In what way, it will be asked, can individual capacity for health be gauged? to what degree can the power to progress or to resist encroachments be strengthened? at what age is intelligent supervision most important? No concise and conclusive answers can be given to these most natural inquiries, but much light has been recently thrown upon the long duration of immaturity and associated instability in mankind; upon the power of self-protection inherent in the body;[118] upon the influence thereon of its environment; upon the penetrating power of heredity; and upon the urgent importance of the adolescent period. Further, it appears that the healthful body is equipped to withstand the attack of the bacteria of most diseases, though the mechanism of self-defence is of more kinds than one. Of the different pathological bacteria identified up to the present, for instance, some appear to be eminently sensible to one kind of action of normal blood fluids, while they are in a much less measure sensible, or are, perhaps, entirely insensible to others; a complication which enhances our respectful admiration for the marvellous and intricate system which provides for our bodily welfare. Obviously, human nature would be practically immune from disease if this protective machinery were always in good working order: unfortunately this is not invariably the case--hence disease. It is the duty of hygiene to insure constant physical equilibrium, but the intricate tactics of Nature are as yet so imperfectly understood that man is not yet an ally of great worth in her operations.