6 



GEOGRAPHICAL DISTRIBUTION 



being transported by tlie winds to immense distances ; but tbougii 

 dispersed, perhaps, over the whole glolje, the}' sprout, are successfully 

 developed onlj^ in the cold climate of the polar regions and of moun- 

 tain-tops. Plants that produce large and heavy pea-like seeds, have a 

 widely different area of diffusion. 



Woald, then, the creation of a single spore, a single grain of micro- 

 scopic dust, have stocked the whole world with Leculea geograpliica ? 

 To me, the cause seems adequate to the effect. Creative power has 

 been exercised within smaller areas than the surface of an ocean 

 islet, in the instance of cave-animals; the Proleas of a Carniolian 

 cave, the blind Coleopterous insect of another European cave, and the 

 blind crustacean, f sh, and Herni pterons insect of a Kentucky cave, 

 having proved each a peculiar species, confined to its own narrow 

 dark subterranean abode. 



Every species of animal and plant appears to have had an original 

 home ; a point from which its generations are continually pressing- 

 outwards, migrating as far as the means of support and the constitu- 

 tional relation to climate permit, unless checked by some direct 

 barrier. 



Next after the alpine microscopic-seeded Cri/pfogamia, certain tiuvia- 

 tile aquatic and subaquatic plants, are regarded as having the most 

 widely-disjoined areas of diffusion. Bat seeds formed for transporta- 

 tion in fresh water would endure transportation in sea-water, and 

 borne by the currents of the ocean beyond the unfavorable belt of 

 warm tropical countries, might obtain foothold in Antarctic territory. 

 Even in these extreme instances, it is by no means necessary to sup- 

 pose plurality of origin; that the same species was created in several 

 distinct localities. Other instances of widely-disjoined area have been 

 variously accounted for; so that the gradual disappearance of difficul- 

 ties tends at least towards an ultimate result, an explanation by a 

 single central home for each species. 



Many animals are found to be diffused in proportion to their means 

 of locomotion; the long-winged birds over larger areas than other birds. 

 Man has diffused himself over the land-surface of the globe by the 

 aid of art, by the invention of navigation. But, in general, among 

 animals and plants, it is the embryo that governs in diffusion : whe- 

 ther the care of parents is required, or otherwise; whether the ova 

 are perishable, or enduring; whether intended to remain on land and 

 in the atmosphere, or in water; whether large or small, heavy or light, 



