loose at the bottom of the sack,
within | within 1869 1872 | | in 1859 1860 1861 1866 |
| have, 1861 1866 1869 1872 | | have 1859 1860 |
| 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 |
| 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 by all naturalists to act as branchiæ. 1872 |
| it need not be 1866 1869 1872 |
| I do not 1859 1860 1861 |
| doubted 1866 1869 1872 | | doubt 1859 1860 1861 |
| the two little 1861 1866 1869 1872 |
| little 1859 1860 |
| aid in 1869 | | aided 1859 1860 1861 1866 1872 |
| the 1859 1860 1861 1866 1869 | | in the 1872 |
| already 1859 1860 1861 1866 1869 | already 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.
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