Under this head come all the facts which are concerned with the external or objective relations of animals - that is to say, their relations to the external conditions in which they are placed.
The geographical distribution of animals is concerned with the determination of the areas within which every species of animal is at the present day confined. Some species are found almost everywhere, when they are said to be "cosmopolitan;" but, as a rule, each species is confined to a limited and definite area. Not only are species limited in their distribution, but it is possible to divide the globe into a certain number of geographical regions or "zoological provinces," each of which is characterised by the occurrence in it of certain associated forms of animal life.
The geographical distribution of land animals is conditioned partly by the existence of suitable surroundings, and partly by the presence of barriers preventing migrations. Thus, certain contiguous regions might be equally suitable for the existence of the same animals, but they might belong to different zoological provinces, if separated by any impassable barrier, such as a lofty chain of mountains. Owing to their power of flight, the geographical distribution of birds is much less limited than that of mammals; and many migratory birds may be said to belong to two zoological provinces. In spite of their powers of locomotion, however, birds are limited by the necessities of their life to definite areas, and a zoological province may be marked by its birds just as well as by its quadrupeds.
The geographical distribution of an animal at the present day by no means necessarily coincides with its former extension in space. Many species are known which now occupy a much more restricted area than they did formerly, owing to changes in climate, the agency of man, or other causes. Similarly, there are species whose present area is much wider than it was originally.
At the present day, naturalists usually adopt either the zoological provinces proposed by Prof. Huxley, or those proposed by Mr Sclater, both arrangements possessing certain features in common. Prof. Huxley proposes to divide the earth's surface into four primary zoological provinces, as follows, each possessing its own "fauna," or characteristic assemblage of animals :
I. Ornithogoea, or the Novo-Zelanian Province, comprising only New Zealand.
II. Antarctogoea, or the Australian Province, comprising Australia, Tasmania, and the Negrito Islands.
III. Dendrogoea, or the Austro-Columbian Province, including South America, Central America, and Mexico.
IV. Arctogoea, including all the rest of the world, and having as sub-provinces
1. North America, north of Mexico.
2. Africa, south of the Sahara.
4. The remainder of the Old World (Europe, Africa north of the Sahara, Asia generally, but without Hindostan, etc.) Mr Sclater, basing his arrangement primarily on the distribution of birds, divides the earth's surface into the following six provinces:
1. The Paloearctic Province, including Europe, Africa north of the Atlas Mountains, and Northern Asia.
2. The AEthopian Province, including Africa south of the Atlas Mountains, and Southern Arabia.
3. The Indian Province, including Asia south of the Himalaya Mountains, Southern China, and the Indian Archipelago.
4. The Australian Province, including Australia, Tasmania, New Guinea, New Zealand, and a large proportion of the islands of the Pacific Ocean.
5. The Nearctic Province, including North America down to the centre of Mexico.
6. The Neotropical Province, including the whole of South America, Central America, and Southern Mexico.
The vertical or bathymetrical distribution of animals relates to the limits of depth within which each marine species of animals is confined. As a rule it is found that each species has its own definite bathymetrical zone, and that its existence is difficult or impossible at depths greater or less than those comprised by that zone. Generalising on a large number of facts, naturalists have been able to lay down and name certain definite zones, each of which has its own special fauna.
The five following zones are those generally accepted :
1. The Littoral zone, or the tract between tide-marks.
2. The Laminarian zone, from low water to 15 fathoms.
3. The Coralline zone, from 15 to 50 fathoms.
4. The deep-sea Coral zone, from 50 to 100 fathoms.
5. To these must now be certainly added a fifth or "Abyssal" zone, extending from 100 fathoms to a depth of 3000 or 4000 fathoms.
Recent researches, however, have rendered it certain that after a certain depth, say 100 fathoms, the bathymetrical distribution of animals is conditioned not by the depth, but by the temperature of the water at the bottom of the sea. Similar forms, namely, are always found inhabiting areas in which the bottom-temperature is the same, wholly irrespective of the depth of water in the particular locality in question. The supply of food, also, and the nature of the habitat, are important elements of the case. In the light, therefore, of these recent facts, it would perhaps be advisable to adopt the views of Dr Gwyn Jeffreys, and to consider that there are only two principal bathymetrical zones - namely, the littoral and the submarine. The researches of the Challenger Expedition have also shown that at depths beyond 500 fathoms, the fauna presents essentially the same features all over the world, deep-sea genera usually possessing a cosmopolitan range.
In addition to the preceding forms of distribution, the zoologist has to investigate the condition and nature of animal life during past epochs in the history of the world.
The laws of distribution in time, however, are, from the nature of the case, less perfectly known than are the laws of lateral or vertical distribution, since these latter concern beings which we are able to examine directly. The following are the chief facts which it is necessary for the student to bear in mind:
1. The rocks which compose the crust of the earth have been formed at successive periods, and may be roughly divided into aqueous or sedimentary rocks, and igneous rocks.
2. The igneous rocks are produced by the agency of heat, are mostly unstratified (i.e., are not deposited in distinct layers or strata), and, with few exceptions, are destitute of any traces of past life.
3. The sedimentary or aqueous rocks owe their origin to the action of water, are stratified (i.e., consist of separate layers or strata), and mostly exhibit "fossils" - that is to say, the remains or traces of animals or plants which were in existence at the time when the rocks were deposited.
4. The series of aqueous rocks is capable of being divided into a number of definite groups of strata, which are technically called "formations."
5. Each of these definite rock-groups, or "formations," is characterised by the occurrence of an assemblage of fossil remains more or less peculiar and confined to itself.
6. The majority of these fossil forms are "extinct" - that is to say, they do not admit of being referred to any species at present existing.
7. No fossil, however, is known, which cannot be referred to one or other of the primary subdivisions of the Animal Kingdom which are represented at the present day.
8. When a species has once died out it never reappears.
9. The older the formation, the greater is the divergence between its fossils and the animals and plants now existing on the globe.
Ideal Section of the Crust of the Earth.
Post-Tertiary and Recent.
Oolitic or Jurassic.
Devonian, or Old Red Sandstone.
10. All the known formations are divided into three great groups, termed respectively Palaeozoic or Primary, Mesozoic or Secondary, and Kainozoic or Tertiary.
The Palaeozoic or Ancient-life period is the oldest, and is characterised by the marked divergence of the life of the period from all existing forms.
In the Mesozoic or Middle-life period, the general fades of the fossils approaches more nearly to that of our existing fauna and flora; but - with very few exceptions - the characteristic fossils are all specifically distinct from all existing forms.
In the Kainozoic or New-life period, the approximation of the fossil remains to existing living beings is still closer, and some of the forms are now specifically identical with recent species; the number of these increasing rapidly as we ascend from the lowest Kainozoic deposit to the Recent period.
Subjoined is a table giving the more important subdivisions of the three great geological periods, commencing with the oldest rocks and ascending to the present day (fig. 5).