In the later chapters of the foregoing treatise I have sought to indicate certain matters of general principle, which many years of study specially devoted to this great movement of contemporary thought have led me to regard as almost certainly sound in themselves, and no less certainly requisite as complements of the Darwinian theory. I will now conclude by briefly summarizing these matters of general principle in the form of twelve sequent propositions. And, in doing so, I may ask it to be noticed that the system which these propositions serve to express may now claim, at the least, to be a strictly logical system. For the fact that, not merely in its main outlines, but likewise in its details, it has been independently constructed by Mr. Gulick, proves at any rate this much; seeing that, where matters of such intricacy are concerned, nothing but accurate reasoning from a common foundation of _data_ could possibly have yielded so exact an agreement. The only difference between us is, that Mr. Gulick has gone into much further detail than I have ever attempted in the way of classifying the many and varied forms of isolation; while I have laid more special stress upon the physiological form, and found in it what appears to me a satisfactory solution of "the greatest of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species"--namely, "the remarkable difference between varieties and species when crossed." GENERAL CONCLUSIONS. 1. NATURAL SELECTION IS PRIMARILY A THEORY OF THE CUMULATIVE DEVELOPMENT OF ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY, OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES IN CASES WHERE ALLIED SPECIES DIFFER FROM ONE ANOTHER IN RESPECT OF PECULIAR CHARACTERS, WHICH ARE ALSO ADAPTIVE CHARACTERS. 2. HENCE, IT DOES NOT FOLLOW FROM THE THEORY OF NATURAL SELECTION THAT ALL SPECIES--MUCH LESS ALL SPECIFIC CHARACTERS--MUST NECESSARILY HAVE OWED THEIR ORIGIN TO NATURAL SELECTION; SINCE IT CANNOT BE PROVED DEDUCTIVELY FROM THE THEORY THAT NO "MEANS OF MODIFICATION" OTHER THAN NATURAL SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF MODIFICATION AS GO TO CONSTITUTE DIAGNOSTIC DISTINCTIONS BETWEEN CLOSELY ALLIED SPECIES; WHILE, ON THE OTHER HAND, THERE IS AN OVERWHELMING MASS OF EVIDENCE TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL NUMBER OF SPECIFIC CHARACTERS" BY CAUSES OF MODIFICATION OTHER THAN NATURAL SELECTION. 3. THEREFORE, AND UPON THE WHOLE, AS DARWIN SO EMPHATICALLY HELD, "NATURAL SELECTION HAS BEEN THE MAIN, BUT NOT THE EXCLUSIVE MEANS OF MODIFICATION."