XXVI INTROnUCTION. 



certain external points of colour, size, or form; "and these 

 may, either, propagate individuals precisely like themselves, 

 or revert to what is usually called the type form of the 

 species. Moreover they breed freely with one another, and 

 their offspring is always fertile. Among domesticated ani- 

 mals and plants, we are familiar with numerous varieties, as 

 here defined ; but, in nature, more especially in the animal 

 kingdom, varieties are rare ; any exceptionally abnor- 

 mal forms produced are few in individuals, and rarely 

 reproduced ; and the variations of all species are bounded 

 within certain fixed limits. In the vegetable world, 

 however, where the plant is so much affected by external 

 circumstances, such as soil, light, heat, supply of water, 

 &c., no doubt varieties' are much more frequent, and of 

 greater divergency from the original type. 



Some naturalists believe that permanent varieties are 

 common in the animal kingdom, and Kaup calls them 

 sub-species. Such persons consider that their differences 

 from other individuals of what they would term the 

 typical form, do not entitle them to the full rank of a 

 species. Others, again, deny that permanent varieties 

 •exist, and state their conviction that even slight differ- 

 ences of colour and size, if found to be constant, are 

 sufficient to constitute such individuals a distinct race or 

 species. When such differences are found to co-exist with 

 a different geographical distribution, I certainly prefer 

 the views of those who look on all permanent distinctions 

 of colour, size, structure, &c., as distinct species, and I 

 believe that no change of climate, or food, or other exter- 

 nal circumstances, will produce any alteration in them or 

 in their descendants, if they remain true to each other ; 

 and as yet I know of no recorded instance where any well 

 marked race has produced offspring differing from their 



