise both the embryonic and the fully developed phases along a specific line of psychogenesis. But on the other hand it is not unnatural that, when the differentiation of the stages is in the focus of thought, these terms should be severally restricted to the highest distinguishable phase of development that at which the process in question reaches maturity. If the progress of thought depends now upon the perception of similarity amid diversities of manifestation, and now upon the distinction of delicate shades of difference, it is inevitable that the preponderance of the one or the other tendency should leave its impress on the language in which that thought is expressed. And where a writer is addressing not only the inner circle of experts but a wider audience of cultured folk, he has to consider the commonly accepted implications of the words he uses and, bearing in mind the fact that even the cultured reader will be more under the sway of these common implications than of the author's most careful definitions, he has to select that usage which will offer the least resistance to the general acceptance of his meaning, and best subserve further progress.
Mr. L. T. Hobhouse in his valuable work on Mind in Evolution distinguishes five stages of correlation in the course of what he terms "Orthogenic Evolution". This he defines as evolution "upwards," assuming at the outset, and contending throughout the work, that it is identical with the evolution of mind or of the conditions which make mind possible. "Doliogenic evolution," as contrasted with orthogenic, is "the growth of any other qualities whatever that assist survival". It must be remembered that Eimer (Verh. der Deutch. Zool. Gesell., 1895) uses the terms "orthogenic" and "orthogenesis" for evolution through use-inheritance and the organic transmission of acquired characters, and therefore on the one hand with implications which Mr. Hobhouse's usage does not carry and on the other without the implications which his definition suggests. Dealing broadly with the adaptation of human and animal action to the requirements of life and growth, and using the term "adaptation" so as to include both racial adaptation, by means of natural selection or otherwise (to which it has been suggested that the term should be restricted),
