406 GEOGRAPHICAL DISTRIBUTION [CHAP. VI 



Letter 315 oceans, that one-fifth less distance, say 4,000 miles instead 

 of 5,000, can explain or throw much light on a material 

 difference in the degree of similarity in the floras of the two 

 regions ? 



I trust you will work out the New Zealand flora, as you 

 have commenced at end of letter : is it not quite an original 

 plan ? and is it not very surprising that New Zealand, so much 

 nearer to Australia than South America, should have an inter- 

 mediate flora ? I had fancied that nearly all the species there 

 were peculiar to it. I cannot but think you make one 

 gratuitous difficulty in ascertaining whether New Zealand 

 ought to be classed by itself, or with Australia or South 

 America — namely, when you seem (bottom of p. 7 of your 

 letter) to say that genera in common indicate only that the 

 external circumstances for their life are suitable and similar. 1 

 Surely, cannot an overwhelming mass of facts be brought 

 against such a proposition ? Distant parts of Australia possess 

 quite distinct species of marsupials, but surely this fact of 

 their having the same marsupial genera is the strongest tie 

 and plainest mark of an original (so-called) creative affinity 

 over the whole of Australia ; no one, now, will (or ought) to 

 say that the different parts of Australia have something in 

 their external conditions in common, causing them to be pre- 

 eminently suitable to marsupials ; and so on in a thousand 

 instances. Though each species, and consequently genus, 

 must be adapted to its country, surely adaptation is manifestly 

 not the governing law in geographical distribution. Is this 

 not so ? and if I understand you rightly, you lessen your own 

 means of comparison — attributing the presence of the same 

 genera to similarity of conditions. 



You will groan over my very full compliance with your 

 request to write all I could on your tables, and I have done it 

 with a vengeance : I can hardly say how valuable I must think 

 your results will be, when worked out, as far as the present 

 knowledge and collections serve. 



1 On Dec. 30th, 1S44, Sir J. D. Hooker replied, " Nothing was 

 further from my intention than to have written anything which would lead 

 one to suppose that genera common to two places indicate a similarity in 

 the external circumstances under which they are developed, though I see 

 I have given you excellent grounds for supposing that such were my 

 opinions." 



