Another consideration of prime importance is too frequently ignored. On Darwinian principles, each step in any development can be made, not because it leads to an advantageous result in the future, but only because it is itself advantageous. At each stage favoured individuals survive others because they are favoured here and now, not because, when the development they promote shall be completed, their remote descendants will be favoured. Hence it must, for instance, be possible to suppose, that all the intermediate forms between two extremes, whereof one is supposed to have originated the other, were, each in its day, so beneficial as to preserve their possessors at the expense of non-possessors. But can this possibly be even imagined? To take one example. We have heard, speaking of embryology, that the feet of lizards and the wings and feet of birds arise from the same fundamental form of limb, whence it is argued that birds and lizards are alike descended from a common sauroid, or lizard-like, ancestor, whose limbs in the case of the former class have developed into wings and into feet of a totally new type,--while scales were developing into feathers, and innumerable alterations of internal structure were simultaneously in progress. But if so, to confine our attention to one particular, it must be true that each of the innumerable minute gradations between the fore-limb of a lizard and the wing of a bird, was in its turn the best kind of member for a creature to possess, giving him a distinct advantage in the struggle for existence. Nothing, however, appears plainer than that this could not possibly have been the case. The limb shaping towards a wing would be a very clumsy and inefficient leg long before it got to the point at which it became of the slightest use for purposes of flight, that is to say before its alteration was accompanied by any utility whatever. We can neither imagine that creatures furnished with limbs of such intermediate forms could have been otherwise than hopelessly handicapped by them, nor do we find anywhere in the rocks any trace whatever of the innumerable series of modifications which would be needed to link by imperceptible gradations legs and wings together.