THE PKINCIPLES OF BIOLOGY. 
335 
2nd.—Another group, generally fresh-water species, are 
widely dispersed by the agency of birds. For an illustration 
of this phenomenon we may again refer to the New Zealand 
Flora. Twenty-two of the native plants of the Canterbury 
Province are common English species, and of these eighteen 
are fresh-water and three littoral forms. 
3rd.—Another group, chiefly littoral forms, are dispersed 
by the agency of ocean currents. 
4th.—Allowance having been made for the three pre¬ 
ceding modes of dispersion, there still remain a large 
majority of which the only explanation that can be given is 
that they have travelled over continuous land. It is to these 
that we must look for information as to the former oscillations 
of land and water. 
Before leaving the New Zealand Flora it will be interest¬ 
ing to note some remarkable features which it presents and 
which show the extreme differentiation of an insular Flora, 
as distinguished from a continental one. Of the 750 
flowering plants in the Canterbury Province, 538 are restricted 
to New Zealand. The number of genera of flowering plants 
and ferns is 275, and of these no fewer than 109 have only 
one species each. Of 857 species of flowering plants and 
ferns, 176 are Australian and 108 American, showing former 
connection with these continents, but very long separation. 
The history of the more recent geological changes in its 
bearing upon the distribution of plants on the European 
Continent is of the highest interest. We know, from the 
evidence of their fossil remains, that in Miocene times a large 
assemblage of plants of a tropical or sub-tropical character 
covered the land of the northern hemisphere from the Tropic 
of Cancer to within ten degrees of the Pole. Palms, figs, 
and laurels grew on the site of the Lake of Geneva, and the 
plane, tulip-tree, walnut and vine, together with the Sequoia 
and Magnolia, flourished in Greenland. After Miocene 
times there was a gradual decline of temperature, which 
culminated in the refrigeration of the glacial epoch, and this 
again has been succeeded by the climatic conditions of the 
modern w T orld. Simultaneously with these variations of 
temperature were oscillations in level of many thousand feet, 
and corresponding changes in the boundary of sea and land. 
Now consider wliat- must have taken place as the climate 
gradually became colder. The plants in any given zone of 
latitude would be sorted by the increasing cold. Those 
unsuited to the colder climate would be driven to the zone 
below, where they would find their proper temperature, but 
