have indeed been discovered, and with great surprise; but in these cases that exceptio firmat regulam applies, since they can be accounted for upon other grounds. Such, for example, is the fact that the tadpoles of the pipa toad have tails and gills, although, unlike all other tadpoles, they do not swim, but await their metamorphosis on the back of the mother; that the male kangaroo has the marsupial bones which in the female carry the pouch; that male mammals have breasts; that the Mus typhlus, a rat, has eyes, although very small ones, without any opening for them in the outer skin, which thus covers them, clothed with hair; and that the moles of the Apennines, and also two fishes —Murena cœcilia and Gastrobrauchus cœcus– are in the same case; of like kind is the Proteus anguinus. These rare and surprising exceptions to the rule of nature, which is otherwise so rigid, these contradictions with itself into which it falls, we must explain from the inner connection which the different kinds of phenomena have with each other, by virtue of the unity of that which manifests itself in them, and in consequence of which nature must hint at some thing in one, simply because another of the same type actually has it. Accordingly the male animal has a rudimentary form of an organ which is actually present in the female. As now here the difference of the sex cannot abolish the type of the species, so also the type of a whole order – for example, of the batrachia – asserts itself even where in one particular species (pipa) one of its determinations is superfluous. Still less can nature allow a determination (eyes) which belongs to the type of a whole division (Vertebrata) to vanish entirely without a trace, even if it is wanting in some particular species (Mus typhlus) as superfluous; but here also it must at least indicate in a rudimentary manner what it carries out in all the others.
Even from this point of view it is to some extent possible to see upon what depends that homology in the skeleton primarily of mammals, and in a wider sense of all vertebrates, which has been so fully explained, especially by Richard Owen in his “Ostéologie comparée,” and on account of which, for example, all mammals have seven cervical vertebræ, every bone of the human hand and arm finds its analogue in the fin of the whale, the skull of the bird in the egg has exactly as many bones as that of the human fœtus, &c. All this points to a principle which is independent of teleology, but which is yet the foundation upon which teleology builds, or the already given material for its works, and just that which Geoffroy St. Hilaire has explained as the “anatomical element.” It is the unité de plan, the fundamental type of the higher animal world, as it were the arbitrarily chosen key upon which nature here plays.
Aristotle has already correctly defined the difference between the efficient cause (causa efficiens) and the final cause (causa finalis) in these words: “Δυο τροποι της αιτιας, το οὑ ἑνεκα και το εξ αναγκης, και δει λεγοντας τυγχανειν μαλιστα μεν αμφοιν.” (Duo sunt causæ modi: alter cujus gratia, et alter e necessitate; ac potissimum utrumque eruere oportet.) De part. anim., i. 1. The efficient cause is that whereby something is, the final cause that on account of which it is; the phenomenon to be explained has, in time, the former behind it, and the latter before it. Only in the case of the voluntary actions of animal beings do the two directly unite, for here the final cause, the end, appears as the motive; a motive, however, is always the true and proper cause of the action, is wholly and solely its efficient cause, the change preceding it which calls it forth, by virtue of which it necessarily appears, and without which it could not happen; as I have shown in my prize essay upon freedom. For whatever of a physiological nature one might wish to insert between the act of will and the corporeal movement, the will always remains here confessedly that which moves, and what moves it is the motive coming from without, thus the causa finalis; which consequently appears here as causa efficiens. Besides, we know from what has gone before that the bodily movement is one with the act of will, for it is merely its phenomenal appearance in cerebral perception. This union of the causa finalis with the efficient cause in the one phenomenon intimately known to us, which accordingly remains throughout our typical phenomenon, is certainly to be firmly retained; for it leads precisely to the conclusion that at least in organised nature, the knowledge of which has throughout final causes for its clue, a will is the forming power. In fact, we cannot otherwise distinctly think a final cause except as an end in view, i. e., a motive. Indeed, if we carefully consider the final causes in nature in order to express their transcendent nature, we must not shrink from a contradiction, and boldly say: the final cause is a motive which acts upon a being, by which it is not known. For certainly the termite nests are the motive which has produced the toothless muzzle of the ant-bear, and also its long extensile, glutinous tongue: the hard egg-shell which holds the chicken imprisoned is certainly the motive for the horny point with which its beak is provided in order to break through that shell, after which it throws it off as of no further use. And in the same way the laws of the reflection and refraction of light are the motive for the wonderfully ingenious and complex optical instrument, the human eye, which has the transparency of its cornea, the different density of its three humours, the form of its lens, the blackness of its choroid, the sensitiveness of its retina, the contracting power of its pupil, and its muscular system, accurately calculated according to those laws. But those motives acted before they were apprehended; it is not otherwise, however contradictory it may sound. For here is the transition of the physical into the metaphysical. But the latter we have already recognised in the will; therefore we must see that the will which extends an elephant's trunk towards an object is the same will which has also called it forth and formed it, anticipating objects.
It is in conformity with this that in the investigation of organised nature we are entirely referred to final causes, everywhere seek for these and explain everything from them. The efficient causes, on the contrary, here assume only a quite subordinate position as the mere tools of the final causes, and, just as in the case of the voluntary movement of the limbs, which is confessedly effected by external motives, they are rather assumed than pointed out. In explaining the physiological functions we certainly look about for the efficient causes, though for the most part in vain; but in explaining the origin of the parts we again look for them no more, but are satisfied with the final causes alone. At the most we have here some such general principle as that the larger the part is to be the stronger must be the artery that conducts blood to it; but of the actually efficient causes which bring about, for example, the eye, the ear, the brain, we know absolutely nothing. Indeed, even in explaining the mere functions the final cause is far more important and more to the point than the efficient; therefore, if the former alone is known we are instructed and satisfied with regard to the principal matter, while, on the other hand, the efficient cause alone helps us little. For example, if we really knew the efficient cause of the circulation of the blood, as we do not, but still seek it, this would help us little unless we knew the final cause, that the blood must go into the lungs for the purpose of oxidation, and again flow back for the purpose of nourishing; but by the knowledge of this, even without the knowledge of the efficient cause, we have gained much light. Moreover, I am of opinion, as was said above, that the circulation of the blood has no properly efficient cause, but that the will is here as immediately active as in muscular movement where motives determine it by means of nerve conduction, so that here also the movement is called forth directly by the final cause; thus by the need of oxidation in the lungs, which here to a certain extent acts as a motive upon the blood, yet so that the mediation of knowledge is in this case wanting, because everything takes place in the interior of the organism. The so-called metamorphosis of plants, a thought lightly thrown out by Kaspar Wolf, which, under this hyperbolic title, Goethe pompously and with solemn delivery expounds as his own production, belongs to the class of explanations of organic nature from the efficient cause; although ultimately he only says that nature does not in the case of every production begin from the beginning and create out of nothing, but as it were, writing on in the same style, adds on to what already exists, makes use of the earlier forms, developed, and raised to higher power, to carry its work further: just as it has done in the ascending series of animals entirely in accordance with the law: Natura non facit saltus, et quod commodissimum in omnibus suis operationibus sequitur (Arist. de incessu animalium, c. 2 et 8). Indeed, to explain
