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Updated: June 10, 2025
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.
We see the same fact in ascending mountains, and sometimes it is quite remarkable how abruptly, as Alph. De Candolle has observed, a common alpine species disappears. The same fact has been noticed by E. Forbes in sounding the depths of the sea with the dredge.
Such characters are not of course of specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally enter into specific definitions. De Candolle then goes on to say that he gives the rank of species to the forms that differ by characters never varying on the same tree, and never found connected by intermediate states.
The fact of the larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit could hardly be transported by any other means; and Alph. de Candolle has shown that such plants generally have restricted ranges. But seeds may be occasionally transported in another manner.
In certain Umbelliferae the exterior seeds, according to Tausch, are orthospermous, and the central one coelospermous, and this is a character which was considered by De Candolle to be in other species of the highest systematic importance.
This is strikingly shown, as remarked by Alph. de Candolle, in large groups of terrestrial plants, which have only a very few aquatic members; for these latter seem immediately to acquire, as if in consequence, a very wide range. I think favourable means of dispersal explain this fact.
De Candolle has well remarked in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species. To give a single instance: in the last edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United States, 260 naturalised plants are enumerated, and these belong to 162 genera.
But M. Alphonse de Candolle quite as high authority as we can quote has come to the conclusion that marine currents, and all other suggested means of distant transportation, "have played only a very small part in the actual dispersion of species," even across narrow channels and the near arms of seas.
The Spaniard withdrew from Spain, bringing with him his botanical treasure, and took refuge at Marseilles, where he met De Candolle, who, on looking over his Mexican flora, said it was admirably well done for Mexicans, who had no access to European books, and he pointed out its deficiencies; they worked at it for eighteen months, when De Candolle was to return to Geneva, and the Spaniard said to him, "Take the book as far as I am concerned, I give it to you, but if my government should reclaim it, you will let me have it."
Such characters, of course, are not specific, although many of them are such as would have been expected to be constant in the same species, and are such as generally enter into specific definitions. Variations of this sort, De Candolle, with his usual painstaking, classifies and tabulates, and even expresses numerically their frequency in certain species.
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