DISTRIBUTION OF PLANTS IN BRITAIN. 711 



of the mammaliferous crag with the third. The date of the fourth is 

 too evident to be questioned ; and the glacial region in which it flour- 

 ished is to be regarded as a local climate, of which no true traces, as 

 far as animal life is concerned, exist southwards of the second and 

 third barriers. This was the newer Pliocene epoch. The period of 

 the fifth flora was that of the post-tertiary, when the present aspect 

 of things was organised. Adopting such a view of the relations of 

 these floras in time, the greatest difficulties in the way of changes of 

 the earth's surface and destruction of barriers, deep sea being found 

 where land (probably high land) was, are removed when we find that 

 those greater changes must have happened during the epoch imme- 

 diately subsequent to the Miocene period ; for we have imdoubted 

 evidence that elsewhere, during that epoch, the Miocene sea-bed was 

 raised 6000 feet in the chain of Taurus, and the barriers forming the 

 westward boundary of the Asiatic Eocene lakes so completely anni- 

 hilated, that a sea several hundred fathoms deep now takes their pro- 

 bable place. The changes required for the events which are supposed 

 to be connected with the peculiar distribution of the British flora are 

 not greater than these. . The distribution of endemic animals, espe- 

 cially that of the terrestrial moUusca, seems to support these views." 



D'Archiac says that in a botanical point of view it would perhaps 

 be desirable to determine whether the external circumstances under 

 which these five floras of &reat Britain now live, such as latitude, alti- 

 tude, temperature, winds, humidity or dryness, exposure, nature of the 

 soil, greater or less distance from the coast, etc., are altogether insuffi- 

 cient to explain their different characters. We know that plants have 

 very different geographical limits. Thus there are some which we 

 meet with over an extent of 25° in latitude, and much more in longi- 

 tude, whUe others occupy only zones extremely restricted in both 

 senses ; it would therefore be useful to study the five British floras 

 in this point of view. The radiation of plants from a centre is by no 

 means satisfactorily proved ; and it may be asked, for example, What 

 is the original centre from which the species common to North America 

 and southern Europe could have radiated? D'Archiac thinks that 

 inconvenience arises from an attempt to give an account of facts 

 hitherto inexplicable in our science, by drawing from ' another science 

 suppositions made, as it appears, with the sole view of these explana- 

 tions, and for which there is no sufficient authority. Proofs drawn 

 from geology must rest on more certain data, he thinks, than those 

 which have been adduced by Professor Forbes. 



On ascending lofty mountains in Britain, there is a marked 

 variation in the nature of the vegetation. On Ben-muich-dhui, which 

 attains an elevation of upwards of 4296 feet, Watson gives a full list 

 of the species observed in succession. On leaving the plants of the 

 low country we flnd Myrica Gale, extending on this mountain to 1400 



