60 THE ORIGIN OF SPECIES 
enough deducible from general considerations ; but 
there are, in addition, phenomena exhibited b 
species themselves, and yet not so much a part 0 
their very essence as to have required earlie 
mention, which are in the highest degree 
plexing, if we adopt the popularly accepte 
hypothesis. Such are the facts of distribution i 
space and in time; the singular phenomen 
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 ; 
that of homology, or of the community of 
structural plan exhibited by large groups 
species differing very widely in their habits 
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 gencra, or family groups, as those 
which now inhabit the same great geographical 
area, Thecrocodilian reptiles which existed in the 
earliest secondary epoch were similar in general 
structure to those now living, but exhibit mace & 
1 Recent investigations tend to show that this statement is 
not strictly accurate. —1870. 
