the neck, which never vary as to number and which, in the mole, were contracted so as to be no longer recognisable, are now prolonged to such a degree, that here, as everywhere else, the neck acquires the same length as the fore-legs, in order to enable the head to reach down to drinking-water. But where, as is the case when it appears as the elephant, a long neck could not have borne the weight of the enormous, unwieldy head--a weight increased moreover by tusks a yard long--the neck remains short, as an exception, and a trunk is let down as an expedient, to lift up food and draw water from below and also to reach up to the tops of trees. In accordance with these transformations, we see in all of them the skull, the receptacle containing the understanding, at the same time proportionately expand, develop, curve itself, as the mode of procuring nourishment becomes more or less difficult and requires more or less intelligence; and the different degrees of the understanding manifest themselves clearly to the practised eye in the curves of the skull. [240] "Principes de Philosophie Zoologique," 1830. Now, in all this, that _anatomical element_ we have mentioned above as fixed and invariable, certainly remains in so far an enigma, as it does not come within the teleological explanation, which only begins after the assumption of that element; since the intended organ might in many cases have been rendered equally suitable for its purpose even with a different number and disposition of bones. It is easy to understand, for instance, why the human skull should be formed out of eight bones: that is, to enable them to be drawn together by the fontanels during birth; but we do not see why a chicken which breaks through its egg-shell should necessarily have the same number of skull-bones. We must therefore assume this anatomical element to be based, partly on the unity and identity of the will to live in general, partly on the circumstance, that the archetypal forms of animals have proceeded one from the other,[241] wherefore the fundamental type of the whole race was preserved. It is this anatomical element which Aristotle means by his ἀναγκαία φύσις, and the mutability of its shapes according to different purposes he calls τὴν κατὰ λόγον φύσιν,[242] and explains by it how the material for upper incisors has been employed for horns in horned cattle. Quite rightly: since the only ruminants which have no horns, the camel and the musk-ox, have upper incisors, and these are wanting in all horned ruminants. [241] "Parerga," vol. ii. § 91; § 93 of the 2nd edition. [242] See Aristotle, "De Partibus Animalium," iii. c. 2 _sub finem_: πῶς δὲ τῆς αναγκαίας φύσεως κ. τ. λ.