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 times—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 
