See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1860
1861
1866
1869
1872

sack, including 1859 1860 1861
of the sack, including 1866
of the sack, together with 1869 1872

large folded branchiæ. 1859 1860
in the same relative position, large, much folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1861
in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1866
in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and by all other naturalists who have treated on the subject. 1869
in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered by all naturalists to act as branchiæ. 1872

I do not 1859 1860 1861
it need not be 1866 1869 1872

little 1859 1860
the two little 1861 1866 1869 1872

8 blocks not present in 1859 1860 1861 1866 1869; present in 1872
There is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction. This has lately been insisted on by Prof. Cope and others in the United States. It is now known that some animals are capable of reproduction at a very early age, before they have acquired their perfect characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from the mature form, the character of the species would be greatly changed and degraded. Again, not a few animals, after arriving at maturity, go on changing in character during nearly their whole lives. With mammals, for instance, the form of the skull is often much altered with age, of which Dr. Murie has given some striking instances with seals; every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, as they grow older. Prof. Cope states that the teeth of certain lizards change much in shape with advancing years; with crustaceans not only many trivial, but some important parts assume a new character, as recorded by Fritz Müller, after maturity. In all such cases,— and many could be given,— if the age for reproduction were retarded, the character of the species, at least in its adult state, would be modified; nor is it improbable that the previous and earlier stages of development would in some cases be hurried through and finally lost. Whether species have often or ever been modified through this comparatively sudden mode of transition, I can form no opinion; but if this has occurred, it is probable that the differences between the young and the mature, and between the mature and the old, were primordially acquired by graduated steps.

Subtitle not present 1859 1860 1861 Special Difficulties of the Theory of Natural Selection. 1872
it is 1859 1860 1861
for it is 1866 1872
for it is im- 1869

but, as Owen and others have 1859 1860 1861
As Owen has 1866 1869
OMIT 1872

called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. These cirripedes have no branchiæ, the whole surface of the body and sack, including the small frena, serving for respiration. The Balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack,
within
in
the well-enclosed shell; but they
have,
have
large folded branchiæ. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiæ of the other family; indeed, they graduate into each other. Therefore I do not
doubted
doubt
that little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly
aid in
aided
in the
the
act of respiration, have been gradually converted by natural selection into branchiæ, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and they have
already
already
suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiæ in this latter family had originally existed as organs for preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ could not
possibly
possibly
have been produced by
successive,
successive
small, transitional
transitional
gradations,
yet
yet,
undoubtedly
undoubtedly,
serious
grave
cases of difficulty
occur.
occur,
some
some
of
of
which
which
will
will
be
be
discussed
discussed
in
in
my
my
future
future
work.
work.
One of the
most serious
gravest
is that of neuter insects, which are often
very
very
differently constructed from either the males or fertile females; but this case will be treated of in the next chapter. The electric organs of fishes offer another case of special difficulty; it is
possible
impossible
to conceive by what steps these wondrous organs have been
produced.
produced;
but, as Owen and others have
remarked,
remarked,