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ration^ as a mere matter of experience ; and, secondly, 

 on some of the causes to wtich the physiologist would, 

 in many instances, endeavour to refer it. 



The former of these considerations (namely, the fact 

 of specific instability as ordinarily noticed) nobody -will 

 be inclined to dispute : and yet it is abundantly evident 

 that it cannot be taken into accoimt, at any rate satis- 

 factorily, without involving the latter also, — it being 

 scarcely possible to attach the proper value to an effect 

 without first investigating its cause. The importance 

 of assigning its legitimate weight (and that only) to a 

 variety, is perhaps the most difficult task which the 

 natural historian has to accomplish ; since on it depends 

 the acknowledgment of the specific identity of one 

 object with another, — whilst, to draw the line of separa- 

 tion between varieties and species is indeed a Gordian 

 knot which generations have proved inadequate to untie. 

 Now it is not the object of this pubKcation to attempt 

 to throw positively new light upon a subject which has 

 ever been one of the main stumbling-blocks in the lower 

 sciences, and which is perhaps destined to be so to the 

 end ; still less would I wish to imply that the causes of 

 variation are altogether overlooked in these days of 

 accurate inquiry, — when thousands are accumulating 

 data, in all parts of Europe, destined to be wielded by 

 the master's hand whensoever the harvest-time shall 

 have arrived : but I do, nevertheless, beheve that there 

 exists a growing tendency, especially in some portions of 

 the Continent, to regard every difference (if at all perma- 



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