DISTRIBUTION OF PLANTS. 115 



thirty feet thick, and many miles in" extent. Still more ex- 

 tensive deposits are found in the West. In the Quaternary 

 period the plants were almost wholly identical with those 

 now living. 



7. The present time exhibits a vegetation of higher mor- 

 phological and physiological development than in the pre- 

 ceding ages ; besides, the species are exceedingly numerous. 

 All countries are not yet fully explored and there is no means, 

 therefore, of knowing the number of species of plants which 

 exist. Even in countries long known new species, especially 

 of the lower plants, are constantly being found. It is certain 

 that there are at least nearly three hundred thousand species. 

 The great majority are limited to a more or less restricted area 

 of the globe, yet each species manifests a disposition to spread 

 itself as widely as possible. This may, in fact, be regarded as 

 a further expression of the fundamental instinct of race-per- 

 petuation, which naturally follows the primary instinct of 

 self-preservation ; that is, as in the earlier stages of the indi- 

 vidual the struggle is for existence, necessitated by the environ- 

 ment; so later the effort, equally imperative, is to perpetuate 

 the kind. The adaptations for migration of the species are 

 seen plainly in the contrivances for wide dispersion of the 

 seed, an account of which has been given in the latter part of 

 Chapter IX. 



8. The barriers to indefinite migration of the species are 

 high mountain ranges, great bodies of water, character of the 

 soil, amount and distribution of rain-fall, variation in temper- 

 ature, and in case of parasites, absence of the proper host- 

 plants. Some species are able to migrate over considerable 

 elevations of land, a few travel over wide areas of water, but 

 these are impassable barriers to the great majority of plants. 

 Some species thrive only on calcareous soils, others are con- 

 fined to arenaceous or other kinds. Aquatic and terrestrial 

 plants each demand and cannot transcend their proper hab- 

 itat. Some terrestrial species are limited to sheltered and moist 

 regions, others grow only in exposed situations. Many are 

 peculiar to prairie regions, others inhabit deserts. The limits 



