Variation, Geographical Distribution, and Succession. 89 



of the current are seen by noticing how it carries along natu- 

 rahsts of widely different views and prepossessions, some faster 

 and further than others, but all in one way. The tendency is, 

 we may say, to extend the law of continuity, or something ana- 

 logous to it, from inorganic to organic nature, and in the latter 

 to connect the present with the past in some sort of material 

 connexion. The generalization may, indeed, be expressed so as 

 not to assert that the connexion is genetic, as in INIr. Wallace^s 

 formula: "Every species has come into existence coincident 

 both in time and space with preexisting closely allied species.^' 

 Edward Forbes, who may be called the originator of this whole 

 line of inquiry, long ago expressed a similar view. But the only 

 material sequence we know, or can clearly conceive, in plants and 

 animals is that from parent to progeny ; and, as DeCandolle im- 

 plies, the origin of species and that of races can hardly be much 

 unlike, nor governed by other than the same laws, whatever 

 these may be. 



The progress of opinion upon this subject in one generation 

 is not badly represented by that of DeCandolle himself, who is 

 by no means prone to adopt new views without much considera- 

 tion. In an elementary treatise, published in the year 1835, he 

 adopted and, if we rightly remember, vigorously maintained, 

 Schouw's idea of the double or multiple origin of species, at 

 least of some species — a view which has been carried out to its 

 ultimate development only perhaps by Agassiz, in the denial of 

 any necessary genetic connexion 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 ' Geo- 

 graphic Botanique,^ the multiple hypothesis, although in prin- 

 ciple not abandoned, is seen to lose its point, in view of the 

 probable high antiquity of existing species. The actual vegeta- 

 tion of the world being now regarded as a continuation, through 

 numerous geological, geographical, and more recently historical 

 changes, of anterior vegetations, the actual distribution of plants 

 is seen to be a consequence of preceding conditions and geo- 

 logical considerations ; and these alone may be expected to ex- 

 plain 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 con- 

 generic 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, DeCandolle 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 



