MECHANICS AND USEFUL AKTS. 31 



united. Nor is this mode of explanation limited to the case of the above 

 regions; for in the "Flora Indica." Dr. Hooker, in conjunction with his 

 fellow traveller, Dr. Thomson, has discussed the same problem with regard to 

 the whole of India, extending from Afghanistan to the Malayan peninsula. 

 And amongst the many services rendered to the Natural Sciences by these 

 indefatigable botanists, one of the greatest I conceive to be, that they have 

 not only protested against that undue multiplication of species, which had 

 taken place by exalting minute points of difference into grounds of radical and 

 primary distinction, but that they have also practically illustrated their views 

 with respect to the natural families which have been described by them in the 

 volume alluded to. They have thus contributed materially to remove another 

 difficulty which stood in the way of the adoption of the theory of specific 

 centres, I mean the replacement of forms of vegetation in adjoining coun- 

 tries by others, not identical, but only as it should seem allied : for it follows 

 from the principles laid down by these authors, that such apparently distinct 

 species may after all have been only varieties, produced by the operation 

 of external causes acting upon the same species during the long periods of 

 time. 



But if this be allowed, what limits, it may be asked, are we to assign to 

 the changes which a plant is capable of undergoing, and in what way can 

 we oppose the principle of the transmutation of species, which has of late 

 excited so much attention, and the admission of which is considered to in- 

 volve such startling consequences ! I must refer you to the writings of 

 modern physiologists for a full discussion of this question. All that I shah 1 

 venture to remark on the subject is, that had not Nature herself assigned 

 certain boundaries to the changes which plants are capable of undergoing, 

 there would seem no reason why any species at all should be restricted 

 within a definite area, since the unlimited adaptation to external conditions 

 which it would then possess, might enable it to diffuse itself throughout the 

 world, as easily as it has done over that portion of space within which it is 

 actually circumscribed. Dr. Hooker instances certain species of Coprosma, of 

 Celmisia, and a kind of Australian Fern, the Lamaria procera. which have 

 undergone such- striking changes in their passage from one portion of the 

 Great Pacific to another, that they are scarcely recognisable as the same, 

 and have actually been regarded by preceding botanists as distinct species. 

 But he does not state that any of these plants have ever been seen beyond 

 the above-mentioned precincts ; and yet if Nature had not imposed some 

 limits to their susceptibility of change, one does not see why they might not 

 have spread over a much larger portion of the earth, in a form more or less 

 modified by external circumstances. The younger De Candolle, in his late 

 admirable treatise, has enumerated about 117 species of plants which have 

 been thus diffused over at least a third of the surface of the globe, but these 

 apparently owed 4heir power of transmigration to their insusceptibility of 

 change, for it does not appear that they have been much modified by the 

 effect of climate or locality, notwithstanding the extreme difference in the 

 external conditions to which they were subjected. On the other hand, it 

 seems to be a general law that plants whose organization is more easily 



