| → are 1869 1872 |
| seem to me 1859 1860 1861 1866 |
|
| → in any one region for the immigration of 1860 1861 1866 1869 1872 |
| for the coming in of quite 1859 |
|
| → from other regions, 1860 1861 1866 1869 1872 |
| by immigration, 1859 |
|
|
represented in the diagram; for the groups will have been more
they will have endured for extremely unequal lengths of time, and will have been modified in various degrees. As we possess only the last volume of the geological record, and that in a very broken condition, we have no right to expect, except in
rare cases, to fill up
intervals in the natural system, and thus
distinct families or orders. All that we have a right to
that those
which
within known geological
undergone much modification, should in the older formations make some slight approach to each other; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups; and this by the concurrent evidence of our best palæontologists
frequently
the case. |
|
| Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms,
→are
explained in a satisfactory manner. And they are wholly inexplicable on any other view. |
|
| On this same theory, it is evident that the fauna
any
period in the earth's history will be
in general character between that which preceded and that which succeeded it.
the species which lived at the sixth great stage of descent in the diagram are the modified offspring of those which lived at the fifth stage, and are the parents of those which became still more modified at the seventh stage; hence they could hardly fail to be nearly intermediate in character between the forms of life above and below. We must, however, allow for the entire extinction of some preceding forms, and
→in any one region for the immigration of
new forms
→from other regions,
and for a large amount of
during the long and blank intervals between the successive formations. Subject to these allowances, the fauna of each geological period undoubtedly is intermediate in character, between the preceding and succeeding faunas. I need give only one instance, namely, the manner in which the fossils of the Devonian system, when this system was first discovered, were at once recognised by palæontologists as intermediate in character between those of the overlying carboniferous, and underlying Silurian
But each fauna is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive formations. |
|
| It is no real objection to the truth of the
that the fauna of each period as a whole is nearly intermediate in character between the preceding and succeeding faunas, that certain genera
|
