VARIATION AND DISTRIBUTION OF SPECIES. 141 



carried out to its ultimate development only perhaps by 

 Agassiz, in the denial of any necessary genetic connection 

 among the individuals of the same species, or of any original 

 localization more restricted than the area now occupied by 

 the species. But in 1855, in his " Geographie Botanique," 

 the multiple hypothesis, although in principle not abandoned, 

 is seen to lose its point, in view of the probable high antiquity 

 of existing species. The actual vegetation of the world being 

 now regarded as a continuation, through numerous geological, 

 geographical, and more recently historical, changes, of ante- 

 rior vegetations, the actual distribution of plants is seen to be 

 a consequence of preceding conditions and geological consid- 

 erations, 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 distribution but the origin of congeneric 

 species is regarded 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 Candolle 

 hazards no opinion. It may, however, be inferred that he 

 looks upon " natural selection " (which he rather underrates) 

 as a real, but insufficient, cause ; while some curious remarks 

 (pp. 57, 58), upon the number of monstrosities annually pro- 

 duced, 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 direction re- 

 ferred to, it will be interesting to compare Sir Charles Lyell's 

 well-known chapters of twenty or thirty years ago, in winch 

 the permanence of species was ably maintained, with his treat- 

 ment of the same subject in a work just issued in England, 

 which, however, has not yet reached us. 



A belief in the derivation of species may be maintained 

 along with a conviction of great persistence of specific char- 

 acters. This is the idea of the excellent Swiss vegetable palae- 

 ontologist Heer, who imagines a sudden change of specific 

 type at certain periods, and perhaps is that of Pictet. Fal- 

 coner adheres to somewhat similar views in his elaborate 



