Suppose that by upheavals, occurring, as they are known to do, at longintervals, the East Indian Archipelago were raised into a continent, anda chain of mountains formed along the axis of elevation. By the first ofthese upheavals, the plants and animals of Borneo, Sumatra, New Guinea, andthe rest, would be subjected to slightly-modified sets of conditions. Theclimate of each would be altered in temperature, in humidity, and in itsperiodical variations, while the local differences would be multiplied. Themodifications would effect, perhaps inappreciably, the entire Flora and Faunaof the region. The change of level would entail additional modifications,varying in different species, and also in different members of the same species,according to their distance from the axis of elevation. Plants growing onlyon the sea-shore in special localities, might become extinct. Others, livingonly in swamps of a certain humidity, would, if they survived at all, probablyundergo visible changes of appearance. While more marked alterations wouldoccur in some of the plants that spread over the lands newly raised out ofthe water. The animals and insects living on these modified plants, wouldthemselves be in some degree modified by changes of food, as well as by changesof climate; and the modifications would be more marked where, from the dwindlingor disappearance of one kind of plant, an allied kind was eaten. In the lapseof the many generations arising before the next upheaval, the sensible orinsensible alterations thus produced in each species, would become organized-- in all the races which survived there would be more or less adaptationto the new conditions. The next upheaval would superinduce further organicchanges, implying wider divergences from the primary forms; and so repeatedly.
Now, however, observe that this revolution would not be a substitution ofa thousand modified species for the thousand original species; but in placeof the thousand original species there would arise several thousand species,or varieties, or changed forms. Each species being distributed over an areaof some extent, and tending continually to colonize the new area exposed,its different members would be subject to different sets of changes. Plantsand animals migrating towards the equator would not be affected in the sameway with others migrating from it. Those which spread towards the new shores,would undergo changes unlike the changes undergone by those which spreadinto the mountains. Thus, each original race of organisms would become theroot from which diverged several races, differing more or less from it andfrom one another; and while some of these might subsequently disappear, probablymore than one would survive into the next geologic period. Not only wouldcertain modifications be thus caused by changes of physical conditions andfood, but also, in some cases, other modifications caused by changes of habit.
The fauna of each island, peopling, step by step, the newly-raised tracts,would eventually come in contact with the faunas of other islands; and somemembers of these other faunas would be unlike any creatures before seen.
