In some cases no doubt the process of
modification will be confined to a single line of descent, and the number
of modified descendants will not be increased; although the amount of
divergent modification may have been augmented. This case would be
represented in the diagram, if all the lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way the English
racehorse and English pointer have apparently both gone on slowly diverging
in character from their original stocks, without either having given off
any fresh branches or races.
After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram to
be excessively small, these three forms may still be only well-marked
varieties; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into doubtful or at least into well-defined species: thus the
diagram illustrates the steps by which the small differences distinguishing
varieties are increased into the larger differences distinguishing species.
By continuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and m14, all descended from (A).
Pages:
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201