| → now extinct species 1859 1860 1861 1866 |
| extinct species 1869 |
| species embedded in each formation 1872 |
|
| → has been simultaneous 1859 1860 1861 1866 |
| embedded 1869 |
| has been simultaneous. 1872 |
|
| → formation. 1859 1860 1861 1866 |
| formation has been simultaneous. 1869 |
| OMIT 1872 |
|
| → strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
| → degree. 1859 1860 1861 1866 |
| in each successive so-called formation. 1869 1872 |
|
|
nor disappearance of
many
→now extinct species
→has been simultaneous
→formation.
|
|
| Species
different genera and classes have not changed at the same rate, or in the same degree. In the
tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. Falconer has given a striking instance of a similar fact,
an existing crocodile
with many
lost mammals and reptiles in the sub-Himalayan deposits. The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly. The productions of the land seem to
at a quicker rate than those of the sea, of which a striking instance has
been observed in Switzerland. There is some reason to believe that
high in the
change more quickly than those that are low: though there are exceptions to this rule. The amount of organic change, as Pictet has remarked,
not
→strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly
the same
→degree. Yet if we compare any but the most closely related formations, all the species will be found to have undergone some change. When a species has once disappeared from the face of the earth, we have
to believe that the same identical form
reappears. The strongest apparent exception to this latter
is that of the so-called "colonies" of M. Barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but
explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems
satisfactory.
|
