THE BIOLOGICAL CONQUEST OF THE Would r, 



that short time, the naked land had been partially ropoijulatcd with 

 these several low forms of life, by spores or seeds blown througli tho 

 air, or floated in water from the nearest islands, wliich wore aboiil 

 fifteen miles away. Twenty-three years after the explosion, I'rofessor 

 Ernst visited Krakatao and reijorted a forest of cocoinit pahns and 

 figs growing near the shore line, a luxuriant jungle in the interior, and 

 considerable animal life, represented by species that could either fly 

 or drift to the island on floating wood. Ernst estimated that within 

 another fifty years this island would differ in no respect from its 

 neighbors, a prediction, however, wliich seems doomed to failure of 

 confirmation because the volcano has again gone on a rampage. 



Variations in temperature, brought about by the changing seasons, 

 are a factor in the movements of animals. This is particularly true 

 in the case of the annual migrations of such animals as crabs, lobsters, 

 and squid, which go into deep water in winter, returning to shallow 

 shore-water in spring. Movements apparently dependent to some 

 extent upon temperature occur in the case of many marine fishes, and 

 birds, certain butterflies, and bats, that go north and south according 

 to the season. Many animals mo\T up and down mountain slojjes 

 probably for the same reason. 



Sometimes other factors than scarcity of food, em'ironmental 

 changes, or seasonal differences cause migration. I^emmings. for 

 instance, small rodents living in the mountainous districts of Scan- 

 dinavia, at intervals of from five to twenty years suddenly mo\'e 

 forth in vast numbers, with no apparent Pied Pi])er of Hamelin to 

 lead them, but always in the same general direction, swimming rivers 

 and lakes, overcoming all sorts of obstacles, and eventually ending 

 the mysterious trek in the ocean. Although they feed on the way 

 and consume enormous amounts of food material, the search for food 

 is not sufficient to explain their fatal pilgrimages. 



The relation of different degrees of salinity to th(> breeding habits 

 of food-fishes jjrobably influences their distribution also, by det(>r- 

 mining the character of organisms in their feeding grounds. Tetters- 

 son found that herring only enter the Baltic when the .salinity g<'ts 

 to a certain degree, whereas Galtsoff found that in America tli(> mi- 

 gration of mackerel is due not so much to salinity as to temperature. 

 Thus, different factors appear to influence different species in deter- 

 mining their movements. 



Birds, becau.se of their ability to fly, are better aide to seek out :i 

 favorable place for abode than most animals. Many tlifferent reasons 



