climate, imkins: it possible for plants as well as animals to li\e anywhere upon 

 the earth, as they cannot now. Then extensive migrations north and south were 

 net necessary, but instead there were roaming about in all directions, or great 

 invasions of new regions by hosts of animals of one kind. 



As the land sank away here and there, and the sea covered it, barriers were 

 thus formed to further roamings, except by the birds of strong flight or animals 

 that could swim long distances, and there could no longer be an intermingling of 

 the animals of the whole land surface of the world. Since all animals are inclined 

 to change somewhat to meet or keep pace with the changes that are going on in 

 vegetation and the general physical conditions of the earth, those that have been 

 separated in this way Avill grow more and more unlike. In some such isolated 

 regions there may not be much change in their environment and so they will 

 change but little, if at all, and so will not keep pace with those in other regions 

 where life is a constant struggle with others for supremacy. It is just as true in 

 the natural world as in the commercial, that competition is necessar\' for the high- 

 est development. It is probably true that the disturbances which caused the land 

 to sink in places and so disconnect what had been connected lands, possibly a 

 splitting up of one great flat land mass, also brought about the changes which 

 made out of one great tropical world the one that we know with its frigid, temper- 

 ate and tropical zones. So that just at the time when the animals of the different 

 regions were separated from each other forever there came these changes in 

 physical conditions which would make them change to meet the new conditions. 

 But that is a long story for the geologist to tell. Of course the sinking of the land 

 in different regions occurred at different times, probably thousands of years apart 

 in many cases. And the changes from tropical to temperate and frigid must have 

 been very gradual also, or there w^ould have been no animals left alive in the 

 northern and southern regions. Only those near the equator could have lived. 



Probably New Zealand was the first considerable land mass to be separated 

 absolutely and for all time from all other land, because here we find the lowest 

 type of birds and lower animals. There are no terrestrial indigenous mammals 

 even. Such birds as were not able to fly across the now wide stretches of ocean 

 did not continue to develop rapidly because there was little change in their en- 

 vironment and because there was little or no competition with other similar forms. 

 So today we find them either very similar to what they were when their island 

 home was made an island home, or else even degenerated into flightless creatures. 

 Australia seems to have been the next tract of land cut oft", for here, too, we meet 

 with the lower forms which show the lack of the keen competition which their 

 relatives further north had to sustain. When North America was cut off from 

 Siberia, marking the close of more or less extensive interchange of communication 

 of the animals of both regions, there was little difference in their animal life; but 

 following this separation there came about a more rapid change in the Orient than 

 in the Occident. It may not be quite clear why this was so, but that it was cannot 

 be doubted, for some of the lower forms of animals which still inhabit America 

 have been completely destroyed in the Orient. At the time of their separation 

 these forms were found in both places. What seems a probable explanation of 



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