THE DISTRIBUTION OF ANIMALS AND PLANTS 421 



definite laws, if that really differs in its nature from certain 

 forms of the hypothesis of evolution. We have also learned 

 that, time being given, animals and plants manifest wonderful 

 powers of migration, that they can vary within considerable 

 limits without ceasing to be practically the same species, and 

 that under certain conditions they can endure far longer in 

 some places than in others. We also see evidence that it is 

 not on limited islands, but on the continents, that land animals 

 and plants have originated, and that swarms of new and 

 vigorous species have issued from the more northern regions 

 in successive periods of favourable Arctic climate. The last 

 of these new swarms or "centres of creation," that with 

 which man himself is more closely connected, belongs to the 

 Palearctic region. We have already seen that in every geo- 

 logical period, when the submerged continental plateaus were 

 pervaded by the warm equatorial waters, multitudes of new 

 marine species appear. In times when, on the contrary, the 

 colder Arctic currents poured over these submerged surfaces, 

 carrying mud and stones, great extinction took place, but 

 certain northern forms of life swarmed abundantly, and when 

 elevation took place, marine species became extinct or were 

 forced to migrate. Everywhere and at all times multiplication 

 of species was promoted by facilities for expansion. The great 

 limestones of our continents, full of corals and shells of new 

 species, belong to times when the ocean spread itself over the 

 continental plateaus, affording wide, untenanted areas of 

 warm and shallow water. The introduction of new faunas 

 and floras on the land belongs to times when vast supplies of 

 food for plants and animals and favourable conditions of 

 existence were afforded by the emergence of new lands 

 possessing fertile soils and abundantly supplied with light, 

 heat, and moisture. Thus geological and geographical facts 

 concur with ordinary observation and experience in reference 

 to varietal forms, in testifying that it is not mere struggle for 



