Volume 2

Principles of geology, being an attempt to explain the former changes of the earth's surface, by reference to causes now in operation / By Charles Lyell, esq.

  • Lyell, Charles, Sir, 1797-1875.
Date:
1830-1833
Catalogue details

Licence: Public Domain Mark

Credit: Principles of geology, being an attempt to explain the former changes of the earth's surface, by reference to causes now in operation / By Charles Lyell, esq. Source: Wellcome Collection.

    88/366 (page 66)
    68 they present you with the forms of those of birds ; finally, they acquire, at the era of birth, and sometimes later, the permanent forms which the adult mammalia present. The cerebral hemispheres, then, only arrive at the state which we observe in the higher animals by a series of successive metamorphoses. If we reduce the whole of these evolutions to four periods, we shall see that in the first are born the cere¬ bral lobes of fishes, and this takes place homogeneously in all classes. The second period will give us the organization of reptiles ; the third the brain of birds ; and the fourth the com¬ plex hemispheres of mammalia. If we could develop the different parts of the brain of the inferior classes, we should make in succession a reptile out of a fish, a bird out of a reptile, and a mammiferous quadruped out of a bird. If, on the contrary, we could starve this organ in the mammalia, we might reduce it successively to the condition of the brain of the three inferior classes. Nature often presents us with this last phenomenon in mon¬ sters, but never exhibits the first. Among the various defor¬ mities which organized beings may experience, they never pass the limits of their own classes to put on the forms of the class above them. Never does a fish elevate itself so as to assume the form of the brain of a reptile ; nor does the latter ever attain that of birds ; nor the bird that of the matnmifer. It may happen that a monster may have two heads, but the con¬ formation of the brain always remains circumscribed narrowly within the limits of its class *. Bearing of these discoveries on the theory of progressive development.—It will be observed, that these curious pheno¬ mena disclose, in a highly interesting manner, the unity of plan that runs through the organization of the whole series of ver- tebrated animals ; but they lend no support whatever to the notion of a gradual transmutation of one species into another, least of all of the passage, in the course of many generations, * E. R. A. Serres, Anatomie Compare du Cerveau, illustrated by numerous plates, tome i., 1824.