WHAT IS A SPECIES?

39

the orders. Mr Bates, who spent eleven years in the region of the Amazons, and carefully studied the variation and distribution of insects, has shown that many species of butterflies, 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 sharp dividing-lines. With plants there is the same kind of difficulty; and botanists are all at war about even the common bramble, some of them attempting to make out many species of it, and others maintaining that they are but many varieties of one species. If it be granted that such cases are exceptions to the rule that ordinarily there is not much difficulty in distinguishing one species from another-it is maintained, on the other hand, that a true rule or law embraces all apparent exceptions; and that, therefore, it cannot have been Nature's intention to make sharplydefined species, with limits never to be transgressed.

In the third place, a grand fact in natural history is the subordination of group under group. The original idea in classification was to arrange animals in linear order according to the degree in which they possessed some single attribute, as books may be put in the order of their dates in single file, or grouped as works in one volume, works in two volumes, works in three volumes, &c. Linnæus grouped animals in six classes, calling

them

Cl. I. MAMMALIA. Ord. Primates, Bruta, Feræ, Glires, Pecora, Belluæ, Cete.

Cl. II. AVES. Ord. Accipitres, Picæ, Auseres, Gallæ, Gallinæ, Passeres.

Cl. III. AMPHIBIA. Ord. Reptiles, Serpentes, Nantes. Cl. IV. PISCES. Ord. Apodes, Jugulares, Thoracici, Abdominales.

Cl. V. INSECTA. Ord. Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.

Cl. VI. VERMES. Ord. Intestina, Mollusca, Testacea, Lithophyta, Zoophyta.

This arrangement is based on apparent gradations of rank; and the placing of the orders in the several classes similarly betrays an endeavour to make successions, beginning with the most superior forms, and ending with the most inferior. While the general and vague idea of perfection determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. Not only Linnæus, but his opponents who proposed other systems, were under the impression that animals were to be arranged together in classes, orders, genera, and species, according to their more or less close external resemblance-a conception which survived till the time of Cuvier. Naturalists, says Agassiz, were bent upon establishing one continued uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was, Natura non facit saltum. They called their system la chaine des étres.1 Now at length, after various improvements, associated with the great names of Cuvier, Lamarck, Von Baer, and others, naturalists have been gradually compelled to arrange living things in groups within groups, and we get the following representation of zoological affinities: 2

1 Agassiz's Essay on Classification, and H. Spencer's Principles of Biology. 2 Spencer, based on Huxley.

In the fourth place, closely allied species are found in closely adjoining localities, as though some attempt at a classification were being made by Nature herself. The naturalist, in travelling, for instance, from north to south, never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He hears from closely allied yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. Humming birds which are confined to the great continent of America, with its adjacent islands, show us particular species adapted to every region where a flowering vegetation can subsist; and very often little groups of two or three allied species in the same or closely adjoining districts. The plains near the Straits of Magellan are inhabited by one species of Rhea, and northward the plains of La Plata by another

species of the same genus; and not by a true ostrich or emeu, like those found in Africa and Australia under the same latitude. Each great river has its peculiar genera of fishes; and in more extensive genera its groups of closely allied species. The inhabitants of islands are commonly allied closely to those of the nearest mainland without being actually the same species. In the Galapagos Archipelago, between 500 and 600 miles from the shores of South America, almost every product of the land and water bears the unmistakable stamp of the American continent-the naturalist feels that he is standing on American land. There are twenty-six land-birds in the archipelago, and twenty-one of them, or perhaps twenty-three, are ranked as distinct species; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of voice, is manifest. So it is with the other animals of these islands, and with nearly all the plants. Similarly throughout nature -every class and order of animals would supply instances; and the question is forced upon us, Why are these things so? Why are the genera of Palms and of Orchids in almost every case confined to one hemisphere? Why are the closely allied species of brown-backed Trogons all found in the East, and the green-backed in the West? Why are the Macaws and Cockatoos similarly restricted? Was there no law that regulated their creation and dispersion?1

In the fifth place, it is found that while every creature is suited to its station or habitat, some habitats are not supplied with the highest forms of life they could nourish. Some plants and animals can only live in the air, others 1 See Wallace's Natural Selection, chap. i.

HABITATS NOT OCCUPIED.

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only in the water; and of the latter, some are restricted to salt-water and others to fresh-water. There are

faunas and floras 1 peculiar to low regions, and others peculiar to high regions; a journey up a high mountain often carrying the traveller through zones of life resembling those passed through in going from the tropics towards the poles. The range of each kind of living thing is limited by climate; the temperature it requires is only found between certain isothermal lines; the requisite humidity or dryness of the air is only found in certain areas. Carnivorous animals, of course, cannot exist out of regions tenanted by creatures numerous and large enough to serve for prey; and granivorous animals are confined within tracts which produce plants fit for them to feed on. But the converse of these facts is not true that is to say, there are many regions quite as well suited for the maintenance of certain living forms as the districts where those forms at present flourish, and yet those regions are peopled by inferior types. We need only refer to the extraordinary manner in which European productions have recently spread over New Zealand, and have seized on places which must have been previously occupied, thus proving themselves to be better fitted for the habitat than are the native productions. The English dock is to be found in New Zealand in every river bed, extending into the valleys of the mountain rivers, until these become mere torrents; the sow-thistle is spread all over the country; the water-cress threatens to choke the still rivers; in fact, the young native vegetation appears to shrink from competition with these more vigorous intruders. The

1 The entire animal life of a district constitutes its fauna, the entire vegetable life its flora.