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best marked affinities between the regions are those between the Nearctic and Palæarctic,—the Oriental and Australian, -the Australian and Neotropical,—which appear to be about equal in each case. Next comes that between the Ethiopian and Oriental on the one side, and the Ethiopian and Neotropical on the other, which also appear about equal. Then follows that between the Nearctic and Neotropical regions; and lastly, and far the least marked, that between the North Temperate and South Temperate regions. That the relation between the Ethiopian and Neotropical region should be so comparatively well marked, is unexpected; but we must consider that in such a comparison as the present, we probably get the result, not of any recent changes or intermigrations, but of all the long series of changes and opportunities of migration that have occurred during many geological epochs,-probably during the whole of the Tertiary period, perhaps extending far back into the Secondary age.

It appears evident that Insects exhibit in a very marked degree in their actual distribution, the influence both of very ancient and very modern conditions of the earth's surface. The effects of the ancient geographical features of the earth, are to be traced, in the large number of cases of discontinuous and widely scattered groups which we meet with in almost every family, and which, to some extent, obscure the broader features of distribution due to the period during which the barriers which divide the several primary regions have continued to exist. And this, which we may consider as the normal distribution, is still further obscured in those cases where the barriers between existing regions are of such a nature as to admit of the free passage of insects or their larva in a variety of ways, and (what is perhaps of more importance) in which the physical features on both sides of the barrier are so nearly identical, as to admit of the ready establishment of such immigrants as may occasionally arrive. These conditions concur, for some families of insects, in the case of the Oriental and Australian portions of the Malay Archipelago: and it is there that the normal distribution has been sometimes greatly obscured, but never, as we have sufficiently shown, by any means obliterated.

VOL. II.-33

CHAPTER XXII.

AN OUTLINE OF THE GEOGRAPHICAL DISTRIBUTION OF MOLLUSCA.

The Mollusca being for the most part marine, it does not enter into the plan of this work to go into much detail as to their distribution. The orders and families will, however, be passed briefly in review, and all terrestrial and fresh-water groups discussed in somewhat more detail; with the object of showing how far their distribution accords with that of the higher animals, and to what extent the anomalies they present can be explained by peculiarities of organisation and habits. If the views advocated in our fifth chapter are correct, the regions there marked out must apply to all classes of animals; and it will be the task of the students of each group, to work out in detail the causes which have led to any special features of distribution. All I can hope to do here, is to show, generally and tentatively, that such a mode of treatment is possible; and that it is not necessary, as it is certainly not convenient or instructive, to have a distinct set of “Regions" established for each class or order in the Animal and Vegetable Kingdoms.

For all the Marine groups I have merely summarised the information contained in Mr. Woodward's Manual of the Mollusca, but in the case of the Land Shells I have consulted the most recent general works, and endeavoured to give an accurate, though doubtless a very incomplete, account of the most interesting facts in their distribution. As their classification is very unsettled, I have followed that of the two latest great works, by Martens and Pfeiffer.

CLASS.- CEPHALOPODA.

Order 1.-DIBRANCHIATA.

(1 Genus,

FAMILY 1.-ARGONAUTIDÆ. "Paper Nautilus."

4 Species).

DISTRIBUTION.—Open seas of all warm regions. Two species fossil in Tertiary deposits.

FAMILY 2.-OCTOPODIDÆ. Polypi.”

Species).

(7 Genera, 60

DISTRIBUTION.—Norway to New Zealand, all tropical and temperate seas and coasts.

FAMILY 3.-TEUTHIDÆ. Squids or Sea-pens." (16 Genera,

102 Species.)

DISTRIBUTION.—Universal, to Greenland ; 2 other genera are fossil, in the Lias and Oolite.

FAMILY 4.-SEPIADÆ. “Cuttle Fish.” (1 Genus, 30 Species).

DISTRIBUTION.—All seas : 4 other genera are fossil, in Eocene and Miocene deposits.

FAMILY 5.-SPIRULIDÆ. (1 Genus, 3 Species).

DISTRIBUTION.—All the warmer seas.

FAMILY 6.- BELEMNITIDÆ. Fossil. (6 Genera, 100

Species). DISTRIBUTION.- Lias to Chalk in Europe, India and North America.

Order 11.-TETRABRANCHIATA.

FAMILY 7.-NAUTILIDÆ. (1 Genus, 3 Species, Living; 4

Genera, 300 Species, Fossil). DISTRIBUTION.-Indian and Pacific Oceans; and the fossil species from the Silurian Period to the Tertiary, in all parts of the world.

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DISTRIBUTION.—Upper Silurian to Chalk. Found at 16,000 feet elevation in the Himalayas.

CLASS.-GASTEROPODA.

Order I.-PROSOBRANCHIATA.

FAMILY 1.-STROMBIDÆ. (4 Genera, 86 Species.)

DISTRIBUTION.—The Strombidæ, or Wing-shells, inhabit tropical and warm seas from the Mediterranean to New Zealand ; most abundant in the Indian and Pacific Oceans. There are nearly 200 fossil species, from the Lias to Miocene and recent deposits.

FAMILY 2.-MURICIDÆ (12 Genera, 1000 Species.)

DISTRIBUTION.--All seas, most abundant in the Tropics, Trichotropis is confined to Northern seas; Murex and Fusus are cosmopolitan. There are about 700 fossil species, ranging from the Oolite to the Miocene and recent formations.

FAMILY 3.-BUCCINIDÆ. (24 Genera, 1100 Species.) DISTRIBUTION.—The Buccinidæ, or “ Whelks,” range over the whole world, but some of the genera are restricted. Buccinum inhabits the north and south temperate seas; Monoceros the West Coast of America; Cassidaria the Mediterranean ; Phos, Harpa, Eburna, and Ricinula, are confined to the Pacific; Dolium inhabits the Mediterranean as well as the Pacific. There are about 350 fossil species, mostly from the Eocene and Miocene beds.

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