74 THE POPULAR SCIENCE MONTHLY. 



their own ingenious contrivances for floating with the wind and tide, 

 and for catching on to every moving object, all have been carefully- 

 observed and faithfully chronicled. 



The first important truth enforced by these observations is that all 

 organic life on the earth is, in a generic or tribal sense at least, migra- 

 tory and nomadic. The individuals may be rooted and stationary, 

 but the tribe is traveling, constantly leaving old fields and surround- 

 ings and as constantly arriving in new ones, sometimes crowded out, 

 sometimes starved out, and sometimes invited out, but always moving 

 — moving on to a new environment, better suited, taking all things into 

 consideration, to satisfy the pressing needs of, and to develop and 

 raise in the scale of being, both the individual and the species. 



A second great truth taught by examining the methods of these 

 movements and studying the causes of this ceaseless tramp of organic 

 life is, that certain essential elements of the environment itself are 

 usually found to be traveling with or a little in advance of the migra- 

 tory species. In other words, the rainfall and isothermal lines, the cli- 

 matic and other conditions of life, are constantly and slowly changing 

 relative to the locality, but moving in fact. It has been frequently 

 observed that certain species, occupying some particular territory now, 

 have at some recent time in the past been enabled by such changes 

 to crowd out other occupants of the same territory, and in turn will 

 be undoubtedly, by similar changes and means, crowded out them- 

 selves. All kinds of plants and animals which have remained in one 

 locality until they have lost the means of movement, which can not 

 or will not travel, must sooner or later first degenerate and then be 

 exterminated. For instance, a rain-belt or an area of dew-fall veers 

 slowly but permanently from the north to the south ; an arid soil is 

 made fertile, and a fertile soil is left arid ; the grass and flowering 

 plants in endless variety move with the dew or the rain-belt ; the deer 

 follow the grass, and the wolves follow the deer ; a thousand varieties 

 of insects follow the flowering plants, and the insectivorous birds and 

 other animals, herbivorous and carnivorous, bring up the rear, and so 

 on, through all the interdependencies of life, the change of a single 

 essential condition, the movement of one variety, causes a disturbance 

 and movement of all in the neighborhood. Thence comes all this 

 ceaseless and migratory activity among the flora and fauna of the 

 earth. 



This condition of things would indicate the possibility at least that 

 life upon the earth had in the main commenced in some favored area, 

 and traveled thence far and wide over the surface of the globe, driven 

 out by changes of environment, lessening in efi^ect the favorable con- 

 ditions of its development in the place of its beginning, and ever 

 beckoned on by more favorable conditions in adjacent districts. As 

 there are no plants and no animals, with the exception of man, and 

 possibly his companion the dog, and his pest the rat, that can thrive 



