It is now generally admitted that we may safely reason on such facts as those， which supply a link in the defective geological record. The upward and downward movements which any country has undergone， and the succession of such movements， can be determined with much accuracy； but geology alone can tell us nothing of lands which have entirely disappeared beneath the ocean. Here physical geography and the distribution of animals and plants are of the greatest service. By ascertaining the depth of the seas separating one country from another， we can form some judgment of the changes which are taking place. If there are other evidences of subsidence， a shallow sea implies a former connexion of the adjacent lands； but iā this evidence is wanting， or if there is reason to suspect a rising of the land， then the shallow sea may be the result of that rising， and may indicate that the two countries will be joined at some future time， but not that they have previously been so. The nature of the animals and plants inhabiting these countries will， however， almost always enable us to determine this question. Mr. Darwin has shown us how we may determine in almost every case， whether an island has ever been connected with a continent or larger land， by the presence or absence of terrestrial Mammalia and reptiles. What he terms "oceanic islands "possess neither of these groups of animals， though they may have a luxuriant vegetation， and a fair number of birds， insects， and landshells； and we therefore conclude that they have originated in mid-ocean， and have never been connected with the nearest masses of land. St. Helena， Madeira， and New Zealand are examples of oceanic islands. They possess all other classes of life， because these have means of dispersion over wide spaces of sea， which terrestrial mammals and birds have not， as is fully explained in Sir Charles Lyell's "Principles of Geology，" and Mr. Darwin's "Origin of Species." On the other hand， an island may never have been actually connected with the adjacent continents or islands， and yet may possess representatives of all classes of animals， because many terrestrial mammals and some reptiles have the means of passing over short distances of sea. But in these cases the number of species that have thus migrated will be very small， and there will be great deficiencies even in birds and flying insects， which we should imagine could easily cross over. The island of Timor (as I have already shown in Chapter XIII) bears this relation to Australia； for while it contains several birds and insects of Australian forms， no Australian mammal or reptile is found in it， and a great number of the most abundant and characteristic forms of Australian birds and insects are entirely absent. Contrast this with the British Islands， in， which a large proportion of the plants， insects， reptiles， and Mammalia of the adjacent parts of the continent are fully represented， while there are no remarkable deficiencies of extensive groups， such as always occur when there is reason to believe there has been no such connexion. The case of Sumatra， Borneo， and Java， and the Asiatic continent is equally clear； many large Mammalia， terrestrial birds， and reptiles being common to all， while a large number more are of closely allied forms. Now， geology has taught us that this representation by allied forms in the same locality implies lapse of time， and we therefore infer that in Great Britain， where almost every species is absolutely identical with those on the Continent， the separation has been very recent；while in Sumatra and Java， where a considerable number of the continental species are represented by allied forms， the separation was more remote.
From these examples we may see how important a supplement to geological evidence is the study of the geographical distribution of animals and plants， in determining the former condition of the earth's surface； and how impossible it is to understand the former without taking the latter into account. The productions of the Aru Islands offer the strangest evidence， that at no very distant epoch they formed a part of New Guinea； and the peculiar physical features which I have described， indicate that they must have stood at very nearly the same level then as they do now， having been separated by the subsidence of the great plain which formerly connected them with it.