Hence both polyp and medusa present characters for classification, and a given species, genus or other taxonomic category may be defined by polyp-characters or medusa-characters or by both combined.
The former is concerned with the division of the earths surface into major districts characterized by particular plants or taxonomic groups of plants, with the subdivision of these floristic districts, and with the geographical distribution (both past and present) of the various taxonomic units, such as species, genera, and families.
Between them arises a median crest, which varies much in extent and composition, and is of considerable taxonomic value.
The humerus with its crests, ridges and processes, presents so many modifications characteristic of the various groups of birds, that its configuration alone is not only of considerable taxonomic value but that almost any genus, excepting, of course, those of Passeres, can be " spotted " by a close examination and comparison of this bone.
The extent of the origin of this muscle from the sternum, on which it leaves converging, parallel or diverging impressions, is of some taxonomic value.
14), mainly from a taxonomic FIG.
The results are as interesting from a morphological point of view (showing the subtle and gradual modifications of these organs in their various adaptations), as they are sparse in taxonomic value, far less satisfactory than are those of the hind-limb.
It is not the quantity but the quality of the anatomical and bionomic characters which determines their taxonomic value, and a few fundamental characters are better indications of the affinities of given groups of birds than a great number of agreements if these can be shown to be cases of isomorphism or heterophyletic, convergent analogy.
Such excessive multiplication of the larger taxonomic divisions shows an imperfect sense of proportion, for if the term " class " be allowed its usual zoological value, no student can fail to recognize that the Hexapoda form a single welldefined class, from which few entomologists would wish to exclude even the Apterygogenea.
He was quite aware of the taxonomic value of the vocal organs of some groups of birds, presently to be especially mentioned, and he had himself ascertained the presence and absence of caeca in a not inconsiderable number of groups, drawing thence very justifiable inferences.
Indeed he was so much prepossessed in favour of a classification based on the structure of the digestive organs that he could not bring himself to consider vocal muscles to be of much taxonomic use, and it was reserved to Johannes Muller to point out that the contrary was the fact.
At any rate he wavered in his estimate of their taxonomic value, for he gave an alternative proposal, arranging all the genera in a single series, a proceeding in those days thought not only defensible and possible, but desirable or even requisite, though now utterly abandoned.
The taxonomic position of the Palamedeidae, for all will allow to the screamers the rank of a family at least, has been much debated.
These distinctions can be recognized on petrified specimens, as well as on the casts, but their taxonomic value is somewhat doubtful.
This distinction, however, has proved to have no constant taxonomic value, for both arrangements may occur on different parts of the same specimen.
The characters shown by such specimens, however, when, as is usually the case, they are in the barren state, are notoriously unstable, or of small taxonomic value, among recent plants.
Of the Ophioglossaceae there are no satisfactory examples; one of the few fossils compared with a recent species, Ophioglossum palmatum, was described several years ago from Triassic rocks under the name Cheiropteris, but the resemblance is one of external form only, and practically valueless as a taxonomic criterion.
Huxley, " On the Classification of Birds and on the Taxonomic Value of the Modifications of certain of the Cranial Bones..."
The taxonomic importance of the configurations of the palate was first pointed out by J.