Darwin to contain twenty species of plants, and these twenty species
belonged to eighteen genera and to eight orders, showing how greatly
they differed from each other. Farmers find that a greater quantity of
hay is obtained from ground sown with a variety of genera of grasses,
clover, etc., than from similar land sown with one or two species only;
and the same principle applies to rotation of crops, plants differing
very widely from each other giving the best results. So, in small and
uniform islands, and in small ponds of fresh water, the plants and
insects, though few in number, are found to be wonderfully varied in
character.
The same principle is seen in the naturalisation of plants and animals
by man's agency in distant lands, for the species that thrive best and
establish themselves permanently are not only very varied among
themselves but differ greatly from the native inhabitants. Thus, in the
Northern United States there are, according to Dr. Asa Gray, 260
naturalised flowering plants which belong to no less than 162 genera;
and of these, 100 genera are not natives of the United States.
Pages:
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206