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134 know or believe to have had such a common descent. The specific identity of two or more individuals admits, therefore, but very rarely of positive proof; we must judge of it by inductive evidence, selecting by the careful consideration of what characters are known, especially in allied species, to remain permanent generation after generation, unaltered by change of soil, climate, or other circumstances, and what are the variations occasioned by causes which we can appreciate, or which are known to occur without assignable cause. The conclusions to be derived from such evidence will not, indeed, always be decisive, and different persons will often form different judgments ; but that is an unavoidable consequence of the imperfection of the human mind.

My own attention was first directed to the variations to which plants of the same species are liable, under different circumstances, in the year 1820. I had then become tolerably familiar with the common plants of France, in the West, in Upper Languedoc, and in the central Pyrenees; and, settling for some years in the neighbourhood of Montpelier, I was struck with the different aspect assumed by several of the same species in this very different soil and climate. In the first instance, I did indeed believe that many of these were representative, not identical species; but I could not but observe even then that, in many cases, species really the same underwent considerable modifications, through the influence of soil and climate. In 1823 I collected, with my friend, Dr. Arnott, a considerable number of Scotch specimens, which, two years later, after our Pyrenean tour, we had the opportunity of comparing with a similar vegetation grown in mountains of twice or thrice the elevation of the Scotch ones, but under a difference of latitude of 12 to 13 degrees; whilst, on the same Pyrenean chain, we were several times struck with the differences exhibited by plants of the same species growing in the cool northern, or the hot southern valleys. In 1821, on my father's estate near Montpelier, a considerable extent of the botanically rich waste lands, called garrigues, was walled in, to allow the natural wood to grow up; and, during the few succeeding years, I could observe a gradual, but in many instances very striking, change take place in the character and aspect of the wild plants protected by the enclosure. In 1837, when at Trieste, I visited a similar enclosure, on a larger scale, and of many years' standing, at Lippiza, near that town, and observed very marked differences in the individuals of some species, when growing within or without these walls. From the time, indeed, when I first began to collect notes on the vegetation of Southern Europe, some of which I embodied in my "Catalogue des Plantes des Pyrenées et du Bas-Languedoc," published in 1826, my attention has been much directed to the modifications of specific types, in all my herborisations in various parts of Europe, and, more especially, in the mountains of Scotland, the Pyrenees, Central France, and Tyrol; in the lower hills and plains of France, Britain, Sweden, Germany, Italy, and Sicily; and on the coasts of Britain, Western and Southern France, and various parts of the Mediterranean, Adriatic, and Black Seas. The preparation of large collections for distribution has given me opportunities of studying