BRITISH ASSOCIATION TOE THE ADVANCEMENT OE SCIENCE. 57 



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 Aus- 

 tralian Fern, the Lomaria procera, which have undergone such striking changes 

 in their passage from one portion of the Great Pacific to another, that they are 

 scarcely recognizable 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 the suscptibility 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 Caudolle, in his late ad- 

 mirable treatise already referred to, 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 their 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 affected by external agencies become 

 from that very cause, more circumscribed in their range of distribution ; simply 

 because a greater difference in the circumstances under which they would be placed 

 brought with it an amount of change in their structure which exceded the limits 

 prescribed to it by Nature. In short, without pretending to do more than to divine 

 the character of those impediments, which appear ever to prevent the changes of 

 which a plant is suceptible from proceeding beyond a certain limit, we seem to catch 

 a glimpse of a general law of Nature, not limited to one of her kingdoms, but ex- 

 tending everywhere throughout her jurisdiction, — a law, the aim of which may be 

 inferred to be that of maintaining the existing order of the universe, without any 

 material or permanent alteration, throughout all time, until the fiat of Omnipotence 

 has gone forth for its destruction. The will which confines the variations in the 

 vegtable structure within a certain range, lest the order of creation should be die- 

 turbed by the introduction of an indefinite number of intermediate forms is appa- 

 rently the same in its motive as that which brings back the celestial luminaries to their 

 original orbits, after the completion of a cycle of changes induced by their mutual 

 perturbations ; it is the same which says to the ocean, Thus far shalt thou go, and 

 no further ; and to the winds, Your violence, however apparently capricious and 

 abnormal, shall nevertheless be constrained within certain prescribed limits — 



Ni faciat, maria et terras ccelurnque profundum, 

 Quippe ferant rapidi secum, verrantque per auras. 



The whole, indeed, resolves itself into, or at least is intimately connected with, that 

 law of symmetry to which Nature seems ever striving to conform, and which possesses 

 the same significance in the organic world, which the law of definite proportions 

 does in the inorganic. It is the principle which the prophetic genius of Goethe 

 had divined, long before it had been proved by the labours of physiologists to be 

 a reality, and to which the poet attached such importance, that the celebrated dis- 

 cussion as to its merits which took place in 1830 between Cuvier and GeofFry St. 

 Hilaire so engrossed his mind, as to deprive him, as his biographer informs us, of 

 all interest in one of the most portentous political events of modern days which waa 

 enacting at the very same epoch, — I mean the subversion of the Bourbon dynasty. 



