208 BRITISH PLANTS 



according to the length of time which has elapsed since 

 they became severed from the mainland : 



(1) Recent Continental Islands, which have been 

 detached in recent geological limes that is, during the 

 later Tertiary Period. They are, in all cases, included 

 within the 100-fathom line drawn round the coast of the 

 continent to which they belong. Their flora differs little 

 from that of the mainland, except that it contains a fewer 

 number of species, while the number of isolated or peculiar 

 forms is very few indeed e.g., the British Isles, New- 

 foundland, Japan. 



(2) Ancient Continental Islands, on the other hand, have 

 been isolated from the mainland for a much longer period. 

 The separating seas are much deeper ; the depth is always 

 over 100 fathoms, and sometimes nearly 1,000 fathoms 

 that is, more than a mile e.g., Australia, New Zealand, 

 Madagascar. These islands also received their vegeta- 

 tion from the mainland, but their long separation from it 

 has resulted in great differences between the continental 

 and insular floras. But processes of evolution are slow, 

 and long isolation would naturally cause the vegetation 

 of the island to diverge gradually from that of the main- 

 land, because the conditions governing its development 

 would not remain the same in the two regions. On the 

 island the evolution of the flora would be carried out 

 undisturbed by competition from without, for the inter- 

 vening sea forms an effective barrier against extensive 

 migration. The mainland, on the other hand, is con- 

 tinually exposed to invasion, and at all times the native 

 flora has had to defend its territories against the vic- 

 torious advance of colonizing forms. For this reason, 

 races which, through the keen competition, have died out 

 on the continent, might survive on the islands ; while, on 

 the other hand, the more recent arrivals on the mainland * 

 may be absent from the islands. 



A feature of ancient islands is the number of peculiar 

 or endemic forms which occur in their flora. Endemic 

 plants are plants which are found growing in one country 

 or locality, and nowhere else. They may either be the 

 survivors of races, once widely spread, which have long 

 since disappeared in the struggle for existence elsewhere, 

 or they may be entirely new forms, the products of evolu- 

 tion in a particular locality, which have not yet had time 



