我们现在要考虑的部分是有用的地方运动（运动）动物，第一，每一部分为什么是这样，因为它是和他们有什么目的他们;第二，无论是在一年的不同部分之间的同样的生物，以及由不同种类生物的部分再次比较彼此。那么就让我们首先放下多少问题，我们要考虑的。首先是运动是什么必要的动物进展最低分，第二个为什么血红的动物有四点意见，而不是多，但不流血的动物超过4，一般为什么一些动物是无足，其他两足动物，其他四足动物，其他polypods ，为什么都有一个偶数的脚，如果他们在所有的脚，为什么在精细点上的进展取决于更是增多。其次，为何人鸟两足动物，但鱼无足;，为什么人与鸟，尽管都两足动物，有一个相反的腿弯曲。对于他的双腿弯曲男子convexly，鸟有他的弯曲concavely;再次，男子弯曲他的胳膊和腿相反的方向，因为他有他的武器弯曲convexly，但他的腿concavely。一胎生四足动物和相反的方向弯曲，四肢男人的，而在相反的方向彼此，因为他有他的前肢弯曲convexly，他的后腿concavely。同样，四足动物不属于胎生卵生，但有一个独特的弯曲的四肢横向远离身体。同样，为什么四足移动他们的腿纵横交错？我们要研究这些事实的原因，及其他性质相同的人;说，事实是从我们的自然史清楚，我们现在要求的事实原因。 2
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    WE have now to consider the parts which are useful to animals for movement in place (locomotion); first, why each part is such as it is and to what end they possess them; and second, the differences between these parts both in one and the same creature, and again by comparison of the parts of creatures of different species with one another. First then let us lay down how many questions we have to consider. The first is what are the fewest points of motion necessary to animal progression, the second why sanguineous animals have four points and not more, but bloodless animals more than four, and generally why some animals are footless, others bipeds, others quadrupeds, others polypods, and why all have an even number of feet, if they have feet at all; why in fine the points on which progression depends are even in number. Next, why are man and bird bipeds, but fish footless; and why do man and bird, though both bipeds, have an opposite curvature of the legs. For man bends his legs convexly, a bird has his bent concavely; again, man bends his arms and legs in opposite directions, for he has his arms bent convexly, but his legs concavely. And a viviparous quadruped bends his limbs in opposite directions to a man's, and in opposite directions to one another; for he has his forelegs bent convexly, his hind legs concavely. Again, quadrupeds which are not viviparous but oviparous have a peculiar curvature of the limbs laterally away from the body. Again, why do quadrupeds move their legs criss-cross? We have to examine the reasons for all these facts, and others cognate to them; that the facts are such is clear from our Natural History, we have now to ask reasons for the facts. 2