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the great majority of cases, what we term “species” are so well marked and definite that there is no difference of opinion about them ; but as the test of a true theory is, that it accounts for, or at the very
least is not inconsistent with, the whole of the phenomena and apparent anomalies of the problem to be solved, it is reasonable to ask that those who deny the origin of species by variation and selection should grapple with the facts in detail, and show how the doctrine of the distinct origin and permanence of species will explain and harmonize them. It has been recently asserted by Dr. J. E. Gray (in the Proceedings of the Zoological Society for 1863, page 134), that the difficulty of limiting species is in proportion to our ignorance, and that just as groups or countries are more accurately known and studied in greater detail the limits of species become settled. This statement has, like many other general assertions, its portion of both truth and error. There is no doubt that many uncertain species, founded on few or isolated specimens, have had their true nature determined by the study of a good series of examples: they have been thereby established as species or as varieties : and the number of times this has occurred is doubtless very great. But there are other, and equally trustworthy cases, in which, not single species, but whole groups have, by the study of a vast accumulation of materials, been proved to have no definite specific limits. A few of these must be adduced. In Dr. Carpenter’s “Introduction to the Study of the Foraminifera,” he states that “there is not a single specimen of plant or animal of which the range of variation has been studied by the collocation and comparison of so large a number of specimens as have passed under the review of Messrs. Williamson, Parker, Rupert Jones, and myself, in our studies of the types of this group ; ” and the result of this extended comparison of specimens is stated to be, “ The range of variation is so great among the Foraminifera as to include not merely those differential characters which have been usually accounted SPECIFIC, but also those upon which the greater part of the GENERA of this group have been founded, and even in some instances those of its oftDERS ’’ (Foraminifera, Preface, x). Yet this same group had been divided by D’Orbigny and other authors into a number of clearly defined families, genera, and species, which these careful and conscientious researches have shown to have been almost all founded on incomplete knowledge.
Professor DeCandolle has recently given the results of an extensive review of the species of Cupuliferae. He finds that the best-known species of oaks are those which produce most varieties and subvarieties; that they are often surrounded by provisional species; and, with the fullest materials at his command, twothirds of the species he considers more or less doubtful. His general conclusion is, that “in botany the lowest series of groups, SUBVARIETIES, VARIETIES, and RACEs are very badly limited; these can be grouped into SPECIEs a little less vaguely limited, which again can be formed into sufficiently precise GENERA.” This
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general conclusion is entirely objected to by the writer of the article in the “Natural History Review,” who, however, does not deny its applicability to the particular order under discussion, while this very difference of opinion is another proof that difficulties in the determination of species do not, any more than in the higher groups, vanish with increasing materials and more accurate research. Another striking example of the same kind is seen in the genera Rubus and Rosa, adduced by Mr. Darwin himself; for though the amplest materials exist for a knowledge of these groups, and the most careful research has been bestowed upon them, yet the various species have not thereby been accurately limited and defined so as to satisfy the majority of botanists. In Mr. Baker's revision of the British Roses, just published by the Linnaean Society, the author includes under the single species Rosa canina, no less than twenty-eight named varieties, distin guished by more or less constant characters and often confined to special localities; and to these are referred about seventy of the species of Continental and British botanists. Dr. Hooker seems to have found the same thing in his study of the Arctic flora. For though he has had much of the accumulated materials of his predecessors to work upon, he continually expresses himself as unable to do more than group the numerous and apparently fluctuating forms into more or less imperfectly defined species. In his paper on the “Distribution of Arctic Plants,” (Trans. Linn. Soc. xxiii., p. 310) Dr. Hooker says:—“The most able and experienced descriptive botanists vary in their estimate of the value of the “specific term' to a much greater extent than is generally supposed.” . . “I think I may safely affirm that the ‘specific term ' has three different standard values, all current in descriptive botany, but each more or less confined to one class of observers.” . . “This is no question of what is right or wrong as to the real value of the specific term ; I believe each is right according to the standard he assumes as the specific.” Lastly, I will adduce Mr. Bates's researches on the Amazons. During eleven years he accumulated vast materials, and carefully studied the variation and distribution of insects. Yet he has shown that many species of Lepidoptera, which before offered no special difficulties, are in reality most intricately combined in a tangled web of affinities, leading by such gradual steps from the slightest and least stable variations to fixed races and well-marked species, that it is very often impossible to draw those sharp dividing-lines which it is supposed that a careful study and full materials will always enable us to do. These few examples show, I think, that in every department of nature there occur instances of the instability of specific form, which the increase of materials aggravates rather than diminishes. And it must be remembered that the naturalist is rarely likely to err on the side of imputing greater indefiniteness to species than really exists. There is a completeness and satisfaction to the mind in defining and limiting and naming a species, which leads us all to do so. whenever we conscientiously can, and which we know has led many collectors to reject vague intermediate forms as destroying the symmetry of their cabinets. We must therefore consider these cases of excessive variation and instability as being thoroughly well established ; and to the objection that, after all, these cases are but few compared with those in which species can be limited and defined, and are therefore merely exceptions to a general rule, I reply that a true law embraces all apparent exceptions, and that to the great laws of nature there are no real exceptions—that what appear to be such are equally results of law, and are often (perhaps indeed always) those very results which are most important as revealing the true nature and action of the law. It is for such reasons that naturalists now look upon the study of varieties as more important than that of well-fixed species. It is in the former that we see nature still at work, in the very act of producing those wonderful modifications of form, that endless variety of colour, and that complicated harmony of relations, which