If, in the diagram, we suppose the amount
of change represented by each successive group of diverging dotted lines to
be great, the forms marked a14 to p14, those marked b14 and f14, and those
marked o14 to m14, will form three very distinct genera. We shall also
have two very distinct genera descended from (I), differing widely from the
descendants of (A). These two groups of genera will thus form two distinct
families, or orders, according to the amount of divergent modification
supposed to be represented in the diagram. And the two new families, or
orders, are descended from two species of the original genus; and these are
supposed to be descended from some still more ancient and unknown form.
We have seen that in each country it is the species belonging to the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts through one
form having some advantage over other forms in the struggle for existence,
it will chiefly act on those which already have some advantage; and the
largeness of any group shows that its species have inherited from a common
ancestor some advantage in common. Hence, the struggle for the production
of new and modified descendants will mainly lie between the larger groups,
which are all trying to increase in number. One large group will slowly
conquer another large group, reduce its number, and thus lessen its chance
of further variation and improvement.
Pages:
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207