| OCR Text |
Show 64 RECAPITULATION. [Ch. IV. above them. Never docs a fish elevate itself so as to assume the form of the brain of a reptile; nor does the lat.ter ever attain that of birds; nor the bird that of the mamtmfer. It may happen that a monster may have two heads, but the conformation of the brain always remains circumscribed narrowly within the limits of its class*· It will be observed, that these curious phenomena disclose, in a highly interesting manner, the unity of plan that runs throuO'h the orO'anization of the whole series of vertebrated I:) 0 animals; but they lend no support whatever to t 11 e noti.O n o f a o-radual transmutation of one species into another, least of all of o . I the passage, in the course of many generations, from an amma of a more simple, to one of a more complex structure. On the contrary, were it not for the sterility imposed on monsters, as well as on hybrids in general, the argument to be derived from Tieddemann's discoveryJ like that deducible from experiments respecting hybridity, would be in favour of the successive degeneracy, rather than the perfectibility, in the course of ages, of certain classes of organic beings. For the reasons, therefore, detailed in this and the two preceding chapters, we draw the following inferences, in regard to the reality of species in nature. First, That there is a capacity in all species to accommodate themselves, to a certain extent, to a change of external circumstances, this extent varying greatly according to the species. 9ldly. When the cnange of situation which they can endure is great, it is usually attended by some modifi~ations of the form, colour, size, structure, or other particulars; but the mutations thus superinduced are governed by constant laws, and the capability of so varying forms part of the permanent specific character. Sdly. Some acquired peculiarities of form, structure, and instinct, are transmissible to the offspring ; but these consist * E. R. A. Serres, Auatomie Comparee du Cerveau, illustrateu by numerous plates, tom, i., 1824. Ch. IV.j RECAPITtfLATION. 65 of such qualities and attributes only as are intimately related to the natural wants and propensities of the species. 4thly. The entire variation from the original type, which any given kind of change can produce, may usually be effected in a brief period of time, after which no farther deviation can be obtained by continuing to alter the circumstances, though ~ver so gradually,-indefinite divergence, either in the way of Improvement or deterioration, being prevented, and the least po.ssible excess beyond the defined limits being fatal to the existence of the individual. 5thly. The intermixture of distinct species is guarded against by the aversion of the individuals composing them to sexual union, or by the sterility of the mule offspring. It does not appear that true hybrid races have ever been perpetuated for several generations, even by the assistance of man ; for the cases usually cited relate to the crossing of mules with individuals of pur~ species, and not to the intermixture of hybrid with hybrid. 6th1y. Ji.,rom the above considerations, it appears that species have a real existence in nature, and that each was endowed, at the. ti~e ~fits cr~at~on, with the attributes and organization by wluch It Is now d1stmguished. Vor., II. F |
