OCR Text |
Show 364 CLASSIFICATION. [CHAP. XIII. nearly uniform, and common to a great nu1nl>er of forms, and not cmnmon to others, they use it as one of high value; if common to some lesser number, they use it as of subordinate value. This 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 most groups of animals, important organs, such as those for prop~lling the blood, or for aerating it, or those for propagating the race, are found nearly uniform, they are considered as highly serviceable in classification; but in some groups of animals all these, the most ilnportant vital organs, are found to offer characters of quite . subordinate value. We can see why characters derived from the ·embryo should be of equal hnportance 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 this purpose than that of the adult whic_h alone plays its full part in the economy of nature: Yet It has been strongly urged by those great naturalists, Milne Edwards and Agassiz, that embryonic characters are the most important of any in the classification of animals; and this doctrine has very generally been admitted as true. The same fact holds good with flowering plants, of which the two main divisions have been founded on characters derived from the embryo,-on the number and position of the mnbryonic leaves or cotyledons, and on the mode of development of the plumule and radicle. In our discussion on embryology, we shall see why such characters are so valuable, on the view of classification tacitly including the idea of descent. Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters com·mon to all birds ; but in the case of crustaceans, such definition has hitherto been found impossible. There are crustaceans at the opposite CBAP. XIILJ CLASSIFIOA '!'ION. ends of the series vvhich h h 36 . t h ' . ave ardly a h t . 5 mon , ye t e specws at b th c arac er m com-allied to o.thers, and these t~ oth:~~s, from being plainly be recogmsed as unequivocall ' a~d so onwards, can no other class. of th_e Articulat~. belonging to this, and to Geographical distribution h perhaps not quite logically . as ffte!l been used, though cially in very larO'e O'rou s 'r 1~ c assrfic~tion, more espeminck insists on tl1e ~tilipty 0 c osely allied forms. Tem-t . . . or even neces 't f h' we In certain groups of birds . and . SI Y o t IS prac-by several entomologists and b' t . It has been followed F . ll . o anists Ina y, With respect to the . . various groups of species su 1 comparative value of the ilies, sub-fa1nilies and ge~erac ~hs orders, sub-orders, faro-present almost arbitrary. Sev'eraf~f~~em to be, a~ least at as ¥r. Bentham, and others hav . e bes~ b?tanists, such arbitrary value. Instances' co 1~ 8~101~gly Insisted on their and insects, of a group of for: fi e given amongst plants naturaHsts as only a genus an~' thrst ra?ked by practised a sub-family or family . a~d th. len rmsed to the rank of cause further research 'has d t 1St ~as. been done, not bedifferences, at :first overlooke~ ~ ~ blmportant structural lied species with sHght d·ffi u ecause numerous alhave been s~bsequently ~isc~v~~dt grades of difference, All the foregoing rules and ~id d . classification are explained 'f I d s an drfficulties in myself, on the view that th 1 o not greatly deceive on descent with modi:ficati e. ~ht~ra~ systmn is founded naturalists consider as sho~~O' t a t ffi c_haracters which two or more species are those::, h.Uh f nity bet'Y"een ~ny from a common pa;ent d . w IC . lave been InherJted tion is genealoO'ical . tha~ ' In so f~r, all true classi:ficahidden bond w~ich ~at a l?~my:-unity of descent is the seeking, and not smne ura Is s ave been unconsciously enunciation of eneral unkno:v?- plan of creation, or the together and segparat' probi?OSitlons, and the mere putting But I t In_g 0 . ~ects more or less alike ~eve mus explain · · that the arran emen~yf ~eainng mo~e _fully. I be-m due subordinatign and o. i :. groups Within each class, must be strictly genealogicaiei: ~~de!0t;1be 0!~~~rN~ul:t |