284 LAY SERMONS, ADDRESSES, AND REVIEWS. [xil 



Such arguments against the hypothesis of the direct 

 creation of species as these are plainly enough cleducible 

 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 

 perplexing, if we adopt the popularly accepted hypo- 

 thesis. Such are the facts of distribution in space and 

 in time ; the singular phcenomena 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 differ- 

 ences in their vertebra?, nasal passages, and one or two 

 other points. The guinea-pig has teeth which are shed 

 before it is born, and hence can never subserve the 

 masticatory purpose for which they seem contrived, and, 

 in like manner, the female dugong has tusks which 

 never cut the gum. All the members of the same 

 great group run through similar conditions in their 



1 Becent investigations tend to show that this statement is not strictlj 

 accurate. — 1870. 



