Man is a vertebrate animal. This means that he has a locomotive, not protective, skeleton, composed of cartilage--a tough, elastic, organic material, hardened, as a rule, by the deposition of mineral salts, mainly phosphate of lime, in exceedingly fine particles, so as to form a homogeneous, flawless, elastic, tough, light, and unyielding skeleton, held together by firm ligaments. The skeleton is internal, and this fact, as we have seen, gives the possibility of large size. And size is in itself no unimportant factor. Professor Lotze maintains that without man's size and strength, agriculture and the working of metals, and thus all civilization, would have been impossible. But we have already seen that there is an extreme of size, _e.g._, in the elephant, which makes its possessor clumsy, able to exist only where there are large amounts of food in limited areas, slow to reproduce, and lacking in adaptability. This extreme also is avoided in man; in this, as in many other particulars, he holds the golden mean. But we have also seen that large size is, as a rule, correlated with long life and great opportunity for experience and observation. And these are the foundations of intelligence. Hence the deliverance of the higher vertebrate, and especially of man, from any iron-bound subjection to instinct. And here another question of vital importance meets us. Is man's life at present as long as it should or can be? The question is exceedingly difficult, but a negative answer seems more probable. We cannot but hope that, with a better knowledge of our physical structure, a clearer vision of the dangers to which we are exposed, more study of the laws of physiology, heredity, and of our environment, and above all, less reckless disregard of these in a mad pursuit of pleasure, wealth, and position, man's period of mature, healthy, and best activity may be lengthened, perhaps, even a score of years. The mitigation of hurry and worry alone, the two great curses of our American civilization, might postpone the collapse of our nervous systems longer than we even dream. And if we could add even five years to the working life of our statesmen, scholars, and discoverers, the work of these last five years, with the advantage of all previously acquired knowledge and experience, might be of more value than that of their whole previous life. Human advance could not but be greatly, or even vastly, accelerated.