46 Studies in the Theory of Descent. 



of climatic influences ; while under the general 

 designation of " local forms," should be comprised 

 all variations which have their origin in other 

 causes such, for example, as in the indirect action 

 of the external conditions of life, or in circum- 

 stances which do not owe their present existence 

 to climate and external conditions, but rather to 

 those geological changes which produce isolation. 

 Thus, for instance, ancient species elsewhere long 

 extinct might be preserved in certain parts of the 

 earth by the protecting influence of isolation, 

 whilst others which immigrated in a state of 

 variability might become transformed into local 

 varieties in such regions through the action of 

 ' amixia,' l i. e. by not being allowed to cross with 

 their companion forms existing in the other 

 portions of their habitat. In single cases it may 

 be difficult, or for the present impossible, to decide 

 whether we have before us a climatic form, or a 

 local form arising from other causes ; but for this 

 very reason we should be cautious in defining 

 climatic variation. 



The statement that climatic forms, in the true 

 sense of the word, do exist is well known to me, 

 and has been made unhesitatingly by all zoologists ; 

 indeed, a number of authentically observed facts 



1 [The word 'Amixie,' from the Greek a/ut'a, was first adopted 

 by the author to express the idea of the prevention of crossing 

 by isolation in his essay " Uber den Einfluss der Isolirung auf 

 die Artbildung," Leipzig, 1872, p. 49. R.M.] 



