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Show 188 THE DESCENT OF MAN. PART I. mals ns are these two latter groups conjoined. This view has not been accepted! as far as I am a ware, Ly .any naturalist capable of forming an independent judgment, and therefore need not here be further cou-: Sidered. We can understand why a classification founded on any single character or organ-even an organ so won- derfully complex and important as the brain-or on the high development of the ment}1l faculties: is almost sure -to prove unsatisfactory. This principle has indeed been tried with hymenopterous insects; but when thus classed by their habits or instincts, the arrangement proved .thoroughly artificial.3 Classifications may, of course, be based on any character whatever, as on size, colour, or t.he element inhabited; but naturalists have long felt a profound conviction that there is a natural system. This 'flystem, it is now generally admitted, mu;t be, as far as possible, genealogical in arrangement,-that is, the ~a-descendants of the same form must be kept together m one group, separate from the co-descendants of any ·other ~orm; but if the parent-forms are related, so will be their descendants, and the two groups together will form a larger group. The amount of difference between the several groups-that is the amount of modification which each has undergone-will be expressed by such terms a.s genera, families, orders, and classes. As we have no record of the lines of descent, these lines can be discovered only by observing the degrees of resemblance between the beings which are to be classed. For this object numerous points of resemblance are of m~c~ ~m:e i~po~·tance than the amount of similarity ·Or dissimilanty m a few points. If two languages were found to resemble each other in a multitude of 3 Westwood, '1\Iodern Class. of Insects,' vol. ii. 1840, p. 87. Cll.AP. VI. AFFINITIES AND GENEALOGY. 189· words and points of construction, they would be universally recognised as having sprung from a common source, notwithstanding that they differed greatly in some few words or points of construction. But with organic beings the points of resemblance must not consist of adaptations to similar habits of life: two animals may, for instance, have had their whole frames modified for living in the water, and yet they will not be Lrought any nearer to each other in the natural system. Hence we can see bow it is that resemblances in unimportant structures, in useless and rudimentary organs, and in parts not as yet fully developed or functionally active, flre by far the most serviceable for classification· foithey can hardly be due to adaptations within a' late period; and thus they reveal the old lines of descent or of true affinity. We can further see why a great amount of modification iu some one character ought not to lead us to· separate widely any two organisms. A part which already differs much from the same part in other allied forms bas already, according to the theory of evolution,. varied much ; consequently it would (as long as theorganism remained exposed to the same excitin()' conditions) be liable to further variations of the same 0 kind · and these, if beneficial, would be preserved, and thu~ continually augmented. In many cases the continued development of a part, for instance, of the beak of a bird, or of the teeth of a mammal, would not be ad vantageous to the species for gaining its food, or for any other object; but with man we can see no definite limit as far as ad vantage is concerned, to the continued de~ velopment of the brain and mental faculties. Therefore in determining the position of man in the natural o:rgen. ealogical system, the extreme development of his bram ought not to outweigh a multitude of resem- |