views, but that they should follow even in his further conquests the great leader who by a single blow won the victory for their side. But the end which he points out to us is high above the doctrine of descent, which doctrine, in so far as it attempts to explain the evolution of organic nature solely by its laws of development, is in fact still of little avail to us.
We must remark, in the first place, that what morphologists denominate laws are not laws at all in the sense of theoretical science. These so-called laws are simply rules deduced from a greater or less number of cases, and like grammatical rules only serve to classify and explain, by a process of vicious circle-reasoning, other facts embraced within the same definition. Even Kepler's laws were but rules of this kind, until Newton deduced them from the universal law of gravitation, and so raised them to the dignity of laws. But now that they are firmly based on the principle of gravitation, the whole doctrine of the movements of the heavenly bodies may be inferred from Kepler's laws with the highest attainable degree of certitude; and our longing to know the causes of things is as fully appeased by this explanation as the nature of the human understanding will permit. We know, with that kind of certitude which we denominate absolute, that, like the planets of our own solar system, so those of unseen suns move in ellipses whose radii vectores describe equal areas in equal times, and that the squares of their revolution times are as the cubes of their distances from their suns.
It is very different with the laws of organic structure. If in a Jurassic rock we find a fragment of a rhombic enameled scale, we infer with a very high degree of probability that the fish of whose panoply this scale was a part thousands of years ago had an independently pulsating aorta-peduncle. If on breaking up a shapeless piece of fossil bone we discover a spiral auditory cochlea, we know that the animal of whose skull the fragment was a part was a mammal. It is no small triumph that we dare make such assertions as these. Still, there is not absolute certitude here. Even the most firmly established laws of organic structure possess only a greater or less probability. Absolute characters are in systematics the philosopher's stone. True, in some cases the probability grounded on laws of organic development borders on certitude. That we shall never find a centaur, pegasus, griffin, a configuration like that of an angel or a demon, whether living or fossil, we may affirm with almost the same certainty as that a planet which has never been observed will obey the laws of Kepler. Whether we can with equal certitude affirm that never will a vertebrate be found in which by a transposition of the central nervous system the posterior and the anterior roots of the spinal-cord nerves will have interchanged functions, may perhaps be open to doubt, however improbable such a thing may be. Would the comparative anatomist ever have supposed a priori that such a structure could exist as that of the Pleuronectæ Then, in the Invertebrata the uncertainty of the laws of organic structure is
