Indeed, in extreme cases, where experiments on the brain have been tried on lower animals, it is found that it can be entirely removed without destroying life, or affecting many of the actions which require perception and volition. Thus, when the brain has been entirely removed from a pigeon, it smoothes its feathers with its bill when they have been ruffled, and places its head under its wing when it sleeps; and a frog under the same conditions, if held by one foot endeavours to draw it away, and if unsuccessful, places the other foot against an obstacle in order to get more purchase in the effort to liberate itself. So much for the organ of mind; the other factor, that of outward stimulus, is still more obvious. If thought cannot exist without grey nerve-tissue, neither can it without impressions to stimulate that tissue. A perfect brain, if cut off from all communication with the external universe, could no more think and have perceptions, than impressions from without could generate them without the appropriate nerve-tissue. Once generated, the mind can store them up by memory, control them by reason, and gradually evolve from them ever higher and higher ideas and trains of reasoning, both in the individual and the species:—in the individual passing from infancy to manhood, partly by heredity from ancestors, and partly by education—using the word in the large sense of influences of all sorts from the surrounding environment; in the species, by a similar but much slower development from savagery to civilisation. Thus the whole fabric of arithmetic, algebra, and the higher calculi are built up from the primitive perception of number. The earliest palæolithic savage must have been conscious of a difference between encountering one or two cave-bears or mammoths; and some existing races of savages have hardly got beyond this primitive perception. Some Australian tribes, it is said, have not got beyond three numerals, one, two, and a great number. But by degrees the perceptions of number have become more extensive and accurate, and the number of fingers on each hand has been used as a standard of comparison. Thus ten, or two-hand, the number of fingers on the two hands has gradually become the basis of arithmetical numeration, and from this up to Sir W. Hamilton’s ‘Quaternions’ the progression is regular and intelligible. But Newton could never have invented the differential calculus and solved the problem of the heavens, if thousands of centuries before some primitive human mind had not received the perception that two apples or two bears were different from one.