Of course the
development or the extension of the range of any such insects, and any
one of many other causes which we see actually at work around us,
would readily account for the destruction of some given species or
even for the destruction of several species in a limited area of
country.
When whole faunal groups die out over large areas, the question is
different, and may or may not be susceptible of explanation with the
knowledge we actually possess. In the old arctogaeal continent, for
instance, in what is now Europe, Asia, and North America, the glacial
period made a complete, but of course explicable, change in the faunal
life of the region. At one time the continent held a rich and varied
fauna. Then a period of great cold supervened, and a different fauna
succeeded the first. The explanation of the change is obvious.
But in many other cases we cannot so much as hazard a guess at why a
given change occurred. One of the most striking instances of these
inexplicable changes is that afforded by the history of South America
towards the close of the tertiary period. For ages South America had
been an island by itself, cut off from North America at the very time
that the latter was at least occasionally in land communication with
Asia.
Pages:
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189