Such changes do not take place in a few generations.  For their completion hundreds, perhaps thousands, of years must elapse.  The descendants of the blacks who were carried from Africa to America as slaves two centuries and a half ago, save where their color has been modified by a mixed parentage, are still black.  Already the influence of new climatic surroundings and of association has wrought great changes upon them:  they are no longer savages.  But their complexion is as dark as that of their kidnapped forefathers.  Their original physical condition remains almost unaltered, and with it many mental characteristics:  their love of display and of bright colors, their fondness for tune and the power of music to move them, their weird and fantastic belief in ghosts and spirits, in signs, omens and charms, and many other traits, still bear witness to their savage origin.  But even these are fading away, and these men are slowly but not the less surely becoming civilized and white.

The point of departure for every structural change in a living organism lies in the apparatus by which nutrition is maintained; and this in the higher classes is the blood.  Most complex and wonderful of fluids, it contains in unexplained and inscrutable combination salts of iron, lime, soda and potassa, with water, oil, albumen, paraglobulin and fibrinogen, which united form fibrine—­in fact, at times, some part of everything we eat and all that goes to form our bodies, which it everywhere permeates, vitalizes and sustains.  Borne in countless numbers in its ever-ebbing and returning streams are little disks, flattened, bi-concave, not larger in man than one-three-thousandth of an inch in diameter, called red corpuscles, whose part it is to carry from the lungs to the tissues pure oxygen, without which the fire called life cannot be sustained, and back from the tissues to the lungs carbonic acid, one of the products of that fire; and larger, yet marvelously small, bodies called leucocytes or white corpuscles, whose precise origin and use to this day, in spite of all the labor that has been spent upon their study, remain unknown.  But that which makes the blood wonderful above all other fluids is its vitality.  Our common expression, “life’s blood,” is no idle phrase.  The blood is indeed the very throne of life.  If its springs are pure and bountiful, if its currents flow strong and free, muscle, bone and brain grow in symmetry and power, and there is cunning to devise and the strong right arm to execute.  But if it be thin and poor, and its circulation feeble and uncertain, the will flags, the mind is weak and vacillating, the muscles grow puny, and the man becomes an unresisting prey to disease and circumstance.  If it escape through a wound, strength ebbs with it, until at length life itself flows out with the unchecked crimson stream.  Thus, then, by acting upon the blood, climate has wrought and is working such changes upon man.  But why are constantly-acting causes so slow in producing their effects?  How is it that countless generations must pass away before purely climatic causes, potent as they are, begin to manifest themselves in physical changes in the races of men exposed to them?