| → OMIT 1872 |
| chanced to have 1859 1860 1861 1866 1869 |
|
| → Consequently if it 1872 |
| So that if such species 1859 1860 1861 |
| So that, if such species 1866 1869 |
|
| → the deposition of any 1872 |
| any 1859 1860 1861 1866 1869 |
|
| → older and underlying 1869 1872 |
| underlying 1859 1860 1861 1866 |
|
| → bed; 1859 1860 1869 1872 |
| and older bed; 1861 1866 |
|
| → by intermediate varieties with 1872 |
| with 1859 1860 1861 1866 1869 |
|
| → respects. 1866 1869 1872 |
| points of structure. 1859 1860 1861 |
|
| → OMIT 1872 |
| be compelled to 1859 1860 1861 1866 1869 |
|
|
had not the trees
→OMIT
been preserved:
Lyell and
found carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearing strata, one above the
at no less than sixty-eight different levels. Hence, when the same species
at the bottom, middle, and top of a formation, the probability is that
not lived on the same spot during the whole period of deposition, but
disappeared and reappeared, perhaps many times, during the same geological period.
→Consequently if it
were to undergo a considerable amount of modification during
→the deposition of any
one geological
a section would not
include all the fine intermediate gradations which must on
theory have
but abrupt, though perhaps
slight, changes of form. |
|
| It is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to connect them together by
intermediate
from the reasons just
we can seldom hope to effect in any one geological section. Supposing B and C to be two species, and a third, A, to be found in an
→older and underlying
→bed;
even if A were strictly intermediate between B and C, it would simply be ranked as a third and distinct species, unless at the same time it could be
closely connected
→by intermediate varieties with
either one or both
Nor should it be forgotten, as before explained, that A might be the actual progenitor of B and C, and yet
not
necessarily be strictly intermediate between them in all
→respects. So that we might obtain the parent-species and its several modified descendants from the lower and upper beds of
formation, and unless we obtained numerous transitional gradations, we should not recognise their
and should consequently
→OMIT
rank them
as distinct species. |
|
| It is notorious on what excessively slight differences many palæontologists have founded their species; and they do this the more readily if the specimens come from different sub-stages of the same formation. Some experienced conchologists are now sinking many of the very fine species of
and others into the rank of varieties; and on this view we do find the kind of evidence of change which on
theory we ought to find. Look again at the later tertiary deposits, which include many shells believed by the majority of naturalists to be identical with existing species; but some excellent naturalists, as Agassiz and Pictet, maintain that all these tertiary species are specifically distinct, though the distinction
|
