now it's jolly difficult to follow an argument of that time when it's read — i appreciate that — but what i think darwin is actually saying is this, that it's true that vertebrates, for example— all vertebrates, from fish to ourselves — have a common pattern of a rigid rod down the middle of the back, segmented muscles either side of it, a mouth at the front, a hollow nerve chord on top, two pairs of fins or legs derived from them, but not three pairs or one pair, but two pairs and so on.