CHANGES OF SPECIES. 279 



There can be no doubt that in the course of ages the species 

 of animals and plants have changed ; but how this alteration has 

 been effected, and why the older species have died out, and how 

 the new ones have been produced, is still a mystery. Nothing 

 but hypotheses are presented to elucidate the problem. 



A withdrawal of the conditions of life brings about the ex- 

 tinction of ancient species. For instance, when there has been 

 an upheaval of a district above the sea-level, the whole animal 

 population of the water in the localities so raised up must have 

 died off; and in like manner land animals must have perished in 

 places which had sunk beneath the water. Species with a small 

 area of distribution have thus, without doubt, become extinct, 

 and, under similar circumstances, may still die out *. But these 

 changes apparently have never affected the whole earth, and 

 never thus produced a complete destruction of all living crea- 

 tures. If we picture to ourselves the multifarious transforma- 

 tions which Switzerland has experienced in the lapse of ages, 

 we shall always find, from the Trias to the Drift-period, firm 

 land upon which terrestrial life could thrive, and also, up to the 

 beginning of the Miocene period, sea in which marine animals 

 could live. It may therefore be imagined that there has been 

 no external cause for the important changes which have taken 

 place in the organized world of Swiss regions. 



We may conceive that to every species, as to every individual, 

 a determinate age is assigned, and that a species must disappear 



* Thus at the present day a beautiful large snail {Helix sulplicata, Sow.) 

 occurs only upon a small rock in the sea near Porto Santo. Formerly it was 

 abundant upon that island, and occurs in its drift sand. If this rock should 

 fall into the sea, this species would be extinct. The Seychelles palm (Lado- 

 icea sechcllaruni) now only occurs rarely on the Seychelles Islands; and merely 

 a few examples of the dragon-tree live in Madeira and the Canary islands : 

 these remarkable trees are therefore approaching extinction. 



[Dr. Le Maout and M. Decaisne, in their ' General system of Botany,' trans- 

 lated by Mrs. Hooker (p. 851), remark of the Dracesna draco that " the 

 dragon-tree of Orotava is visited by all travellers in the Canary Islands. Its 

 trunk below the lowest branches is 80 feet in height ; and ten men holding 

 hands can scarcely encircle it. When Teneriffe was discovered, in 1402, 

 tradition affirms that it was already as large as it is now, a tradition con- 

 firmed by the slow growth of the young dragon-tree of the Canaries, of which 

 the age is exactly known ; whence it has been calculated that the dragon-tree 

 of Orotava is the oldest plant now existing on the globe." EDITOR.] 



