American region; so that the eastern and western shores of the Atlantic appear decidedly more distinct than the eastern and western coasts of America. The extensive Oriental region is by far the richest.
From the distribution of the species (not genera) of barnacles, or cirrhipeds, which are an aberrant group of Crustacea, Darwin considered that the ocean might be divided into the following regions, viz.:—(1) The North Atlantic, comprising North America and Europe down to N. lat. 30°; (2) The West American, from Bering Strait to Tierra del Fuego; (3) The Malayan, from India to New Guinea, and (4) The Australian, comprising Australia and New Zealand: the Malayan and Australian regions being the richest in cirrhipeds.
One of the earliest students of the geographical distribution of marine animals was Dr S. P. Woodward, who, in his Manual of Mollusca, proposed a scheme of zöo-geographical regions. He adopted three main divisions for the warmer parts of the ocean, namely, the Atlantic, the Indo-Pacific and the West American; and these Wallace was inclined to regard as the only valid marine molluscan regions.Molluscan regions. The Indo-Pacific region extends from the Red Sea and the east coast of Africa to the eastern Pacific islands, and corresponds to Prof. Dana’s Oriental region for Crustacea, many species ranging over nearly the whole area. The Atlantic region unites the fauna of the east coast of America with that of West Africa and South Europe, but has considerable affinity with that of West America, many genera being common to both areas. Several genera appear restricted to the north temperate zone, which in Wallace’s opinion should perhaps form a distinct region. Numerous genera are confined to the Indo-Pacific region. The Atlantic coasts have few peculiar genera of importance, while the west coast of America has hardly any, the difference of its fauna from that of the Atlantic on the one side and the Pacific on the other being chiefly specific. It is stated that while there is not a single species common to the east and west coasts of tropical South America, the corresponding coasts of North America have a large number in common, while others are so closely representative as to be almost identical. Inclusive of an Arctic province of somewhat doubtful value, Dr Woodward’s three main regions were divided into 18 subregions; but, according to a somewhat modified later scheme, these may be arranged in four main groups, as follows:—
| Regions. | Subregions. | |||||||||
| A. Atlantic and Circumpolar |
| |||||||||
| B. Indo-Pacific |
| |||||||||
| C. Australian |
| |||||||||
| D. American |
|
Fish Regions.— From the distribution of shore-haunting fishes, Dr A. C. L. G. Günther[1] suggested the following marine zoological regions, the characteristic family and generic types of which we are prevented by limitations of space from discussing:—
I. Arctic Ocean.
II. Northern Temperate Zone.
- A. Temperate N. Atlantic.
- 1. British district.
- 2. Mediterranean district.
- 3. N. American district.
- B. Temperate N. Pacific.
- 1. Kamchatkan district.
- 2. Japanese district.
- 3. Californian district.
III. Equatorial Zone.
- A. Tropical Atlantic.
- B. Tropical Indo-Pacific.
- C. Pacific Coast of Tropical America.
- 1. Central American district.
- 2. Galapagos district.
- 3. Peruvian district.
IV. Southern Temperate Zone.
- 1. Cape of Good Hope district.
- 2. South Australian district.
- 3. Chilean district.
- 4. Patagonian district.
V. Antarctic Ocean.
Mammalian Regions.— The last scheme of marine zoological regions necessary to mention is one proposed by P. L. and W. L. Sclater[2] on the distributional evidence afforded by seals, sea-cows and cetaceans. According to this, we have the following six regions, viz.:—
(i) Arctatlantica (North Atlantic), characterized by the presence of seals of the subfamily Phocinae, with the genera Halichoerus peculiar to it, and Phoca common to it and No. iv.; the absence of sea-cows; and (it is stated) by the bottle-nosed whales (Hyperoodon) being peculiar to this area.
(ii) Mesatlantica (Mid Atlantic), the sole habitat of the monk-seal (Monachus) and the manatis (Manatus).
(iii) Philopelagica (Indian Ocean, &c.), characterized by the presence of dugongs and the absence of seals,
(iv) Arctirenia (North Pacific), agreeing with No. i. in the possession of Phoca but distinguished by also having sea-bears and sea-lions (Otariidae); formerly the habitat of the northern sea-cow (Rhytina), and now of the grey whale (Rhachianectes).
(v) Mesirenia (Mid Pacific), without Phocinae or sea-cows, but with the elephant-seal (Macrorhinus), from the south, and also Otariidae.
(vi) Notopelagica (Southern Ocean), with four peculiar genera of seals (Phocidae), numerous sea-bears and sea-lions, and two peculiar genera of cetaceans, the pigmy-whale (Neobalaena) and Arnoux’s beaked-whale (Berardius).
To explain the absence of sea-bears and sea-lions from the North Atlantic, and likewise the existence of manatis on both Atlantic coasts, the authors of this scheme call in the aid of a land-connexion between Africa and South America, which presented a barrier to the northward progress of the former, while its coasts afforded a means of dispersal to the latter. As the Otariidae are at present unknown previous to the Miocene, such an explanation, if valid, requires the persistence of the ancient land-bridge across the Atlantic to a much later date than is commonly supposed.
III. Distribution in Time
The subject of the distribution of animals in time, i.e. the relative dates of their first appearance on the earth, and in the case of extinct forms the length of their sojourn there, can be treated but briefly; reference being restricted to the larger groups, and not even all of these being mentioned. The dates of appearance and disappearance of the various groups are only relative, for although many more or less vague attempts have been made to determine the age of the earth, there is no possibility of indicating in years the length of time occupied by the deposition of any one stratum or series of strata. All that can be attempted is to say that one stratum (and consequently the remains of animals that may be entombed in it) is older or younger, as the case may be, than another. For the sequence and names of the various strata, or time-periods of the geological record, see Geology. An important factor in regard to the past history of animals is the imperfection of the geological record. Recent discoveries have rendered this imperfection much less marked than was formerly the case. There are, however, still many very serious gaps; and we have, for instance, no definite information as to where and when the transformation from reptiles into mammals took place.
It may nevertheless be emphatically affirmed that the geological, or rather the palaeontological, record indicates a gradual progression in the status of animals from the lowest to the highest fossiliferous strata. That is to say, the earlier animals were creatures of comparatively low grade (although certain representatives of such groups may have attained a relatively high degree of specialization), and that as we ascend the geological ladder higher and higher types of animals make their appearance, till the series culminates in man himself—the crowning effort of creation, in the modern evolutionary signification of that term. It is not, indeed, to be supposed that the higher groups made their appearance exactly according to their relative grades (or what we regard as such); all that can be affirmed is that in the main the higher forms have made their appearance later than the lower. The record is thus almost exactly what it might be expected to be on the theory of evolution; while it also accords fairly well—if regarded with sufficient breadth of view—with the Biblical narrative of creation.
A brief survey of the time-distribution of the leading groups of animals may now be undertaken, commencing with the highest and concluding with the lowest groups.
