518 X. GENERAL REMARKS. ' . 



a few natural groups, based on some important resem- 

 blances in their distribution, with some important dif- 

 ferences to distinguish one group from another. The 

 resemblances on which the types have been formed for 

 Britain are those of site and direction. A considerable 

 number of species either restricted to, or radiating /rom, 

 the same geographic site, are considered to constitute 

 a true type of distribution ; those absent from, or ra- 

 diating into, that site, will belong to one or more other 

 types. 



Such eclectic groups will doubtless always bear an 

 intimate relation to the climate and other conditions of 

 physical geography in the country where they are found 

 or formed. — The direction of their decrease may not un- 

 reasonably be held to suggest that of their immigration 

 into the country, if immigrants ; though there seems 

 much likelihood of many important exceptions to the 

 general accuracy and applicability of the suggestion. 



To suppose that groups thus distinguishable from 

 each other bj'^ present distribution, are also distinct by 

 enormous differences of age, is a flight of imagina- 

 tion, not necessarily unsound, but as yet supported 

 on no basis of sufificientl}'^ reliable facts in evidence of 

 its soundness. 



To suppose that the present occurrence of a dozen or a 

 score of species on the hills of two distant countries, at 

 present disjoined -by an intervening ocean, is proof suffi- 

 cient of former junction, seems quite beyond the bounds 

 of legitimate reasoning. It implies that all lands have 

 been joined to all other lands, at one time or other, since 

 existing species appeared on the earth ; A to B, B to C, 

 C to D, and so on. — It all-but-unavoidably implies also 

 that various species were formerly able to endure cli- 

 mates very dissimilar from the climates to which those 



