192 BARWimANA. 



continuation, through numerous geological, geograplii- 

 cal, and more recently historical changes, of anterior 

 vegetations, the actual distribution of plants is seen to 

 be a consequence of preceding conditions ; and geologi- 

 cal considerations, and these alone, may be expected 

 to explain all the facts — many of them so curious and 

 extraordinary — of the actual geographical distribution 

 of the species. In the present essay, not' only the dis- 

 tribution but the origin of congeneric species is re- 

 garded as something derivative ; whether derived by 

 slow and very gradual changes in the course of ages, 

 according to Darwin, or by a sudden, inexplicable 

 change of their tertiary ancestors, as conceived by 

 Heer, De CandoUe hazards no opinion. It may, how- 

 ever, be inferred that he looks upon " natural selection " 

 as a real, but insufficient cause ; while some curious 

 remarks upon the number of monstrosities annually 

 produced, and the possibility of their enduring, may 

 be regarded as favorable to Heer's view. 



As an index to the progress of opinion in? the di- 

 rection referred to, it will be interesting to compare 

 Sir Charles Ly ell's well-known chapters of twenty or 

 thirty years - ago, in which the permanence of species 

 was ably maintained, with his treatment of the same 

 subject in a work just issued in England, which, how- 

 ever, has not yet reached us, 



A belief of the derivation of species may be main- 

 tained along with a conviction of great persistence of 

 specific characters. This is the idea of the excellent 

 Swiss vegetable palseontologist, Heer, who imagines 

 a sudden change of specific type at certain periods, 

 and perhaps is that of Pictet. Falconer adheres to 



