Volume Ii Part 14 (1/2)
Upon the whole, then, and with regard to the direct action of external conditions, I conclude--not only from general considerations, but also from special facts or instances quite sufficient for the purpose--that these must certainly give rise to immense numbers of somatogenetic species on the one hand, and probably to considerable numbers of blastogenetic species on the other; that in neither case is there any reason for supposing the distinctively ”specific characters” to be other than ”neutral” or ”indifferent”; while there are the best of reasons for concluding the contrary. So that, under this division of our subject alone (B), there appears to be ample justification for the statement that ”a large proportional number of specific characters” are in reality, as they are in appearance, dest.i.tute of significance from a utilitarian point of view.
(C.)
Thus far in the present chapter we have been dealing exclusively with the case of ”climatic variation,” or change of specific type due to changes in the external conditions of life. But it will be remembered that, in the preceding chapter, allusion was likewise made to changes of specific type due to internal causes, or to what Darwin has called ”the nature of the organism.” Under this division of our subject I mentioned especially s.e.xual Selection, which is supposed to arise in the aesthetic taste of animals themselves; Isolation, which is supposed to originate new types by allowing the average characters of an isolated section of an old type to develop a new history of varietal change, as we shall see more fully in the ensuing part of this treatise; and the Laws of Growth, which is a general term for the operation of unknown causes of change incidental to the living processes of organisms which present the change.
Now, under none of these divisions of our subject can there be any question touching the criterion of Heredity. For if new species--or even single specific characters of new species--are ever produced by any of these causes, they must certainly all ”reproduce their like.” Therefore the only question which can here obtain is as to whether or not such causes ever do originate new species, or even so much as new specific characters. Mr. Wallace, though not always consistently, answers this question in the negative; but the great majority of naturalists follow Darwin by answering it in the affirmative. And this is enough to show the only point which we need at present concern ourselves with showing--viz. that the question is, at the least, an open one. For as long as this question is an open one among believers in the theory of natural selection, it must clearly be an unwarrantable deduction from that theory, that all species, and _a fortiori_ all specific characters, are necessarily due to natural selection. The deduction cannot be legitimately drawn until the possibility of any other cause of specific modification has been excluded. But the bare fact of the question as just stated being still and at the least an open question, is enough to prove that this possibility has not been excluded. Therefore the deduction must be, again on this ground alone (C), unwarrantable.
Such are my several reasons--and it is to be observed that they are all _independent_ reasons--for concluding that it makes no practical difference to the present discussion whether or not we entertain Heredity as a criterion of specific distinction. Seeing that our species-makers have paid so little regard to this criterion, it is neither absurd nor preposterous to have adduced, in the preceding chapter, the facts of climatic variation. On the contrary, as the definition of ”species” which has been practically followed by our species-makers in No. 3, and not No. 4, these facts form part and parcel of our subject. It is perfectly certain that, in the vegetable kingdom at all events, ”a large proportional number” of specifically diagnostic characters would be proved by experiment to be ”somatogenetic”; while there are numerous constant characters cla.s.sed as varietal, although it is well known that they are ”blastogenetic.” Moreover, we can scarcely doubt that many specific characters which are also hereditary characters owe their existence, not to natural selection, but to the direct action of external causes on the hereditary structure of ”germ-plasm”; while, even apart from this consideration, there are at least three distinct and highly general principles of specific change, which are accepted by the great majority of Darwinists, and the only common peculiarity of which is that they produce hereditary changes of specific types without any reference to the principle of utility.
CHAPTER X.
CHARACTERS AS ADAPTIVE AND SPECIFIC (_concluded_).
Our subject is not yet exhausted. For it remains to observe the consequences which arise from the dogma of utility as the only _raison d'etre_ of species, or of specific characters, when this dogma is applied in practice by its own promoters.
Any definition of ”species”--excepting Nos. 1, 2, and 5, which may here be disregarded--must needs contain some such phrase as the one with which Nos. 3 and 4 conclude. This is, that peculiar characters, in order to be recognized as of specific value, must present neither more nor less than ”some certain degree of distinctness.” If they present more than this degree of distinctness, the form, or forms, in question must be ranked as generic; while if they present less than this degree of distinctness, they must be regarded as varietal--and this even if they are known to be mutually sterile. What, then, is this certain degree of distinctness? What are its upper and lower limits? This question is one that cannot be answered. From the very nature of the case it is impossible to find a uniform standard of distinction whereby to draw our boundary lines between varieties and species on the one hand, or between species and genera on the other. One or two quotations will be sufficient to satisfy the general reader upon this point.
Mr. Wallace himself alludes to ”the great difficulty that is felt by botanists in determining the limits of species in many large genera,”
and gives as examples well-known instances where systematic botanists of the highest eminence differ hopelessly in their respective estimates of ”specific characters.” Thus:--
”Mr. Baker includes under a single species, Rosa canina, no less than twenty-eight named varieties distinguished by more or less constant characters, and often confined to special localities, and to these are referred about seventy of the species of British and continental botanists. Of the genus Rubus or bramble, five British species are given in Bentham's _Handbook of British Flora_, while in the fifth edition of Babington's _Manual of British Botany_, published about the same time, no less than forty-five species are described. Of willows (Salix) the same two works enumerate fifteen and thirty-one species respectively. The hawkweeds (Hieracium) are equally puzzling, for while Mr. Bentham admits only seven British species, Professor Babington describes no less than seventy-two, besides several named varieties[125].”
[125] _Darwinism_, p. 77.
Mr. Wallace goes on to quote further instances, such as that of Draba verna, which Jordan has found to present, in the south of France alone, no less than fifty-two permanent varieties, which all ”come true from seed, and thus present all the characteristics of a true species”; so that, ”as the plant is very common almost all over Europe, and ranges from North America to the Himalayas, the number of similar forms over this wide area would probably have to be reckoned by hundreds, if not by thousands[126].”
[126] _Darwinism_, p. 77.
One or two further quotations may be given to the same general effect, selected from the writings of specialists in their several departments.
”There is nothing that divides systematists more than what const.i.tutes a genus. Species that resemble each other more than other species, is perhaps the best definition that can be given.
This is obviously an uncertain test, much depending on individual judgement and experience; but that, in the evolution of forms, such difficulties should arise in the limitation of genera and species was inevitable. What is a generic character in one may be only a specific character in another. As an ill.u.s.tration of the uncertain importance of characters, I may mention the weevil genus _Centrinus_ in which the leading characters in the cla.s.sification of the family to which it belongs are so mixed that systematists have been content to keep the species together in a group that cannot be defined.... No advantage or disadvantage is attached, apparently, to any of the characters. There are about 200 species, all American.
The venation of the wings of insects is another example of modifications without serving any special purpose. There is no vein in certain Thripidae, and only a rudiment or a single vein in Chalcididae. There are thousands of variations more or less marked, some of the same type with comparatively trivial variation, others presenting distinct types, even in the same family, such genera, for example, as _Polyneura_, _Tettigetra_, _Huechys_, &c. in the Cicadidae.
Individual differences have often been regarded as distinctive of species; varieties also are very deceptive, and races come very near to species. A South-American beetle, _Arescus histrio_, has varieties of yellow, red, and black, or these colours variously intermixed, and, what is very unusual, longitudinal stripes in some and transverse bars in others, and all taken in the same locality.
Mr. A. G. Butler, of the British Museum, is of opinion that 'what is generally understood by the term species (that is to say, a well-defined, distinct, and constant type, having no near allies) is non-existent in the Lepidoptera, and that the nearest approach to it in this order is a constant, though but slightly differing, rare or local form--that genera, in fact, consist wholly of a gradational series of such forms (Ann. Mag. Nat. Hist. 5, xix.
103)[127].'”
[127] Pascoe, _The Darwinian Theory of the Origin of Species_, 1891, pp. 31-33, and 46.
So much as regards entomology, and still living forms. In ill.u.s.tration of the same principles in connexion with palaeontological series, I may quote Wurtenberger, who says:--
”With respect to these fossil forms [i.e. mult.i.tudinous forms of fossil Ammonites], it is quite immaterial whether a very short or a somewhat longer part of any branch be dignified with a separate name, and regarded as a species. The p.r.i.c.kly Ammonites, cla.s.sed under the designation of Armata, are so intimately connected that it becomes impossible to separate the accepted species sharply from one another. The same remark applies to the group of which the manifold forms are distinguished by their ribbed sh.e.l.ls, and are called Planulata[128].”
[128] _Neuer Beitrag zum geologischen Beweis der Darwinischen Theorie_, 1873.