CHECKS ON COLONIZATION. 167 



to the new country have established a predominance 

 over the native species ; but I question whether, if the 

 course of history had been different, and the con- 

 querors of South America had come from South 

 Africa or South Austraha, bringing with them seeds 

 of those regions, we should not have seen in Patagonia 

 African or Australian plants in the place of the Euro- 

 pean thistles and other weeds now so widely spread. 



If I am not much mistaken as to the history of the 

 introduction of foreign plants into new regions, it very 

 commonly happens that a species which spreads very 

 widely at first becomes gradually restricted in its area, 

 and finally loses the predominance which it seemed 

 to have established. Attention has not, I think, been 

 .suflficiently directed to the fact that the chief limit to 

 the spread of each species is fixed by the prevalence 

 of the enemies to which it is exposed, and that plants 

 carried to a distant region will, as a general rule, enjoy 

 advantages which in the course of time they are likely 

 to lose. Whether it be large animals that eat down 

 the stem — as goats prevent the extension of pines — 

 or birds that devour the fruit, or insects that attack 

 some vital organ, or vegetable parasites that dis- 

 organize the tissues, the chances are great that in a 

 new region the species will not find the enemies that 

 have been adapted to check its extension in its native 

 home. Of the marvellous complexity of the agencies 

 that interact in the life-history of each species we 

 first formed some estimate through the teachings of 

 Darwin ; but to follow out the details in each case will 

 be the work of successive generations of naturalists. 

 We cannot doubt that in a new reijion new enemies 



