The science of life : fully illustrated in tone and line and including many diagrams

THE EVIDENCE FROM LIVING THINGS

and Jameson had been collecting and studying the fossils of Australian caves and breccia beds dating from the Pleistocene (VE); and these creatures, often now wholly extinct, all revealed in their structure that they belonged to the marsupials, and only to those groups of marsupials still living in Australia. We may mention here the Thylacoleo, as big as a leopard, which is a phalanger adapted to a flesh diet ; and the gigantic Diprotodon, almost as large as the Megatherium, which was closely akin to the kangaroos, though far too big to hop. Clearly the facts are parallel to those which impressed Darwin on the pampas. The marsupial stock which we find in Australia to-day (V F) must have been there for long periods, and it has evolved and changed its composition since Pleistocene (V E) times.

Africa is characterized by no such primitive types of animals as either South America or Australia. It seems to have been cut off from the main centres of mammalian evolution by the Sahara for a long time. It received its first land-mammals in the Oligocene (V B), after marsupials and the first clumsy Eocene (V A) placentals had disappeared from Asia and Europe. After this first irruption the way was again closed, to open again (probably on the eastern side of the Sahara, across to Asia by Arabia and Syria) only in the Pliocene (V D). The second and larger irruption which then followed gave Africa the bulk of its existing animal types ; since then there has been evolution in many details, but no great changes. The door was then again closed, or at most left ajar, so that Africa thus became an Ark for a large sample of the Pliocene Old World mammals. Some, like deer and bears, had failed to find the door before it shut again, and there are none of them in Africa ; but the rest throve and multiplied in the broad, equatorial stretches, while the drought and cold of the Ice Age dealt hardly with their congeners who had stayed in the north.

Just as stretches of sea act as barriers to purely terrestrial animals, so stretches of land bar the migrations of the inhabitants of the waters. The upheaval which in Miocene times (V C) created the Isthmus of Panama and the land-bridge between the two Americas, put an impassable barrier between the marine animals of Atlantic and Pacific, at least between those which could

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: . . = . . ; the Fig. 147. Diprotodon, the extinct giant wombat, from t Pleistocene (V E) of Australia, as it probably appeared

not face the cold of southern seas. As a result, no fewer than six hundred cases are known of pairs of fish-species from the coasts of Panama, one member of each pair from its Pacific, the other from its Atlantic shores, the two closely related, but different in some trivial but constant character. Original identity explains the similarities; independent evolution for some twenty million years has produced the differences.

Another example, very similar in its watery Way to the case of the Australian land-masses, is provided by Lake Baikal, the great, isolated sheet of fresh water that lies in Southern Siberia. Since it was formed, possibly in the Mesozoic Era (IV), certainly before the earlier part of the Cenozoic (V), it has been without any close connection with

in life.

Like so many large animals in different parts of the world, this creature died out during the last Ice Age.

Inset, a small spaniel, to give the scale.

any other large body of water, fresh or salt. The huge lake, over four hundred miles long and in some places nearly five thousand feet deep, holds out the most varied opportunities to water-living animals ; but most of the kinds of animals which elsewhere take advantage of similar opportunities were absent in it from the start. As a result, other types, which elsewhere remain monotony and feebly developed, have here blossome out in extraordinary variety and fill the rgst important places in the economy of the lake. This is so with certain kinds of fish, but pre-eminently so with a particular een family, the Gammarids. To this belongs t e familiar sand-hopper which swarms under moist seaweed on our sandy beaches, together with a goodly number _ of other ee, freshwater as well as marine. Members o

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