42 



cited no great disturbing power has been made evident, 

 the aberrations to which we have appealed being, 

 most of them, comparatively minute. This, however, is 

 simply in harmony with the belief which we have already 

 expressed, that climatal causes, when taken singly and 

 alone, are not of primary importance whilst discussing 

 the question of specific modification. It remains for us, 

 in the following sections, to inquire, whether there are 

 any other elements at work from which greater results 

 are to be expected. Meanwhile, let us not forget that 

 differences may be, in the strictest sense, significant, 

 even whilst small ; and that it is their constancy, rather 

 than their magnitude, which more particularly concerns 

 us in the present treatise, seeing that it is with reference 

 to those distinctions which are less conspicuous that the 

 greatest amount of misunderstanding (through the fact 

 of their being fixed) usually prevails ; whilst it is our 

 main object to show that dissimilarities do not neces- 

 sarily imply the specific isolation of the creatures which 

 display them, merely because they are, in their several 

 localities, permanent. 



II. Temporary heat or cold, of an unusual degree. 



It is perhaps unnecessary that the action of temporary 

 heat and cold, of an unusual degree, should be considered 

 under a separate head from that of climatal causes gene- 

 rally ; nevertheless, since the latter are, in a certain 

 sense, permanent in their operation, it may be thought 



