It comes out pretty clearly from our short study of the infective diseases that some of the microbes that cause these, such as the bacillus of tubercle, only feed on unhealthy human tissue, while the greater number of them kill, if anything, the weak rather than the strong. They are, therefore, on the whole, and as a natural consequence, our race friends rather than our foes, and if we attempt seriously to do away with their selective influence--_viz._, the elimination of the weak and the preservation of the strong--we must supply this selective influence by one equally potent, or the race will tend to deteriorate. What can be done in the future, and what it is expedient for us to do at the present time, will be more fully discussed in a subsequent chapter; but I may state at once that _pari passu_ with our endeavour to prevent these diseases must be our efforts to enlist the co-operation of the human charity that would avert suffering in such selection as shall necessitate the birth of future generations from the healthiest and best of those amongst us. As selection is the race-changer, we must replace the selection of the microbe by the selection of human forethought. A number of diseases, which are due probably to innate family predispositions, are known to us. Of these diabetes, hæmophilia, and some others are of comparative rarity, and may be left on one side in this necessarily contracted sketch. Others, such as cancer and constitutional weakness of the respiratory and other organs, as well as insanity, are frequent enough to merit our close attention. Of cancer we at present know too little, and I propose to leave it on one side. Of inherited weakness of special systems we have many examples, such as a delicate respiratory or digestive mucous membrane, inherited variations in the mechanism of assimilation, and also gout and obesity--in fact, innate delicacy of all kinds, which renders their possessor less able to cope with his natural surroundings, let these be what they may. There is hardly a family that can boast of the complete want of hereditary weakness, and among the children of particular families, where these weaknesses exist, some show the taint more than others. In times of hardship, cold, exposure, coarse food, etc., these weaker ones perish, and the race is consequently propagated from the stronger ones. Within certain limits cold, exposure and coarse food are compatible with great physical excellence, for the cold and exposure, hurtful to the sickly, braces and hardens the more robust, and coarse but nutritive food supplies him with energy and strengthens the powers of digestion. The finest races have been bred by hardship. It is proverbial to speak of “the hardy mountaineer,” and one cannot look at a lowland Scot without feeling that his stock had, in days gone by and for many centuries, run the gauntlet of oatmeal porridge and cold east wind.