14 INTRODUCTION 



tion of parts, and size. In some species one or more 

 groups termed varieties may be recognised, and are dis- 

 tinguished from the other forms included in the species 

 by some slight, but fairly well-marked and constant 

 modification. Varieties are frequently connected with the 

 special physical or biological conditions under which they 

 are living. The varieties in some species pass into one 

 another by intermediate forms ; but others appear to be 

 fairly distinct and may be regarded as incipient species. 



Sometimes two groups of individuals resemble each 

 other so closely that they might be regarded as belonging 

 to the same genus or to the same species, but they 

 appear to have descended from different ancestors since 

 they are found to differ in development (ontogeny) or in 

 their palgeontological history ; this phenomenon, of forms 

 belonging to different stocks, approaching one another in 

 character, is known as convergence or heterogenetic homceo- 

 morphy, and may occur either at the same geological 

 period or at widely separated intervals. 



Similarly, animals belonging to two distinct groups 

 may, when subjected to similar conditions, show corre- 

 sponding modifications, though they do not really approach 

 one another in essential characters ; thus parallel modifi- 

 cation may occur in independent groups. 



