Forms of animal life : being outlines of zoological classification based upon anatomical investigation and illustrated by descriptions of specimens and of figures / by George Rolleston.

  • Rolleston, George, 1829-1881.
Date:
1870
Catalogue details

Licence: Public Domain Mark

Credit: Forms of animal life : being outlines of zoological classification based upon anatomical investigation and illustrated by descriptions of specimens and of figures / by George Rolleston. Source: Wellcome Collection.

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    then found to be correlated with the complete or nearly complete disappearance of such structures, must be regarded as entailing a true morphological degradation. Sharply circumscribed outlines are, in the second place, as com¬ monly wanting in the classifications we have to deal with as are precisely graduated scales of dignity. The boundaries of species, of orders, of classes, and, in more than one instance, even of sub-king¬ doms, may be closely apposed not only at many single points but even along considerable lengths and depths, so that in not a few cases it is a matter of difficulty to decide whether a particular organism or set of organisms shall be placed within the one or the other of the thus complexly approximated groups. The distances, thirdly, which intervene between the various sub¬ kingdoms at their points of widest separation from each other are exceedingly unequal. And in particular, it may be said that, in spite of recent discoveries1*, it would still appear that the Vertebrate Sub-kingdom lies at a greater distance from the group made up by all the other Sub-kingdoms than that by which any one of these is separated from its nearest neighbour. Groups, fourthly, which would be allowed on all hands to possess the same morphological or qualitative rank are found to differ very widely as to the numbers of the objects they severally include; and if, by the aid of diagrams, we represent to ourselves the quan¬ titative relations which the corresponding divisions in almost any two of the animal sub-kingdoms hold to each other as wholes of ‘ extension’ or of ‘ denotation,’ we are at once struck by the great inequality of size indicated by the figures thus constituted. A similar result would ensue upon the application of a similar process to many non-biological classifications; the especial significance which these differences possess in organic classifications depends upon two singular but suggestive facts, actual observation having shown, firstly, that the poorer a species is numerically the more aberrant is it ordinarily found to be from the type of the group to which it is subordinated ; and secondly, that with a paucity of individuals or of species, as the case may be, a similar paucity of the localities on the earth’s surface in which they are now to be *> For a summary of these discoveries, see the Quarterly Journal of Microscopical Science, January, 1870.