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174 an animal locomotive power? Now, it may be supposed that the generative and the nutritive functions, which are innate in all living beings, originate the motion concerned in the processes of growth and decay, which equally belong to them all; and with respect to breathing and expiration, sleep and watching, which are subjects of much difficulty, we shall enter upon the consideration of them hereafter. Let us, however, consider what confers upon an animal the power of progression.

Now, it clearly is not the nutritive faculty—for the movement of progression is ever for some end, and is associated either with imagination or appetite; and then no being moves unless urged to it by desire or fear, excepting, indeed, there be impulse from without; plants, besides, were nutrition the cause, should be locomotive, and possess some organ to fit them for that kind of movement.

Neither can it be the sentient faculty—for there are many creatures which are sentient, and yet stationary throughout their existence; and if nature do nothing in vain, and never, except in the case of beings dwarfed or deformed, omits anything necessary to existence, the creatures alluded to are perfect creatures; and as proof of this they are reproductive, are capable of development and subject to decay, so that they also ought to have organs to fit them for progression.