160 ON BIO-GEOLOGY. [vii. 



though there is plenty of food for them outside it, 

 simply because they do not increase fast enough to 

 require to spread out in search of more food. Thus I 

 should explain a case which I heard of lately of Antho- 

 cera trifolii, abundant for years in one corner of a 

 certain .field, and only there; while there was just as 

 much trefoil all round for its larvae as there was in the 

 selected spot. I can, I say, only give hints : but they 

 will suffice, I hope, to show the path of thought into 

 which I want young naturalists to turn their minds. 



Or, again, you will have to inquire whether the 

 species has not been prevented from spreading by some 

 natural barrier. Mr. Wallace, whom you all of course 

 know, has shown in his "Malay Archipelago " that a 

 strait of deep sea can act as such a barrier between 

 species. Moritz Wagner has shown that, in the case 

 of insects, a moderately-broad river may divide two 

 closely-allied species of beetles, or a very narrow 

 snow-range, two closely-allied species of moths. 



Again, another cause, and a most common one, is : 

 that the plants cannot spread because they find the 

 ground beyond them already occupied by other plants, 

 who will not tolerate a fresh mouth, having only just 

 enough to feed themselves. Take the case of Saxifraga 

 hypnoides and 8. umbrosa, " London pride." They 

 are two especially strong species. They show that, 

 S. hypnoides especially, by their power of sporting, 

 of diverging into varieties ; they show it equally by 

 their power of thriving anywhere, if they can only get 

 there. They will grow both in my sandy garden, under 

 a rainfall of only 23 inches, more luxuriantly than in 

 their native mountains under a rainfall of 50 or 60 

 inches. Then how is it that S. hypnoides cannot get 



