It will be observed that Darwin and his followers take divergent variations of offspring simply as a known fact, upon which natural selection operates to produce progressive modification; and, as the cause of variation in offspring is wholly unknown, such variations are often spoken of as fortuitous. But, of course, it is well understood that nothing in Nature is really fortuitous. They may, however, for all purposes of natural selection be thus regarded until we know their cause. It is evident, then, that if we, with Darwin, take natural selection, as the most important known factor, the really most important cause of evolution is the _cause_ of varieties. This is the _unknown_ fundamental factor. As Darwin reduced Agassiz’s three formal laws of succession to more general laws of life, and thus made one important step in the advance of biological science, so he who shall explain the _cause_ of divergent variation will make another important step by reducing the phenomena to still more general and fundamental laws of life. In conclusion, let me again impress upon the reader that all the doubt and discussion, above described, as to the factors of evolution, is entirely aside from the truth of evolution itself, concerning which there is no difference of opinion among thinkers. CHAPTER III. THE GRADES OF THE FACTORS OF EVOLUTION AND THE ORDER OF THEIR APPEARANCE. We have given in the previous chapter six factors of evolution--viz.: 1. _Pressure of the environment._ 2. _Use and disuse of parts._ 3. _Natural selection._ 4. _Sexual selection._ 5. _Physiological selection._ 6. _Reason._ Let us now compare these as to their grade in the scale of energy and as to the order of their introduction.