380 EVOLUTION [CHAP. V 



Letter 290 breeds which are changed through " unconscious selection " 

 by man. 1 



When such great lengths of time are considered as are 

 necessary to change a specific form, I greatly doubt whether 

 more or less rapid powers of multiplication have more than 

 the most insignificant weight. These powers, I think, are 

 related to greater or less destruction in early life. 



P. Ixxix. I still think you rather underrate the import- 

 ance of isolation. I have come to think it very important 

 from various grounds ; the anomalous and quasi-extinct 

 forms on islands, etc., etc., etc. 



With respect to areas with numerous " individually 

 durable ' forms, can it be said that they generally present 

 a " broken ' surface with " impassable barriers " ? This, no 

 doubt, is true in certain cases, as Teneriffe. But does this 

 hold with South-W'est Australia or the Cape ? I much 

 doubt. I have been accustomed to look at the cause of so 

 many forms as being partly an arid or dry climate (as 

 De Candolle insists) which indirectly leads to diversified [?] 

 conditions ; and, secondly, to isolation from the rest of the 

 world during a very long period, so that other more dominant 

 forms have not entered, and there has been ample time for 

 much specification and adaptation of character. 



P. lxxx.--I suppose you think that the Restiaceae, 

 Proteaceas, 2 etc., etc., once extended over the world, leaving 

 fragments in the south. 



You in several places speak of distribution of plants as if 

 exclusively governed by soil and climate. I know that you 

 do not mean this, but I regret whenever a chance is omitted 

 of pointing out that the struggle with other plants (and hostile 

 animals) is far more important. 



I told you that I had nothing worth saying, but I have 

 given you my thoughts. 



1 Bentham had said : " We must also admit that every race has 

 probably been the offspring of one parent or pair of parents, and con- 

 sequently originated in one spot." The Duke of Argyll inverts the 

 proposition. 



2 It is doubtful whether Bentham did think so. In his 1870 address 

 he says : " I cannot resist the opinion that all presumptive evidence is 

 against European Proteaceas, and that all direct evidence in their favour 

 has broken down upon cross-examination." 



