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Show 506 MR. ST. G. MIVART ON THE LEMURS. [May 20, Manifestly, then, very great caution is necessary in discriminating between genetic characters and characters purely or mainly adaptive. Experience has more and more persuaded me that the number of similar structures which have arisen independently is prodigious. The elaborate investigations of my friend Mr. Parker constantly bring before us an increasing number of complex cross relations and more and more entangled interdependencies; and I am convinced that by means of such careful and minute researches many of the genealogical trees which have been developed with the rapidity of the fabled " bean-stalk " are destined to enjoy an existence little less ephemeral. The notion that "similarity of structure" necessarily implies "genetic affinity" can no longer be ranked as a biological axiom. If, then, it is so difficult to decide as to which characters are genetic and which adaptive, the second question can be answered at once. Evidently anatomical science does not now enable us to group even the Mammalia by genetic characters; yet surely the main features of Mammalian classification may be considered to be satisfactorily established. The third question concerns the exclusion from any order of all species which cannot be supposed to have sprung from an ancestor common to them and to all the other species. To confine our attention to the Mammalia, can it be considered certain that the Baleenoidea and the Delphinoidea sprang from an ancestor which at the same time was the ancestor of no species belonging to any other special order of existing Mammals? And if we could demonstrate that such had not been the case, would that be a reason for breaking up the very natural and, on the whole, homogeneous order Cetacea ? Again, can we feel any certainty that Orycteropus has descended from the same stock as that whence the American Edentates descended ? yet who would place it in a separate order ? Once more, it may well be that the Artiodactyla and Perisso-dactyla are entirely independent genetically beyond the fact that they are both Mammals ; yet no one can deny that the Ungulata form a very natural group. As to the fourth question-whether, namely, no common descendants should be classed in two different orders,-it seems reasonable that convenience should determine our practice. If the number of species of any one group is overwhelming, and if the complicated subdivisions of its families, subfamilies, and genera are very great, surely, then, convenience should determine us to subdivide them into two or even more orders. Similarly as convenience may induce us to separate into distinct ordinal groups, so convenience may reasonably induce us to unite in one group forms which, whether descended from a common ancestor or not, undeniably constitute a well-defined and convenient aggregation. As has been said, it may be that the characters which unite the Artiodactyla with the Perissodactyla are merely adaptive functional |