| Comparison with 1866 |
|
the parts of one species with those of another and
distinct species,
we can indicate but
few serial homologies;
that
is, we are seldom enabled to say that one part or organ
is homologous with another in
the same individual. And we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part,
as we find in the other great classes of the animal and vegetable kingdoms. ↑| 6 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | But morphology is a much more complex subject than it at first appears, as has lately been well shown in a remarkable paper by Mr. E. Ray Lankester, who has drawn an important distinction between certain classes of cases which have all been equally ranked by naturalists as homologous.
He proposes to call the structures which resemble each other in distinct animals, owing to their descent from a common progenitor with subsequent modification,
homogenous;
and the resemblances which cannot thus be accounted for, he proposes to call
homoplastic
. For instance, he believes that the hearts of birds and mammals are as a whole homogenous,— that is, have been derived from a common progenitor; but that the four cavities of the heart in the two classes are homoplastic,— that is, have been independently developed.
Mr. Lankester also adduces the close resemblance of the parts on the right and left sides of the body, and in the successive segments of the same individual animal; and here we have parts commonly called homologous, which bear no relation to the descent of distinct species from a common progenitor.
Homoplastic structures are the same with those which I have classed, though in a very imperfect manner, as analogous modifications or resemblances.
Their formation may be attributed in part to distinct organisms, or to distinct parts of the same organism, having varied in an analogous manner; and in part to similar modifications, having been preserved for the same general purpose or function,— of which many instances have been given.
|
|
|
Naturalists frequently speak of the skull as formed of metamorphosed vertebræ:
the jaws of crabs as metamorphosed legs; the stamens and pistils of
flowers as metamorphosed leaves; but it would in these
cases probably
be more correct, as Professor Huxley has remarked, to speak of both skull and vertebræ, both
jaws and legs,
&c., | &c., 1866 1869 1872 | | &c.,— 1859 1860 1861 |
| other 1866 | | other, 1859 1860 1861 1869 1872 |
| in their present state, but 1866 |
| but 1859 1860 1861 |
| as they now exist, but 1869 1872 |
| and simpler element. 1866 1869 1872 |
| element. 1859 1860 1861 |
| Most naturalists, 1866 1869 1872 | | Naturalists, 1859 1860 1861 |
| such modifications having actually 1866 |
| a modification of this nature having 1859 1860 1861 |
| this having 1869 1872 |
| probably would 1866 1869 1872 | | would probably 1859 1860 1861 |
| though simple legs, is 1866 |
| legs, or from some simple appendage, is 1859 1860 1861 |
| though extremely simple legs, is 1869 |
| though extremely simple legs, is in part 1872 |
|
|
Embryology
and
Development
.
|
This is one of the most important departments
of natural history. Herein are included the ordinary
metamorphoses of insects, with which every one is familiar. These
are generally effected somewhat
abruptly by a few stages
and in a concealed manner;
but the transformations are in reality numerous and graduated. For instance, Sir J. Lubbock has recently shown that a
certain ephemerous insect (Chlöeon) during its development
moults
above
twenty times, and each time undergoes a certain amount of change; in such
cases
we probably behold
the act of metamorphosis in its natural or primary progress.
↑| 4 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | Many insects, and especially certain crustaceans, show us what wonderful changes of structure can be effected during development.
Such changes, however, reach their climax in the so-called alternate generations of some of the lower animals.
It is, for instance, an astonishing fact that a delicate branching coralline, studded with polypi and attached to a submarine rock, should produce, first by budding and then by transverse division, a host of huge floating jelly-fishes; and that these should produce eggs, from which are hatched swimming animalcules, which attach themselves to rocks and become developed into branching corallines; and so on in an endless cycle.
The belief in the essential identity of the process of alternate generation and of ordinary metamorphosis has been greatly strengthened by Wagner's discovery of the larva or maggot of a fly, namely the Cecidomyia, producing asexually other larvæ, and these others, which finally are developed into mature males and females, propagating their kind in the ordinary manner by eggs.
|
| 5 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | It may be worth notice that when Wagner's remarkable discovery was first announced, I was asked how was it possible to account for the larvæ of this fly having acquired the power of asexual reproduction.
As long as the case remained unique no answer could be given.
But already Grimm has shown that another fly, a Chironomus, reproduces itself in nearly the same manner, and he believes that this occurs frequently in the Order.
It is the pupa, and not the larva, of the Chironomus which has this power; and Grimm further shows that this case, to a certain extent, "unites that of the Cecidomyia with the parthenogenesis of the Coccidæ;"— the term parthenogenesis implying that the mature females of the Coccidæ are capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the power of ordinary reproduction at an unusually early age; and we have only to accelerate parthenogenetic reproduction by gradual steps to an earlier and earlier age,— Chironomus showing us an almost exactly intermediate stage, viz., that of the pupa— and we can perhaps account for the marvellous case of the Cecidomyia.
|
| Subtitle not present 1866 1869 1872 |
|
Embryology
.— 1859 1860 1861 |
|
It has already been
remarked that various parts and organs of the same individual animal are during an early embryonic period exactly like each other, but become | remarked that various parts and organs of the same individual animal are during an early embryonic period exactly like each other, but become 1866 |
| casually remarked that certain organs 1859 1860 1861 |
| stated that various parts and organs 1869 |
| stated that various parts 1872 |
| adult state widely different and serve for widely different purposes. 1866 |
| individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. 1859 1860 1861 |
| same individual are exactly like each other during an early embryonic period, but in the adult state become widely different and serve for widely different purposes. 1869 |
| same individual which are exactly alike during an early embryonic period, become widely different and serve for widely different purposes in the adult state. 1872 |
|
