same plan with that of other mammals, independently of the uses to which the parts may be put; the occasional reappearance of various structures, for instance, of several muscles which man does not normally possess, but which are common to the Quadrumana, and a crowd of analogous facts,--all point in the plainest manner to the conclusion that man is the co-descendant with other mammals of a common progenitor. Darwin recognized that the high standard of our intellectual powers and moral disposition constitutes the greatest difficulty which presents itself after we have been driven by the mass of biological evidence to accept his conclusion as to the origin of man. Touching this point, he observes: "Every one who admits the principle of evolution must see that the mental powers of the higher animals, which are the same in kind with those of man, though so different in degree, are capable of advancement. Thus the interval between the mental powers of one of the higher apes and of a fish, or between those of an ant and scale-insect, is immense; yet their development does not offer any special difficulty, for with our domesticated animals the mental faculties are certainly variable, and the variations are inherited. No one doubts that their mental faculties are of the utmost importance to animals in a state of nature. Therefore the conditions are favorable for their development through natural selection. The same conclusion may be extended to man; the intellect must have been all-important to him, even at a very remote period, as enabling him to invent and use language, to make weapons, tools, traps, etc., whereby, with the aid of his social habits, he long ago became the most dominant of all living creatures." It is further pointed out that a great stride in the development of man's intellect must have followed as soon as the half-art and half-instinct of language came into use; for the continued use of language must have reacted on the brain, and produced an inherited effect, and this again will have reacted on the improvement of language. The largeness of the brain in man relatively to his body, compared with the size of that organ in the lower animals, is attributable in chief part to the early use of some simple form of language, that engine which affixes signs to all sorts of objects and qualities, and excites trains of thought which would never arise from the mere impression of the senses, or, if they did arise, could not be followed out. The higher intellectual powers of man, such as those of ratiocination, abstraction, self-consciousness, etc., probably follow from the continued improvement and exercise of the other mental faculties.