Although the name of continent was not applied to large portions of land for any physical reasons, it so happens that there is a certain physical similarity or homology between them which is not shared by the smaller islands or peninsulas.
It is the old distinction between homology and analogy on a grand scale.
The homology of members was based, in the first instance, upon similarity of development and upon similar relations to the other parts of the body, that is, upon ontogeny.
Homogeny, in contrast, the " special homology " of Owen, is the supreme test of kinship or of hereditary relationship, and thus the basis of all sound reasoning in phylogeny.
For, whereas to one brilliant suggestion of far-reaching homology another can always be opposed, by the detailed comparison of individual growth-stages in carefully selected series of fossils, and by the minute application to these of the principle that individual history repeats race history, it actually is possible to unfold lines of descent that do not admit of doubt.
It must be pointed out that, however probable Haeckel's theory may be in other respects, there is not the slightest evidence for any such cleft in the umbrella having been present at any time, and that the embryological evidence, as already pointed out, is all against any homology between the stem and a manubrium, since the primary siphon does not become the stem, which arises from the ex-umbral side of the protocodon and is strictly comparable to a stolon.
Thus the histological differentiation of the sporogonium of the higher mosses is one of considerable complexity; but there is here even less reason to suppose that these tissues have any homology (phylogenetic community of origin) with the similar ones met with in the higher plants.
But since the general adoption of the theory of evolution, similarity of descent, that is of p/iylogeny, has come to form an essential part of this conception; in other words, in order that their homology may be established the parts compared must be proved to be homogenetic.
The effect of the phylogenetic factor in homology may be illustrated in the following cases.
Another effect is that different degrees of homology have to be recognized, just as there are different degrees of relationship or affinity between individual plants.
When two organs can be traced along the same line of descent to one primitive form, that is when they are found to be mono phyletic, their homology is complete; when, however, they are traceable to two primitive forms, though these forms belong to the same morphological series, they are polyphyletic and therefore only incompletely homologous.
The relief of the surface typically includes a central plain, Homology sometimes dipping below sea-level, bounded by lateral Homology of con- h i ghlands or mountain ranges, loftier on one side than.
The compound eyes of insects resemble so closely the similar organs in Crustaceans that there can hardly be reasonable doubt of their homology, and the primitively appendicular nature of the eyes in the latter class suggests that in the Hexapoda also they represent the appendages of an anterior (protocerebral) segment.
Sur les animaux sans vertebres, 1816) established the homology of the jaws of all insects whether biting or sucking, deserves especial honour.
Lankester (Homoplasy and Homogeny), " On the Use of the term Homology in Modern Zoology," Ann.
Owen not only occupied himself with the dissection of rare animals, such as the Pearly Nautilus, Lingula, Limulus, Protopterus, Apteryx, &c., and with the description and reconstruction of extinct reptiles, birds and mammals - following the Cuvierian tradition - but gave precision and currency to the morphological doctrines which had taken their rise in the beginning of the century by the introduction of two terms, " homology " and " analogy," which were defined so as to express two different kinds of agreement in animal structures, which, owing to the want of such " counters of thought," had been hitherto continually confused.
Goto, " Homology of Genital Ducts," Centralbl.
This mode of sporeformation is totally different from that in the ascus; hence one of the difficulties of the acceptance of Brefeld's view of the homology of ascus and sporangium.
In the various groups of the Entomostraca, on the other hand, the terms thorax and abdomen, though conveniently employed for purposes of systematic description, do not imply any homology with the regions so named in the Malacostraca.
The structure of the zooid of Heliopora, however, is that of a typical Alcyonarian, and the septa have only a resemblance to, but no real homology with, the similarly named structures in madreporarian corals.
Accepting the homology of these apical systems with the calycinal system, the theory would regard the aboral pole of a sea-urchin or starfish as corresponding in everything, except its relations to the sea-floor, with the aboral pole of a fixed echinoderm.
Aboral) poles of starfish and crinoids are not comparable with one another, and that all conclusions based on the supposed homology of the dorso-central of echinoids and asteroids with that of crinoids are incorrect.
S, Blood-spaces, of which the homology is still uncertain.
All members that belong to the same category are sitting together.