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Show 8 ISLAND LIFE. [rAll'l' I. and in many cases it can never now be obtained owing to the reckless destruction of forests and with thell,l of countless species of plants and animals. In the next place we require a true and natural classification of animals and plants, so that we may know their real affinities; and it is only now that this is being generally arrived at. We further have to make use of tho theory of "descent with modification" as the only possible key to the interpretation of the facts of distribution, and this theory has only been generally accepted within the last twenty years. It is evident that, so long as the belief in "special creations" of each species prevailed, no explanation of the complex facts of distribution could be arrived at or even conceived; for if each species was created where it is now found no further inquiry can take ns beyond that fact, and there is an end of the whole matter. Another important factor in our interpretation of the phenomena of distribution, is a knowledge of the extinct forms that have inhabited each country during tho tertiary and secondary periods of geology. New facts of this kind are daily coming to light, but except as regards Europe, North America, and parts of India, they are extremely scanty; and even in the best-known countries the record itself is often very defective and fragmentary. Yet we have already obtained remarkable evidence of tl1e migrations of many animals and plants in past ages, throwing an often unexpected light on the actual distribution of many groups.1 By this means alone can we obtain positive evidence of the past migrations of organisms; and when, as too frequently is the case, this is altogether wanting, we have to trust to collateral evidence and more or less probable hypothetical explanations. Hardly less valuable is the evidence of stratigraphical geology; for this often shows us what parts of a country have been submerged at certain epochs, and thus enables us to prove that certain areas have boon long isolated and the fauna and flora allowed time for special development. Here, too, our knowledge is exceedingly imperfect, though the blanks upon the geological map of the world ~ The general facts of Palroontology, as bearing on the migrations of ammal groups, are summarised in my Geographical Dist1·ibution of A nimals Vol. I. Chapters VI., VII., and VIII. ' CHAP. I.] IN'l'HODUCTORY. 9 are yearly diminishing in extent. Lastly, as a most valuable supplement to geology, we require to know the exact depth and contour of the ocean-bed, since this affords an important clue to the former existence of now-submerged lands, uniting islands to continents, or affording intermediate stations which have aided the migrations of many organisms. This kind ·of information has only begun to be obtained during the last few years; and it will be seen in the latter part of this volume, that some of the most recent deep-sea soundings have afforded a basis for an explanation of one of the most difficult and interesting questions in geographical biology-the origin of the fauna and flora of New Zealand. Such are the various classes of evidence that bear directly on the question of the distribution of organisms; but there are others of oven a more fundamental character, and the importance of which is only now beginning to be recognised by students of nature. These are, firstly, the wonderful alterations of climate which have occurred in the temperate and polar zones, as proved by the evidences of glaciation in the one and of luxuriant vegetation in the other; and, secondly, the theory of the permanence of existing continents and oceans. If glacial .epochs in temperate lands and mild climates near the poles have, as now believed by men of eminence, occurred several times over in the past history of the earth, the effects of such O'reat and repeated changes, both on the migration, modification, ~nd extinction, of species, must have been of overwhelming importance-of more importance perhaps than even the geological changes of sea and land. It is therefore necessary to consider the evidence for these climatal changes ; and then, by a critical examination of their possible causes, to ascertain whether they were isolated phenomena, were due to recurrent cosmical actions, or were the result of a great system of terrestrial development. The latter is the conclusion we arrive at; and this conclusion brings with it the conviction, that in the theory which accounts for both glacial epochs and warm polar climates, we have the key to explain and harmonize many of the most anomalous biological and geological phenomena, and one which is especially valuable for the light it throws on the |
