vii.] HOW DID THEY GET THERE? 159 



lower than 2000 feet, and is not plentiful even there? 

 it will reply Because in the Craven I can get as much 

 carbonic acid as I want from the decomposing lime- 

 stone ; while on the Snowdon Silurian I get very little ; 

 and I have to make it up by clinging" to the mountain 

 tops, for the sake of the greater rainfall. But if you 

 ask Polypodium calcareum How is it you choose only 

 to grow on limestone, while Polypodium Dryopteris, of 

 which, I suspect, you are only a variety, is ready to 

 grow anywhere ? Polypodium calcareum will refuse, 

 as yet, to answer a word. 



Again I can only give you the merest string of 

 hints you will find in your questionings that many 

 plants and animals have no reason at all to show why 

 they should be in one place and not in another, save 

 the very sound reason for the latter which was 

 suggested to me once by a great naturalist. I was 

 asking Why don't I find such and such a species in my 

 parish, while it is plentiful a few miles off in exactly 

 the same soil ? and he answered For the same reason 

 that you are not in America. Because you have not 

 got there. Which answer threw to me a flood of light 

 on this wiole science. Things are often where they are, 

 simply because they happen to have got there, and not 

 elsewhere. But they must have got there by some 

 means, and those means I want young naturalists to 

 discover; at least, to guess at. 



A species, for instance and I suspect it is a common 

 case with insects may abound in a single spot, simply 

 because, long years ago, a single brood of eggs happened 

 to hatch at a time when eggs of other species, who 

 would have competed against them for food, did not 

 hatch ; and they may remain confined to that spot, 



