156 



From the very fragmentary character of the geologic record yet brought 

 to li\;ht, it is difficult to trace back the entire scries of changes, and the reason 

 is this : we have so few land surfaces presented by rock formations that any- 

 thing like a consecutive record of the Flora that lived in their vicinity cannot be 

 gained. 



Those ancient landways, formed by the junction of continents, and 

 marking the commencement of the tertiary period, were undoubtedly so 

 many means of transport by which the different species or genera of those 

 times, spreading out, crept to their proper grounds : and thus a common Flora 

 is an evidence at once that lands now separated were once parts of the same 

 continent. But the very circumstances which favoured an immigration of 

 certain Flora, such as the Iberian, in early tettiary times, were such as to 

 make the northern forms ascend the summits ; and this is the reason why we 

 find Alpine species still remaining on our mountain heights, while our southern 

 o >asts here and there shelter the extreme northern representatives of species 

 common to the warmer regions of southern Europe. 



And during the succeeding pre-glacial, glacial, and post-glacial periods, 

 whether of submergence by the glacial sea or elevation, so many different 

 pathways were opened up for effecting a migration of plants from one conti- 

 nent to the other, and may explain why hardly a single species of flowering 

 plant (I believe Spironthes gemmipara is the sole exception,) is restricted to 

 our island. The great bulk of our Flora, whether on hills, or plains, or sea 

 coasts, is, in similar situations, spread over Europe, the vast area of tem- 

 perate and North Asia, and follows in the wake of civilisation. 



It is well known that the number of species of the Germanic type lessens 

 as we go westward — that while the Flora peculiar to our western counties may 

 ba traced down the same departments of France to Spain and Portugal, the 

 eastern counties present certain Scandinavian forms, marking a glacial period. 



The shiftings of level evidenced in these formations, involving as they do 

 such changes in the relative proportions of land and water, must have been 

 very influential in favouring the diffusion of some, while opposing obstacles to 

 the spread of other species. 



It has been established by the researches of Professor Edmund Forbes 

 that Mollusca, even in their larval state, cease to exist at certain periods of 

 their metamorphosis if they do not meet with favourable circumstances for 

 their development, and the particular zone of depth to which they are 

 adapted ; in such circumstances they perish and sink to depths below. For the 

 proofs that our islands formed once a part of a great continent we point to our 

 Mediterranean, Germanic, and Iberian flora. 



That ancient tract is now destroyed sed genus immortale manet. Each 

 addition to terra firma has been stamped by a conquest of Flora, "that 

 rver fresh mosaic " continuous throughout all time. 



