LTNNliAN SUCIEXX OF LONDON. XXXVi 



but in islands which have a peculiar vegetation it is less pronounced 

 than in continents. Prom any one point climate is gradually altered, 

 like the radii of a circle, which gradually diverge more and more from 

 each other from the centre to the circumference. In a continent 

 the whole area of the circle may be supposed to be suited to the pro- 

 duction of changes in organization ; in an archipelago it is inter- 

 rupted by the sea, and here, therefore, few similar species have 

 arisen. Another consideration to be taken into account is, that 

 genera when compared with each other are unequally susceptible of 

 change (veranderungsfahig) ; their species, therefore, to keep to the 

 same metaphor, will be found arranged at greater or less distances 

 from each other in the radii of the circle. If the area of the con- 

 tinuous land is small, monotypes will have more readily arisen — ge- 

 nera which, on the one hand, are verj' little or not at aU susceptible 

 of change, and on the other hand can no longer subsist with a certain 

 degree of cHmatological change. If in a more remote geographical 

 distance the more important climatological conditions which these 

 genera require are repeated, we may perhaps find in another part of 

 the globe a second species ; and this generally explains the origin of 

 the species which have been termed representative (vikariirende 

 Arten). A precisely similar climate, however (exactly the same com- 

 plication of the very varied phenomena towards which organisms 

 bear themselves receptively), is never repeated in two distant points 

 of the earth's surface ; and this may be taken as the foundation of the 

 absolute unity of centres of vegetation — that is to say, of the proposi- 

 tion that every species in its wanderings has issued from a single 

 birthplace, which does not exclude the possibility of solitary excep- 

 tions which might be imagined in plants of less receptivity," 



In all this it appears to me that if the writer refuses to admit of 

 a descent from a common parent, we have a right to ask of him 

 what is the previous organization upon which he imagines climate 

 to have worked to produce allied species in one region and representa- 

 tive species in distant regions ? — what are the previous genera which 

 have changed? for upon that seems to hinge the whole of his argument 

 in refutation of Asa Gray's hypothesis explanatory of the original 

 connexion between the East- Asiatic and East- American floras. That 

 every species had arisen in one spot, whether by differentiation or 

 by creation, appears now to be tacitly admitted by all. Asa Gray, 

 in accordance with Darwinian theories, supposes widely spread spe- 

 cies to have been, under the different conditions of distant lands, 

 gradually modified in different directions, so as to have produced 

 distinct varieties or representative species ; Grisebach supposes these 



