many American, African, and Asiatic species in large masses of specimens. In some of the more important monographs I have worked up, I have been enabled to compare the materials of the principal herbaria of Europe; and, since my working-stock has been transferred to Kew, the daily consultation of such a collection as that of Sir William Hooker has contributed very much to confirm my ideas as to the variability and limitation of species; and nothing more so than the extensive and highly instructive series brought from India by Drs. Hooker and Thomson, and the numerous accurate and judicious notes and memoranda so liberally communicated by Dr. Hooker. When, therefore, I speak of having observed a series of specimens collected in various parts of the geographical area of a species, I do not mean (as has been hinted) the examination of a few single specimens from different localities deposited in a herbarium, but the observation of a species in a living wild state in different countries, or the comparison of numerous specimens, either promiscuously collected, or selected, with notes, for the purpose of illustrating variations.
And here I would observe, that the use of herbaria in determining the extent of variability of species requires the greatest caution. Not only are the specimens preserved generally unaccompanied by any notes on the comparative frequency of the form gathered, and others closely resembling it, or on any other local circumstances affecting the question, but they are very likely to lead the botanist astray in these particulars. A collector is naturally struck by a plant differing in aspect from the generality of its species, and gathers it in preference to the forms more familiar to him. The consequence is, that it frequently happens that an accidentally abnormal variety, which may occur only once in the way in nature, having been cut up into a number of specimens, and distributed without notes to various botanists, has, from its presence in so many herbaria, all the appearance of a form abundant in the locality cited.
The experience I have thus obtained has gradually produced in my mind a conviction of the truth of the following axioms:—
Every species has certain determinate limits of variation, which it only exceeds under exceptional circumstances.
The exceptionally abnormal forms thus produced are few in individuals, and are not reproduced; or their race becomes gradually extinguished, when the causes which produce them cease.
Within these limits of variation, a species will, in some countries, or under certain circumstances, produce an indefinite number of individual, or more or less permanent varieties, often passing into each other by almost imperceptible gradations; whilst, in other countries, or under other circumstances, a certain number of these varieties or races will remain, generation after generation, marked by positive, distinctive characters, having at first sight the appearance of real species.
Plants of the same species often breed freely together, the crossbreeding of different individuals sometimes producing a more vigorous offspring than those sprung from a single flower, and being, perhaps, oc-
