Notices of New Books. 7585 



as for example small islands, far isolated from continents, being 

 destitute of mammals and batrachians, incapable of migration across 

 any extent of sea ; the great difference oftentimes observable in the 

 marine floras and faunas of closely-contiguous districts, separated by 

 lofty and contiguous mountain ranges, deserts or even large rivers, 

 acting as barriers to free migration; and the great differences in the 

 marine floras and faunas of some closely-contiguous seas that are 

 separated by barriers of land, preventing free interraigvation ; again, 

 in the affinity of the productions of the same continent or sea, though 

 the species themselves are distinct at different periods and stations, 

 as implying not only genealogical relationship, but an uninterrupted 

 existence, within the district, for a length of time sufficient for the 

 development of the allied species from a common parentage. Chap- 

 ter XI. contains also some interesting details respecting the means of 

 the geographical dispersal of species; but this we shall again refer to 

 when we consider the part of Mr. Darwin's work relating to his expla- 

 nation of difficulties. Chapter XII., on the mutual relation of organic 

 beings, aims at assigning community of descent as the cause of common 

 points of resemblance, the different degrees of resemblance repre- 

 sented by the terras variety, species, genus and class, representing 

 various parallel degrees of consanguinity, Mr. Darwin says that these 

 different grades are difference of measure, not of kind ; that the amount 

 of difference recognised as necessary to define these different groups 

 is arbitrary, and various in different parts of the organic kingdoms, 

 and resembling the unequal amounts of difference that would be 

 observable between the multiplied descendants of large and small 

 families ; and that the genealogical relationship between species and 

 their subordinate varieties being a known fact, and that the relation- 

 ship between species and the higher grades of classification being the 

 same in kind, their nearness of consanguinity in proportion to their 

 resemblance is implied, — that adaptive qualities, or those dependant 

 on, and influenced by, habits of life, are not such as naturalists find 

 most useful in classification ; and this, Mr. Darwin says, is because 

 they have been similarly influenced in unrelated families, instancing 

 the community of habits and external appearance between the mouse 

 and the shrew, the resemblance of the whale to a fish, and the external 

 resemblance in the habits of unrelated genera of plants growing in 

 similar situations, — and that it is a general rule that the less any part 

 of the organisation is connected with special habits, the more 

 important it becomes in classification, implying that there is a hidden 

 bond of union between organisms, independently of the places they 

 VOL. XIX. 2 P 



