It was Herbert Spencer, the English philosopher, who first brought the argument of co-adaptation into the field against my view of the non-inheritance of functionally acquired modifications. He pointed out that many, if not, indeed, most modifications of bodily parts, to be effective, implied further changes, often very numerous, in other parts, and these latter must therefore have changed _simultaneously_ with the part which was being changed under the control of natural selection; this, however, is only conceivable as due to an inheritance of the changes caused by use, since a simultaneous alteration of so many parts through natural selection would be impossible. If, for instance, the antlers of our modern stag were to grow to the size of those of the Giant Stag of the Irish peat-bogs, which measured over ten feet across from tip to tip, this would mean--as has already been shown--a simultaneous thickening of the skull, and to bear the heavy burden, a strengthening of the _ligamentum nuchæ_, of the muscles of the neck and back, of the bones of the legs and their muscles, and, finally, of all the nerves supplying the muscles; and how could all this happen simultaneously with, and in exact proportion to the growth of the antlers, if it depended--as natural selection assumes--on chance variations of all these parts? What if the appropriately favourable variation in one of these organs did not occur? A harmonious variation of all the parts--bones, muscles, nerves, ligaments--which unite in a common activity, is an inadmissible assumption, because, in many cases, such co-operating groups of organs have in the course of evolution developed in opposite directions. In the giraffe, for instance, the fore-legs are longer than the hind-legs, which is the reverse of what obtains in the majority of ruminants; in the kangaroo the hind-legs, on the contrary, have developed to a disproportionate size, while the fore-legs have degenerated into relatively small grasping arms. Co-operating parts, like the fore and hind limbs, may thus follow opposite paths of evolution; their variations need not always be directed to the same end.