The Malay archipelago : the land of the orang-utan ... - Wallace Online

I.] PHYSICAL GEOGRAPHY. 9tliickness of many marine deposits through wide areas showsthat subsidence has often continued (with intermitting periodsof repose) during epochs of immense duration. The depth ofsea produced by such subsidence will therefore generally be ameasure of time ; and in like manner the change which organicforms have undergone is a measure of time. When we makeproper allowance for the continued introduction of new animalsand plants from surrounding countries, by those natural meansof dispersal which have been so well explained by Sir CharlesLyell and Mi'. Darwin, it is remarkable how closely these twomeasures correspond. Britain is separated from the continentby a very shallow sea, and only in a very few cases have ouranimals or plants begun to show a diflference from the correspondingcontinental species. Corsica and Sardinia, divided fromItaly by a much deejDer sea, present a much greater difterencein their organic forms. Cuba, separated from Yucatan by awider and deeper sti'ait, differs more markedly, so that most ofits productions are of distinct and peculiar species ; whileMadagascar, divided from Africa by a deep channel threehundred miles wide, possesses so many jDeculiar features as toindicate separation at a very remote antiquitj^, or even torender it doubtful whether the two countries have ever beenabsolutely united.Eeturning now to the Malay ArchijDelago, we find that all thewide expanse of sea which divides the islands of Java, Sumatra,and Borneo from each other, and from Malacca and Siam, is soshallow that ships can anchor in any part of it, since it rarelj'^exceeds foi'ty fathoms in depth ; and if we go as far as the line ofa hundred fathoms, we shall include the Philippine Islands andBali, east of Java. If, therefore, these islands have been separatedfrom each other and the continent by subsidence of tlie interveningtracts of land, we should conclude that the separation hasbeen comparatively recent, since the depth to wliich the land hassubsided is so small. It is also to be remarked, that the greatchain of active volcanoes in Sumatra and Java furnishes us witha sufficient cause for such subsidence, since the enormous massesof matter they have thrown out would take away tlie foundationsof the surrounding district ; and this may be the true explanationof the often-noticed fact, that volcanoes and volcanic chainsare always near the sea. The subsidence they produce aroundthem will, in time, make a sea, if one does not already exist.^But it is when we examine the zoology of these countries thatwe find what we most require— e\adence of a very strikingcharacter that these great islands must have once formed a partof the continent, and could only have been separated at a veryrecent geological epoch. The elephant and tapir of Sumatra andBorneo, the rhinoceros of Sumatra and the allied species of Java,1 It it now believed by most geologists that subsidence is produced by the weight ofevery fresh deposit of materials either in the sea or on the land. Accumulations ofrock or ashes from volcanoes would, therefore, be Itself a cause of subsidence.