60 THE ORIGIN OF SPECIES n 



enough deducible from general considerations ; but 

 there are, in addition, phenomena exhibited by 

 species themselves, and yet not so much a part of 

 their very essence as to have required earlier 

 mention, which are in the highest degree per 

 plexing, if we adopt the popularly accepted 

 hypothesis. Such are the facts of distribution in 

 space and in time ; the singular phenomena 

 brought to light by the study of development ; 

 the structural relations of species upon which our 

 systems of classification are founded ; the great 

 doctrines of philosophical anatomy, such as 

 that of homology, or of the community of 

 structural plan exhibited by large groups of 

 species differing very widely in their habits and 

 functions. 



The species of animals which inhabit the sea on 

 opposite sides of the isthmus of Panama are 

 wholly distinct ; 1 the animals and plants which 

 inhabit islands are commonly distinct from those 

 of the neighbouring mainlands, and yet have a 

 similarity of aspect. The mammals of the latest 

 tertiary epoch in the Old and New Worlds belong- 

 to the same genera, or family groups, as those 

 which now inhabit the same great geographical 

 area. The crocodilian reptiles which existed in the 

 earliest secondary epoch were similar in general 

 structure to those now living, but exhibit slight 



1 Recent investigations tend to show that this statement is 

 not strictly accurate. 1870. 



