The Chalk period, which we have just
traversed, affords a very clear instance. One of our chief
investigators of the English Chalk, Dr. Rowe, paid particular
attention to the sea-urchins it contains, as they serve well to
identify different levels of chalk. He discovered, not merely
that they vary from level to level, but that in at least one
genus (Micraster) he could trace the organism very gradually
passing from one species to another, without any leap or
abruptness. It is certainly significant that we find such cases
as this precisely where the conditions of preservation are
exceptionally good. We must conclude that species arise,
probably, both by mutations and small variations, and that it is
impossible to say which class of species has been the more
numerous.
There remain one or two conceptions of evolution which we have
not hitherto noticed, as it was advisable to see the facts first.
One of these is the view--chiefly represented in this country by
Professor Henslow--that natural selection has had no part in the
creation of species; that the only two factors are the
environment and the organism which responds to its changes.
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