| → I do not 1859 1860 1861 |
| it need not be 1866 1869 1872 |
|
| → the two little 1861 1866 1869 1872 |
| little 1859 1860 |
|
| ↑ 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 |
|
| → intimate structure closely resembles that of common muscle; 1859 1860 1861 |
| OMIT 1872 |
| is much analogy between them and ordinary muscles, in their manner of action, in the influence on them of the nervous power and other stimulants such as strychnine, 1866 |
| is much analogy between them and ordinary muscles, in their manner of action, in the influence on them of the nervous power and of stimulants such as strychnine, 1869 |
|
| → it has lately been shown that Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteucci asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible. 1861 |
| OMIT 1872 |
| it has lately been shown that Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible. 1859 |
| it has lately been shown that Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteucei asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible. 1860 |
| some believe in their intimate structure. 1866 1869 |
|
| ↑ 2 blocks not present in 1859 1860 1861; present in 1866 1869 1872 |
| We
do not even in all cases
know of what use these organs are; though in
the Gymnotus and Torpedo they no doubt serve as powerful means of defence
and perhaps for securing prey; yet in the Ray
an
analogous organ in the tail,
even when
greatly irritated,
manifests, as lately observed by Matteucci, but little electricity;
so little
that it can hardly be of much
use for these ends.
Moreover, in the Ray, besides the organ just referred to, there is, as Dr. R. M'Donnell has shown, another organ near the head, not known to be electrical, but which apparently is
the real homologue of the electric battery in the torpedo.
|
|
| ↑ 3 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| It is generally admitted that there exists between these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in the manner in which they are acted on by various reagents.
It should, also, be especially observed that muscular contraction is accompanied by an electrical discharge; and, as Dr. Radcliffe insists, "in the electrical apparatus of the torpedo during rest, there would seem to be a charge in every respect like that which is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which attends upon the action of muscle and motor nerve."
Beyond this we cannot at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed.
|
|
| ↑ 1 blocks not present in 1859 1860 1861 1872; present in 1866 1869 |
| And
lastly,
as we know nothing about the lineal progenitors of any of
these fishes, it must be admitted that we are too ignorant to be enabled to affirm that no transitions are possible, through which the electric organs might have been developed.
|
|
| → The electric organs 1859 1860 1861 |
| These same organs at first appear to 1866 1869 |
| These organs appear at first to 1872 |
|
| → which 1859 1860 1861 |
| fish, of which 1866 1869 1872 |
|
