CHAP. XXI. LIMITATION OF SPECIES. 419 



species of plant, nor for the amount of variation it has 

 undergone, nor will it indicate the time Avheu it first ap- 

 peared, nor the form it had when created.* 



To what an extent the limits of species are indefinable, is 

 evinced, he says, by the singular fact that, among those 

 botanists who believe them to be immutable, the number of 

 flowering plants is by some assumed to be 80,000, and by 

 others over 150,000. The general limitation of species to 

 certain areas suggests the idea that each of them, with all 

 their varieties, have sprung from a common parent, and have 

 spread in various directions from a common centre. The 

 frequency also of the grouping of genera within certain 

 geographical limits is in favor of the same law, although 

 the migration of species may sometimes cause apparent 

 exceptions to the rule, and make the same types apj)ear to 

 have originated independently at different spots.f 



Certain genera of plants, which, like the brambles, roses, 

 and willows in Europe, consist of a continuous series of 

 varieties, between the terms of which no intermediate forms 

 can be intercalated, may be supposed to be on the increase, 

 and therefore undergoing much variation ; whereas genera 

 which present no such perplexing gradations ma}^ be those 

 which have been losing species and varieties by extinction. 

 The annihilation of the intermediate forms which ones 

 existed, makes it an easy task to distinguish those which 

 remain. 



It had usually been supposed by the advocates of the 

 immutability of species, that domesticated races, if allowed to 

 run wild, always revert to their parent type. Mr. Wallace 

 had said, in reply, that a domesticated species, if it loses the 

 protection of man, can only stand its ground in a wild state 

 by resuming those habits and recovering those attributes 



* Hooker, Introductory Essay, Flora of Australia. t Ibid. p. 13. 



