134 Bibliographical Notices. 



Moreover, these differences may be positive or negative ; that is, they 

 may depend upon the presence or absence of certain species. It is 

 very desirable that increased attention be paid to the latter point, 

 which is so often neglected. 



The British flora is not remarkable for the possession of many 

 species peculiar to itself. jSIr, Watson tells us that the flora of 

 Britain is, with very few exceptions, a simple fragment of the 

 European, and has, on that account, been too readily considered as 

 derived from the European continent. Our author says (p. 386), 

 •'It is likely enough that many species have reached the present 

 surface of Britain by a migration from East to West ; but it is not to 

 be too hastily assumed that migration has been exclusively in the 



one longitudinal course other species may have spread from 



West to East. Looking at the present distribution of the European 

 flora, a gradual migration eastward seems as well supported by facts 

 as is a like migration westward. Britain is no exception : there are 

 western species quite absent from the eastern side of Britain ; there 

 are species much more prevalent on the western side of our island ; 

 and there are many plants found in Britain and westein Europe 

 which have early limits eastward on the continent." We believe 

 especial stress should be laid upon the principle expressed in the last 

 sentence, and that it is to a proper observation of the direction and 

 degree of " thinning-out " of species that we must look for forming 

 any opinion upon their probable original home or metropolis. Such 

 seems the course advocated by Mr. Watson (at pp. 442 and 452) 

 when he recommends including in our estimate " all the species of a 

 flora, and placing them fairly under the same arrangements, tests, 

 and aspects," and tells us that " the truest line of eastern and western 

 species would be made out by selecting them in accordance with any 

 decided predominance on the one side, though not absolutely re- 

 stricted thereto." From this point of view, the question of assigning 

 the species to their respective " types of distribution" rises greatly in 

 importance. " The northern and southern tendencies are far more 

 obvious than those in relation to longitude ; even at the present time 

 southern plants appear to be extending northwardly within the limits 

 of our own island, chiefly perhaps through the agency of man :" but 

 the thinning-out northwards becomes still more evident if we turn to 

 the respective numbers of the three floras, which are given (at p. 455) 

 as— South Britain 1280, 3Iid Britain 1148, North Britain 930; 

 whereas West Britain has 1305 species against the 1355 of East 

 Britain. Of these, 50 are western exclusively, and 1 20 peculiar to the 

 east side of Great Britain. Similarly, "South Britain" has 209 

 peculiar species, "North" and "Middle Britain" together 139. 

 This is further shown by the distribution of the local species 

 (p. 443, &c.) ; for out of 77 plants restricted toa single "subprovince," 

 South Britain has 44, Mid and North Britain together 33. So, of 

 the 111 plants peculiar to a single "province," 7Q belong to East 

 Britain, against 35 in West Britain. "The most local species tend 

 in three directions ; namely, to the south-east of England, to the 

 south-west of England, and to the Highlands. In other words, the 



