422 GEOGRAPHICAL DISTRIBUTION [Chap. VI 



Letter 324 almost any European flora as two, three, or more, in a very 

 considerable number of cases. 



In boldly reducing nominal species J. Hooker is doing a 

 good work ; but his vocation — like that of any other reformer 

 — exposes him to temptations and dangers. 



Because you have shown that a and b arc so connected by 

 intermediate forms that we cannot do otherwise than regard 

 them as variations of one species, we may not conclude that c 

 and d, differing much in the same way and to the same degree, 

 are of one species, before an equal amount of evidence is actually 

 obtained. That is, when two sets of individuals exhibit any 

 grave differences, the burden of proof of their common origin 

 lies with the person who takes that view ; and each case must 

 be decided on its own evidence, and not on analogy, if our 

 conclusions in this way are to be of real value. Of course we 

 must often jump at conclusions from imperfect evidence. I 

 should like to write an essay on species some day ; but before 

 I should have time to do it, in my plodding way, I hope you 

 or Hooker will do it, and much better far. I am most glad 

 to be in conference with Hooker and yourself on these 

 matters, and I think we may, or rather you may, in a few 

 years settle the question as to whether Agassiz's or Hooker's 

 views are correct ; they are certainly widely different. 



Apropos to this, many thanks for the paper containing 

 your experiments on seeds exposed to sea water. Why has 

 nobody thought of trying the experiment before, instead of 

 taking it for granted that salt water kills seeds? I shall have 

 it nearly all reprinted in Silliman's Journal as a nut for 

 Agassiz to crack. 



Letter 325 To Asa Gray. 



Down, May 2nd [1856?] 

 I have received your very kind note of April 8th. In 

 truth it is preposterous in me to give you hints ; but it will 

 give me real pleasure to write to you just as I talk to Hooker, 

 who says my questions are sometimes suggestive owing to my 

 comparing the ranges, etc., in different kingdoms of Nature. 

 I will make no further apologies about my presumption ; but 

 will just tell you (though I am certain there will be veiy little 

 new in what I suggest and ask) the points on which I am very 

 anxious to hear about. I forget whether you include Arctic 



