148 ARE SOILS ENRICHED, IMPOVERISHED, pt. in. 



existence is apparently confided to no second causes, 

 but hangs on the fiat of his Ahnighty Creator alone. 



In reference to species bordering on distinct stations 

 Lyell writes : ' In almost every district, especially if it be 

 mountainous, there are a variety of species the limits of 

 whose habitations are conterminous, some being unable 

 to proceed further without encountering too much 

 heat, others too much cold. Individuals which are 

 thus on the borders of the regions proper to their 

 respective species are like the outposts of hostile armies, 

 ready to profit by every slight change of circumstances 

 in their favour, and to advance upon the ground occu- 

 pied by their neighbours and opponents. The prox- 

 imity of distinct climates produced by the inequalities 

 of the earth's surface, brings species possessing very 

 different constitutions into such immediate contact 

 that their naturalisations are very speedy whenever 

 opportunities of advancing present themselves.' Now, 

 these opponents^ these outposts of hostile armies^ ' pos- 

 sessing yerj different constitutions^' and natives of 

 distinct stations^ might, from their perpetual propin- 

 quity, be called by physiologists ' social , plants ; ' as 

 those of the same station are called whose perpetual 

 propinquity is caused by similarity of constitution^ or by 

 any of the many other causes of propinquity. 



This perpetual propinquity physiologists have attri- 

 buted to the inclination of the plants each for the other, 

 instead of both for the soil, or instead of both for the 

 conditions of vegetable hfe existent at the spot : such 



