« AnteriorContinuar »
in both regions.
But the Marsupials of Australia seem to have but a very remote connection with those of South America, and there is at present no palæontological evidence of the former occurrence of the Australian forms, or of forms allied to them, outside of Australia itself.1 On the other hand, the presence of fossil opossums (Didelphyide) in the Eocene beds of France, shows that the South American forms were formerly more widely spread.
Professor Huxley has also cited the Parrots (Psittacomorpha) "as helping, together with the three-toed Ratitæ, to bind together the widely-separated portions of the south world." But on referring to the account of the distribution of the Parrots in Salvadori's recently published catalogue (8), it will be found that out of the six families into which he divides the group, five are practically confined to the Australian Region, and that the remaining one is widely spread throughout the tropical regions of both hemispheres. The most recent arrangement of this family, therefore, gives little support to Professor Huxley's arguments.
Looking, again, to the distribution of the Ratita (wingless birds), we find the Neotropical form (the Rhea) more closely connected with the Ostrich, the Ethiopian form, and that they both differ considerably from the Emus, Cassowaries, and Kiwis, the three Australian representatives of this order. Thus, then, there seems to be
1 Recently Señor Ameghino has described from the Santa Cruz beds of Patagonia, which are probably of Eocene age, certain fossil mammals which he has referred to the Dasyurida, one of the Australian families. Again, Mr. Thomas' Canolestes (see P. Z. S. 1895), is also believed to be allied to the Australian Diprotodonts. If these relationships should turn out to be correct, it will indicate further evidence of some connection between South America and Australia, though at a considerably remote epoch of geological time.
scarcely any ground for connecting the Neotropical and Australian Regions under one name.
Before discussing the other differences between this scheme and that of Huxley, it will be as well to mention the diverging views of some other naturalists. Of these the chief is Professor Heilprin, of Philadelphia, who in his "Geographical and Geological Distribution of Animals," (5)" in accordance with a suggestion by Professor Newton," has proposed to unite the Nearctic and Palearctic Regions into a single realm-the "Holarctic"-and to separate the Pacific Islands from Australia as the "Polynesian Realm." Again, Mr. J. A. Allen, of New York, in his recently published essays (1 and 2), has shown considerable independence of thought in this matter. In the introduction to the later of them, which deals chiefly with the distribution of North American mammals, Mr. Allen gives an account of the influences which, in his opinion, mainly determine the geographical distribution of life, dwelling first on the great importance of temperature and moisture, and afterwards on the inter-relation of landareas, which, he says, is "co-eval and perhaps more than co-ordinate with climate in its influence upon the distribution of life." Next, Mr. Allen treats of the seven primary life-regions, or "realms" as he terms them, into which he proposes to divide the Earth. These are:
1. An Arctic Realm, occupying all the country in both hemispheres north of the isotherm 32° F., this boundary corresponding very closely to that of the northern limit of trees.
2. A North Temperate Realm, occupying the whole of the northern hemisphere between the isotherms of 32° and 70° F.
3. An American Tropical Realm, consisting of Tropical America.
4. An Indo-African Realm, consisting of Africa, except the northern border, and Tropical Asia and its islands.
5. A South American Temperate Realm, embracing extra-tropical South America.
6. An Australian Realm, equivalent to our Australian Region.
7. A Lemurian Realm, containing Madagascar and its islands.
Mr. Allen's views on Distribution have been criticised and answered by another American naturalist, Mr. Gill (4), who has proposed a division of the Earth into nine "realms." These, as will be seen, although not differing in many cases from regions adopted by former authorities, are distinguished by an entirely new set of names, as follows:
(1) The Anglo-gæan (= Nearctic Region).
(5) The Dendro-gæan (=the tropical half of the Neotropical Region).
(6) The Amphi-gæan (=the temperate half of the Neotropical Region).
(7) The Austro-gæan (= Australia, New Guinea, and the adjacent islands).
(8) The Ornitho-gaan (= New Zealand).
(9) The Neso-gæan (= Polynesia).
Dr. Bowdler Sharpe (12) has also recently published his views on the zoo-geographical areas, as worked out from the distribution of birds. Dealing here only with the division of the Earth into Regions, we notice that
although he makes many complimentary allusions to Mr. Allen and his views, he employs in nearly all its entirety the system adopted in this work, with the exception that he recognises an Arctic Sub-region to include the more northerly parts of both the Old and New worlds.
Finally, Professor Newton, who has given us his views on this subject as regards birds (7), adopts the method of divisions followed here with the two following exceptions. In conformity with the suggestion already made to Professor Heilprin, he unites the Palearctic and Nearctic Regions under the title "Holarctic," and he also separates New Zealand from Australia as an independent region.
The chief questions in dispute, therefore, seem to be as follows:
(1) Whether the Palearctic and Nearctic Regions are to be recognised as separate?
(2) Whether Madagascar and New Zealand are to be separated as independent regions from the Ethiopian and Australian Regions respectively?
(3) Whether the Ethiopian and Oriental Regions should be joined to form one region?
(4) Whether there are any good grounds for dividing the Neotropical into two separate regions.
The only way in which questions of this sort could be settled would be by constructing accurate lists of the families and genera of the various classes of the terrestrial faunas of the regions in dispute, and then carefully comparing them, in order to determine the percentage of peculiar species and of absentees. The difficulty of doing this satisfactorily is twofold.
(1) The absence of any definite boundaries to most of
the regions, and hence the difficulty in determining how many of the border-forms, which have obviously intruded from the neighbouring regions, should be counted.
(2) The uncertainty as to the limits of the genera. This uncertainty has been greatly increased of late years by the action of some zoologists in proposing a multitude of unnecessary generic terms.
When these two factors have been settled and the lists constructed, a further difficulty is met with, and this is one which depends very much on the individual fancy of the author, namely, as to the percentage of peculiarity which should be required to constitute a Region.
Taking the first question in dispute, we find that Mr. Allen, in his paper already quoted (2), gives a tabulated list of the genera of his North Temperate realm, dividing them into North American and Eur-asiatic (= Palearctic) forms, and putting the individual genera into three categories, namely, those circum-polar, or common to the Nearctic and Palearctic Regions (numbering thirty-two); those peculiar to each Region (i.e. twenty-nine to the Nearctic and forty-one to the Palearctic); and, finally, those which range further south into the Neotropical Region on the one hand, and into the Oriental and Ethiopian Regions on the other.
Working from these tables we find that 38 per cent. of the Nearctic genera and 42 per cent. of the Palearctic genera are confined to their respective Regions, while 42 per cent. in the case of the Nearctic and 34 per cent. in the case of the Palearctic are common to the two regions. These last percentages include, however, several quite widespread genera which can hardly be called circum-polar-such as Sciurus, Sciuropterus, Lepus, Lutra, Canis, and Felis.
These figures show that there is, as has indeed never