24 MISSISSIPPI STATE GEOLOGICAL SURVEY [Bull. 
beeches and maples in another, and perhaps pine in still an- 
other part. These differences are due to soil variation, which 
will be noticed more fully in the description of the vegeta- 
tional regions of Mississippi. While so far as broad climatic 
influences are concerned, our whole state lies within a wood- 
land formation, diversity of soil conditions has greatly modi- 
fied the results, giving us a checkered and varied vegetation. 
Biotic Factors.—These include the influence of plants up- 
on the growth and distribution of other plants, either of the 
same species or of different species. Many more seeds begin 
growth than ever become mature plants; overcrowding brings 
about a contest among them for space and soil moisture, 
resulting in a weeding out of the weaker, and a ‘‘survival of 
the fittest.”’ New species sometimes invade an area preoccu- 
pied by already established forms, and a struggle begins. If 
the invader is more virile and better adapted to the conditions, 
it eventually drives out the previously established forms. 
So also, animal influences are important in plant distribu- 
tion. Many seeds that would otherwise grow, are destroyed 
by squirrels, seed-eating birds, or by insects; or they may be 
attacked by moulds and their vitality destroyed; or the ma- 
ture plants may themselves be destroyed by parasitic fungi. 
On the other hand many seeds are distributed into new regions 
by birds, squirrels, or by other hairy animals, to the coats of 
which many fruits. provided with hooklets attach themselves, 
the cocclebur being a familiar example. 
Man, himself, is at present perhaps the most active factor 
in influencing plant distribution, especially of certain useful 
species. For his own use he has removed the forests from 
immense areas of the earth; he has started forest fires which 
have destroyed the native growth from other areas; he has 
cultivated the soil in every zone except the Polar Zones, ana 
so disturbed the natural balance of vegetable life that in 
the cultivated areas hosts of exotic weeds have come in and 
crowded out the native species, to say nothing of the pampered 
cultural forms, including cereals, corn, potatoes, fruits, tex- 
tile crops, and numerous others, which he maintains over vast 
areas of the best soils, to the exclusion of the native growth. 
