ILLUSTRATIVE OF NATUEAL SELECTION. 161 



Few naturalists will doubt that all these may and pro- 

 bably have been derived from a common stock, and 

 therefore it seems desirable that there should be a unity 

 in our method of treating them ; either call them all 

 varieties or all species. Varieties, however, continually 

 get overlooked ; in lists of species they are often alto- 

 gether unrecorded ; and thus we are in danger of 

 neglecting the interesting phenomena of variation and 

 distribution which they present. I think it advisable, 

 therefore, to name all such forms ; and those who will 

 not accept them as species may consider them as sub- 

 species or races. 



6. Species. Species are merely those strongly 

 marked races or local forms which when in contact 

 do not intermix, and when inhabiting distinct areas 

 are generally believed to have had a separate origin, 

 and to be incapable of producing a fertile hybrid 

 offspring. But as the test of hybridity cannot be 

 applied in one case in ten thousand, and even if it 

 could be applied would prove nothing, since it is 

 founded on an assumption of the very question to 'be 

 decided and as the test of separate origin is in every 

 case inapplicable and as, further, the test of non- 

 intermixture is useless, except in those rare cases 

 where the most closely allied species are found in- 

 habiting the same area, it will be evident that we 

 have no means whatever of distinguishing so-called 

 "true species" from the several modes of variation 

 here pointed out, and into which they so often pass 

 by an insensible gradation. It is ouite true that, in 



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