About the beginning of the tertiary period, however, just after the chalk was all deposited, and just before the comparatively modern clays and sandstones of the London basin began to be laid down, an arm of the sea broke up the connection which once subsisted between Australia and the rest of the world, probably by a land bridge, via Java, Sumatra, the Malay peninsula, and Asia generally.  ‘But how do you know,’ asks the candid inquirer, ‘that such a connection ever existed at all?’ Simply thus, most laudable investigator—­because there are large land mammals in Australia.  Now, large land mammals do not swim across a broad ocean.  There are none in New Zealand, none in the Azores, none in Fiji, none in Tahiti, none in Madeira, none in Teneriffe—­none, in short, in any oceanic island which never at any time formed part of a great continent.  How could there be, indeed?  The mammals must necessarily have got there from somewhere; and whenever we find islands like Britain, or Japan, or Newfoundland, or Sicily, possessing large and abundant indigenous quadrupeds, of the same general type as adjacent continents, we see at once that the island must formerly have been a mere peninsula, like Italy or Nova Scotia at the present day.  The very fact that Australia incloses a large group of biggish quadrupeds, whose congeners once inhabited Europe and America, suffices in itself to prove beyond question that uninterrupted land communication must once have existed between Australia and those distant continents.

In fact, to this day a belt of very deep sea, known as Wallace’s Line, from the great naturalist who first pointed out its far-reaching zoological importance, separates what is called by science ’the Australian province’ on the southwest from ‘the Indo-Malayan province’ to the north and east of it.  This belt of deep sea divides off sharply the plants and animals of the Australian type from those of the common Indian and Burmese pattern.  South of Wallace’s Line we now find several islands, big and small, including New Guinea, Australia, Tasmania, the Moluccas, Celebes, Timor, Amboyna, and Banda.  All these lands, whose precise geographical position on the map must of course be readily remembered, in this age of school boards and universal examination, by every pupil-teacher and every Girton girl, are now divided by minor straits of much shallower water; but they all stand on a great submarine bank, and obviously formed at one time parts of the same wide Australian continent, because animals of the Australian type are still found in every one of them.  No Indian or Malayan animal, however, of the larger sort (other than birds) is to be discovered anywhere south of Wallace’s Line.  That narrow belt of deep sea, in short, forms an ocean barrier which has subsisted there without alteration ever since the end of the secondary period.  From that time to this, as the evidence shows us, there has never been any direct land communication between Australia and any part of the outer world beyond that narrow line of division.