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Show 414 CLASSIFICATION. CIIAI'. XIII. cation, which is partially revealed to us by our classifi-cations. 1 d . 1 . Let us now consider the rules fol owe In c assi-fi t ca w. n, and the difficulties which .a re encountered on the view that classification either gives some unk.no~n plan of creation, or is simply a ~cheme for enuncmtmg general propositions and of plaCing together the forms most like each other. It might have been thought (and was in ancient times thought) that those parts of the structure which determined the habits of life, ana the general place of each bein? in the eco~omy o~ nat~re, would be of very high Importance In classrficatwn. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any importance. These resemblances, though so intimately connected with the whole life of the being, are ranked as merely "adaptive or analogical characters ; " but to the consideration of these resemblances we shall have to recur. It may even be given as a general rule, th~t the l~ss an~ part of the organisation is concerned wit~ spe?Ial habits, ~he more important it becomes for classrficat1on. As an Instan~e: Owen, in speaking of the dugong, says, "The generative organs being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities. We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character." So with plants, how remarkable it is that the organs of vegetation, on which their whole life dep~md~, .a:e 0 : little signification, excepting in the first main divisJOns ' whereas the organs of reproduction, with their product the seed, are of paramount importance! We must not therefore, in classifying, trust to resem· ' . · 1 · mportant blances in parts of the organisatiOn, 10wever 1 CHAP. XIII. CLASSIFICATION. 415 they may be for the welfare of the being in relation to th~ outer world. Perhaps from this cause it has partly ar1sen, that almo~t all naturalists lay the greatest stress ?n resemblances In organs of high vital or physioloO'ical Importance. No doub~ this v~ew of the classificator; importance of organs whiCh are Important is generally, but by n_o m~ans alwa.Y~, true. But their importance for classification, I believe, depends on their greater constancy throughout large groups of species ; and this consta~ cy depends on such organs having generally been subJec~ed to ~e~s change in the adaptation of the species ~their conditions of life. That the mere physiological Importance of an organ does not determine its classificatory value, is almost shown by the one fact that in allied groups, in which the same organ, as we h:ve every reason .to supp?se, has nearly the same physiological value, ~ts classificatory value is widely different. No naturahs~ can l:ave worked at any group without being struck with this fact; and it has been most fully ack~ owledged in the writings of almost every author. It will suffice to quote the highest authority, Robert Brown, who in speaking of certain organs in the Protea? ere, says their generic importance, "like that of all their parts, not only in this but, as I apprehend, in every natural family, is very unequal, and in some cases seems to be entirely lost." Again in another work he says, the genera of the Connaracero "differ in having one or more ovaria, in the existence or absence of albumen, in the imbricate or valvular rostivation. Any one o~ t~ese characters singly is frequently of more than generic Importance, though here even when all taken together they appear insufficient to separate Cneetis from Connarus." To give an example amongst insects, in one great division of the Hymenoptera, the antennre, as Westwood has remarked, are most constant in structure· ' |