EXTERNAL FACTORS. 505 



competing kinds of plants as are not immediately influenced 

 in the same way. The animals which eat the seeds or browse 

 on the leaves, either of the plant primarily affected or those of 

 its competitors, are severally altered in their states of nutri- 

 tion and in their numbers; and this change presently tells 

 on various predatory animals and parasites. And since each 

 of these secondary and tertiary changes becomes itself a centre 

 of others, the increase or decrease of each species produces 

 waves of influence which spread and reverberate and re- 

 reverberate throughout the whole Flora and Fauna of the 

 locality. 



More marked and multiplied still, are the ultimate effects 

 of those causes which make possible the colonization of neigh- 

 bouring areas. Each intruding plant or animal, besides the 

 new inorganic conditions to which it is subject, is subject to 

 organic conditions different froni those to which it has been 

 accustomed. It has to compete with some organisms unlike 

 those of its preceding habitat. It must preserve itself from 

 enemies not before encountered. Or it may meet with a 

 species over which it has some advantage greater than any 

 it had over the species it was previously in contact with. 

 Even where migration does not bring it face to face with 

 new competitors or new enemies or new prey, it inevitably 

 experiences new proportions among these. Further, an ex- 

 panding species is almost certain to invade more than one 

 adjacent region. Spreading both north and south, or east 

 and west, it will come among the plants and animals, here of 

 a level district and there of a hilly one — here of an inland 

 tract and there of a tract bordered by the sea. And while 

 different groups of its members will thus expose themselves 

 to the actions and reactions of different Floras and Faunas, 

 these different Floras and Faunas will simultaneously have 

 their organic conditions changed by the intruders. 



This process becomes gradually more active and more 

 complicated. Though, in particular cases, a plant or animal 

 may fall into simpler relations with the living things around 



