EXTERNAL FACTOKS. 417 



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 localitj^. 



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 considerably different from those to which 

 it has been habituated. It has to compete with some organ- 

 isms 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 that it had over the species it was previously lq 

 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 expanding species is almost certain to invade 

 more than one adjacent region. Spreading north or south, 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 differ- 

 ent groups of its members will thus expose themselves to 

 the actions and re-actions of different Floras and Faunas, 

 these different Floras and Faunas vdR 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, 

 than those it was before placed in ; yet it is manifest thut, 



