In the species Man the exemplification of this great law is, as I have intimated, essentially psychical, and its application is upon masses, upon ethnic groups. History, the story of man’s progress, deals only with these, not with individuals. Progressive ethnic mental variation is therefore the theme for our immediate consideration, and especially as it is displayed in the processes of natural selection and adaptation. This is the physiology of ethnic psychology, the history of its normal progress to more specialised powers and higher types. I cannot go amiss if I present it with a rather close adherence to the recognised method of natural science; for the impression is constantly gaining ground that the psychical life of Man follows the same laws as does his physical; or, to express the thought more accurately, that the one is the reflex of the other, for we can read both with equal correctness in terms of thought or terms of extension. Such changes may take place in several directions: as in abolishing organs no longer useful; in reducing others which are diminishing in value; in strengthening those which are of immediate utility; and, by correlation, maintaining those relations of parts on which the “type” depends. These changes are not “purposive”; they do not aim toward a future type, though they may result in one. Such a type may be more decadent than its antecedent, and be the prelude to extinction, under this adamantine law of destruction; but if its variations have been physiological and adaptive, they will confer upon it the blessing of life, the gift of length of days. Those changes which strengthen an organ or structure, or tend to develop and preserve new and useful variations are called “progressive”; those which tend to draw individual variation back to the current type or to reduce certain structures or functions are called “regressive” variations. It would seem at first sight that such processes must tend in opposite directions—the one beneficial, the other injurious. In fact, both are preservative; but by contrasted physiological processes. Progressive changes begin in the individual and pass by inheritance into the stock, when they have proved beneficial to it. They continue in action so long as they are useful. When their utility ceases, the energy of the economy is expended elsewhere, on other structures or faculties. The degeneration thus produced is “compensatory.” It does not detract from but adds to the general viability of the organism. What is most marvellous in this process is that the part or power rarely wholly disappears, no matter how long it has been useless. The pineal gland in the human brain is the remains of a third eye with which our ancestors looked out from the top of their heads when they were Silurian fishes; and the appendix vermiformis was an annex to their stomachs when they were herbaceous ruminants!