Contributions to the Theory of Natural Selection Part 7

6. _Species._--Species are merely those strongly marked races or local forms which when in contact do not intermix, and when inhabiting distinct areas are generally believed to have had a separate origin, and to be incapable of producing a fertile hybrid offspring. But as the test of hybridity cannot be applied in one case in ten thousand, and even if it could be applied would prove nothing, since it is founded on an a.s.sumption of the very question to be decided--and as the test of separate origin is in every case inapplicable--and as, further, the test of non-intermixture is useless, except in those rare cases where the most closely allied species are found inhabiting the same area, it will be evident that we have no means whatever of distinguis.h.i.+ng so-called "true species" from the several modes of variation here pointed out, and into which they so often pa.s.s by an insensible gradation. It is quite true that, in 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 a.s.serted 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 a.s.sertions, 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 acc.u.mulation 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 pa.s.sed 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_ ORDERS" (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, two-thirds 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 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_, distinguished 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 acc.u.mulated 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 cla.s.s 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 a.s.sumes as the specific."

Lastly, I will adduce Mr. Bates's researches on the Amazons. During eleven years he acc.u.mulated 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 gratify every sense and give occupation to every faculty of the true lover of nature.

_Variation as specially influenced by Locality._

The phenomena of variation as influenced by locality have not hitherto received much attention. Botanists, it is true, are acquainted with the influences of climate, alt.i.tude, and other physical conditions, in modifying the forms and external characteristics of plants; but I am not aware that any peculiar influence has been traced to locality, independent of climate. Almost the only case I can find recorded is mentioned in that repertory of natural-history facts, "The Origin of Species," viz. that herbaceous groups have a tendency to become arboreal in islands. In the animal world, I cannot find that any facts have been pointed out as showing the special influence of locality in giving a peculiar _facies_ to the several disconnected species that inhabit it.

What I have to adduce on this matter will therefore, I hope, possess some interest and novelty.

On examining the closely allied species, local forms, and varieties distributed over the Indian and Malayan regions, I find that larger or smaller districts, or even single islands, give a special character to the majority of their Papilionidae. For instance: 1. The species of the Indian region (Sumatra, Java, and Borneo) are almost invariably smaller than the allied species inhabiting Celebes and the Moluccas; 2. The species of New Guinea and Australia are also, though in a less degree, smaller than the nearest species or varieties of the Moluccas; 3. In the Moluccas themselves the species of Amboyna are the largest; 4. The species of Celebes equal or even surpa.s.s in size those of Amboyna; 5.

The species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands; 6. Tailed species in India or the Indian region become tailless as they spread eastward through the archipelago; 7. In Amboyna and Ceram the females of several species are dull-coloured, while in the adjacent islands they are more brilliant.

_Local variation of Size._--Having preserved the finest and largest specimens of b.u.t.terflies in my own collection, and having always taken for comparison the largest specimens of the same s.e.x, I believe that the tables I now give are sufficiently exact. The differences of expanse of wings are in most cases very great, and are much more conspicuous in the specimens themselves than on paper. It will be seen that no less than fourteen Papilionidae inhabiting Celebes and the Moluccas are from one-third to one-half greater in extent of wing than the allied species representing them in Java, Sumatra, and Borneo. Six species inhabiting Amboyna are larger than the closely allied forms of the northern Moluccas and New Guinea by about one-sixth. These include almost every case in which closely allied species can be compared.

Species of Papilionidae of the Closely allied species of Java and Moluccas and Celebes (large). the Indian region (small).

Expanse. Expanse.

Inches. Inches.

Ornithoptera (Helena { O. Pompeus 58 Amboyna) 76 { O. Amphrisius 60 Papilio Adamantius } (Celebes) 58 } P. Lorquinia.n.u.s } P. Peranthus 38 (Moluccas) 48 } P. Blumei (Celebes) 54 P. Brama 40 P. Alphenor (Celebes) 48 P. Theseus 36 P. Gigon (Celebes) 54 P. Demolion 40 P. Deucalion (Celebes) 46 P. Macareus 37 P. Agamemnon, var.

(Celebes) 44 P. Agamemnon, var. 38 P. Eurypilus (Moluccas) 40 } P. Jason 34 P. Telephus (Celebes) 43 } P. aegisthus (Moluccas) 44 P. Rama 32 P. Milon (Celebes) 44 P. Sarpedon 38 P. Androcles (Celebes) 48 P. Antiphates 37 P. Polyphontes (Celebes) 46 P. Diphilus 39 Leptocircus Ennius (Celebes) 20 L. Meges 18

Species inhabiting Amboyna Allied species of New Guinea and (large). the North Moluccas (smaller).

Papilio Ulysses 61 { P. Autolycus 52 { P. Telegonus 40 P. Polydorus 49 P. Leodamas 40 P. Deiphobus 68 P. Deiphontes 58 P. Gambrisius 64 { P. Ormenus 56 { P. Tydeus 60 P. Codrus 51 P. Codrus, var.

papuensis 43 Ornithoptera Priamus, Ornithoptera Poseidon, (male) 83 (male) 70

_Local variation of Form._--The differences of form are equally clear.

Papilio Pammon everywhere on the continent is tailed in both s.e.xes. In Java, Sumatra, and Borneo, the closely allied P. Theseus has a very short tail, or tooth only, in the male, while in the females the tail is retained. Further east, in Celebes and the South Moluccas, the hardly separable P. Alphenor has quite lost the tail in the male, while the female retains it, but in a narrower and less spatulate form. A little further, in Gilolo, P. Nicanor has completely lost the tail in both s.e.xes.

Papilio Agamemnon exhibits a somewhat similar series of changes. In India it is always tailed; in the greater part of the archipelago it has a very short tail; while far east, in New Guinea and the adjacent islands, the tail has almost entirely disappeared.

In the Polydorus-group two species, P. Antiphus and P. Diphilus, inhabiting India and the Indian region, are tailed, while the two which take their place in the Moluccas, New Guinea, and Australia, P.

Polydorus and P. Leodamas, are dest.i.tute of tail, the species furthest east having lost this ornament the most completely.

Western species, Tailed. Allied Eastern species not Tailed.

Papilio Pammon (India) P. Thesus (Islands) minute tail.

P. Agamemnon, var. (India) P. Agamemnon, var. (Islands).

P. Antiphus (India, Java) P. Polydorus (Moluccas).

P. Diphilus (India, Java) P. Leodamas (New Guinea).

The most conspicuous instance of local modification of form, however, is exhibited in the island of Celebes, which in this respect, as in some others, stands alone and isolated in the whole archipelago. Almost every species of Papilio inhabiting Celebes has the wings of a peculiar shape, which distinguishes them at a glance from the allied species of every other island. This peculiarity consists, first, in the upper wings being generally more elongate and falcate; and secondly, in the costa or anterior margin being much more curved, and in most instances exhibiting near the base an abrupt bend or elbow, which in some species is very conspicuous. This peculiarity is visible, not only when the Celebesian species are compared with their small-sized allies of Java and Borneo, but also, and in an almost equal degree, when the large forms of Amboyna and the Moluccas are the objects of comparison, showing that this is quite a distinct phenomenon from the difference of size which has just been pointed out.

In the following Table I have arranged the chief Papilios of Celebes in the order in which they exhibit this characteristic form most prominently.

Papilios of Celebes, having the Closely allied Papilios of the wings falcate or with abruptly surrounding islands, with less curved costa. wings and slightly falcate curved costa.

1. P. Gigon P. Demolion (Java).

2. P. Pamphylus P. Jason (Sumatra).

3. P. Milon P. Sarpedon (Moluccas, Java).

4. P. Agamemnon, var. P. Agamemnon, var. (Borneo).

5. P. Adamantius P. Peranthus (Java).

6. P. Ascalaphus P. Deiphontes (Gilolo).

7. P. Sataspes P. Helenus (Java).

8. P. Blumei P. Brama (Sumatra).

9. P. Androcles P. Antiphates (Borneo).

10. P. Rhesus P. Aristaeus (Moluccas).

11. P. Theseus, var. (male) P. Thesus (male) (Java).

12. P. Codrus, var. P. Codrus (Moluccas).

13. P. Encelades P. Leucothoe (Malacca).

It thus appears that every species of Papilio exhibits this peculiar form in a greater or less degree, except one, P. Polyphontes, allied to P. Diphilus of India and P. Polydorus of the Moluccas. This fact I shall recur to again, as I think it helps us to understand something of the causes that may have brought about the phenomenon we are considering. Neither do the genera Ornithoptera and Leptocircus exhibit any traces of this peculiar form. In several other families of b.u.t.terflies this characteristic form reappears in a few species. In the Pieridae the following species, all peculiar to Celebes, exhibit it distinctly:--

1. Pieris Eperia compared with P. Coronis (Java).

2. Thyca Zebuda " " Thyca Des...o...b..si (India).

3. T. Rosenbergii " " T. Hyparete (Java).

4. Tachyris Hombronii " " T. Lyncida.

5. T. Lycaste " " T. Lyncida.

6. T. Zarinda " " T. Nero (Malacca).

7. T. Ithome " " T. Nephele.

8. Eronia tritaea " " Eronia Valeria (Java).

9. Iphias Glaucippe, var. " " Iphias Glaucippe (Java).

The species of Terias, one or two Pieris, and the genus Callidryas do not exhibit any perceptible change of form.

In the other families there are but few similar examples. The following are all that I can find in my collection:--

Cethosia aeole compared with Cethosia Biblis (Java).

Eurhinia megalonice " " Eurhinia Polynice (Borneo).

Limenitis Limire " " Limenitis Procris (Java).

Cynthia Arsinoe, var. " " Cynthia Arsinoe (Java, Sumatra, Borneo)

All these belong to the family of the Nymphalidae. Many other genera of this family, as Diadema, Adolias, Charaxes, and Cyrestis, as well as the entire families of the Danaidae, Satyridae, Lycaenidae, and Hesperidae, present no examples of this peculiar form of the upper wing in the Celebesian species.

_Local variations of Colour._--In Amboyna and Ceram the female of the large and handsome Ornithoptera Helena has the large patch on the hind wings constantly of a pale dull ochre or buff colour, while in the scarcely distinguishable varieties from the adjacent islands of Bouru and New Guinea, it is of a golden yellow, hardly inferior in brilliancy to its colour in the male s.e.x. The female of Ornithoptera Priamus (inhabiting Amboyna and Ceram exclusively) is of a pale dusky brown tint, while in all the allied species the same s.e.x is nearly black with contrasted white markings. As a third example, the female of Papilio Ulysses has the blue colour obscured by dull and dusky tints, while in the closely allied species from the surrounding islands, the females are of almost as brilliant an azure blue as the males. A parallel case to this is the occurrence, in the small islands of Goram, Matabello, Ke, and Aru, of several distinct species of Euploea and Diadema, having broad bands or patches of white, which do not exist in any of the allied species from the larger islands. These facts seem to indicate some local influence in modifying colour, as unintelligible and almost as remarkable as that which has resulted in the modifications of form previously described.