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Show 418 CLASSIFICATION. CHAP. XIII. t t ble themselves about the physiological value no rou . . d fi . of the characters which they use m . e n1ng a group, or in allocating any particular species. If they find a character nearly uniform, and common to a great number of forms, and not common to others, they use it as one of high value ; if common to some lesser number, they use it as of subordinate value. T:us principle has been broadly confessed by some naturalists to be the true one ; and by none more clearly than by that excellent botanist, Aug. St. Hilaire. If certain characters are always found correlated with others, though no apparent bond of connexion can be discovered between them, especial value is set on them. As in roost groups of animals, important organs, such as those for propelling the blood, or for aerating it, or those for propagating the race, are fo':nd neai~ly unifo~·m, ~hey are considered as highly serviceable In classrficatwn; but in some groups of animals all these, the most important vital organs, are found to offer characters of quite subordinate value. We can see why characters derived from the embryo should be of equal importance with those derived from the adult, for our classifications of course include all ages of each species. But it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for thif:! purpose than that of the adult, which alone plays its full part in the economy of nature. Yet it has been strongly urged by those great naturalists, Milne Ed wards and Agassiz, tha~ embryonic characters are the most important of any m the classification of animals; and this doctrine has very generally been admitted as true. rrhe same fa~t bo.l~s good with flowering plants, of which the two ~am diVIsions have been founded on characters denved · from the embryo,-on the number and position of the em- CnAP.XIII. CLASSIFICATION. 419 bryonic leaves or cotyledons, and on the mode of deve-lopment of the plumule and radicle • In our di SCUSS·l On on embryology, w~ shall see why such characters are so valuable, on the VIew of classification tacitly includi the idea of descent. ng ~ur classific~~ions are ?ften plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to all birds · but in the case of crustaceans, such definition has hitherto been found impossible. There are crustaceans at the opposit~ ends of the series, which have hardly a character In common; yet the species at both ends from being plainly allied to others, and these to other~ and so on:wards, can be recognised as unequivocally belo~ging to this, and to no other class of the Articulata. Geographica! distr~bution. has often been used, though perh~ps not quite logically, In classification, more especially In very large groups of closely allied forms. Temminc~ r in~ists on . the utility or even necessity of this practice In certain groups of birds ; and it has been followed by several entomologists and botanists. ~inally, with respect to the comparative value of the varwus groups of species, such as orders sub-orders families, sub-families, and genera, they se~m to be, a~ least at present, almost arbitrary. Several of the best botanists, such as Mr. Bentham and others have strongly insisted on their arbitrary value. In~tances could be given amongst plants and insects, of a group of forms, first ranked by practised naturalists as only a gen~s, and the~ raised to the rank of a sub-family or family; and this has been done, not because further research has detected important structural differences, at. first ?ve1loo~ed, but because numerous allied species, With shghtly different grades of difference, have been subsequently discovered. |