Vestiges of the natural history of creation / ... greatly amended by the author ; an introduction by Rev. George B. Cheever. [Anon].

  • Robert Chambers
Date:
1845
Catalogue details

Licence: Public Domain Mark

Credit: Vestiges of the natural history of creation / ... greatly amended by the author ; an introduction by Rev. George B. Cheever. [Anon]. Source: Wellcome Collection.

    87/344 (page 51)
    The placoids are now slenderly represented by the shark, cestraceon, &c., of modern seas; the ganoids are all but unrepresented in our time. Of both classes, one invaria- ble peculiarity has attracted much attention. “ In all recent fish, with the exception of the shark family, the sturgeon and the long pike, the vertebral column termi- nates at the point where the caudal fin is given off, and this fin is expanded above and below the body, forming what is called a homocercal tail. In all those, without exception, which have been found in strata of the Palaeo- zoic period [placoids and ganoids], the caudal fin is hete- rocercal, being formed of two unequal branches, the upper one expanded immediately from the vertebral column, while the lower one is given off at a point some distance from the extremity.”* Now it is a remarkable fact, that this one-sided tail is a peculiarity in the more perfect fishes (as the salmon) at a certain stage in their embryonic his- tory ; as is also the inferior position of the mouth peculiar to the early fishes. More than this—in the earlier periods of embryonic life, there is no vertebral column. This organ is represented in embryos by a gelatinous cord, called the dorsal cord, which in maturity disappears as the vertebrae are formed upon it. M. Agassiz has satisfied himself that this was the nature of the organization of the early fishes, as it is that of the sturgeon of the present seas. It is not premature to remark how broadly these facts seem to hint at a parity of law affecting the progress book. For more popular descriptions, reference may be made to “New Walks in an Old Field, by Hugh Miller,” Edin., 1842, and to Jameson’s Journal, July and October, 1844. *Ansted’s Geology, i., 185.