Divergent views have been held by different authors both as regards the nature of the cormus as a whole, and as regards the homologies of the different types of appendages borne by it.
From the rough comparison of the skeleton of a bird with that of a man by Pierre Delon, in the 16th century (to go no further back), down to the theory of the limbs and the theory of the skull at the present day; or, from the first demonstration of the homologies of the parts of a flower by C. F.
The introduction of the phylogenetic factor has very much increased the difficulty of determining homologies; for the data necessary for tracing phylogeny can only be obtained by the study of a series of allied, presumably ancestral, forms. One of the chief difficulties met with in this line of research, which is one of the more striking developments of modern morphology, is that of distinguishing between organs which are reduced, and those which are really primitive.
Rolleston, " On the Homologies of Certain Muscles connected with the Shoulder-joint," Trans.
The situation of the lateral nervestems in the different genera with respect to the muscular layers lends definite support to the interpretation of their homologies here given and forms the basis of Burger's classification.
These homologies of the hemipterous jaws were determined by J.
It is not altogether easy to recognize the homologies of the endites and exites even within the order Phyllopoda, and the identification of the two distal endites as corresponding to the endopodite and exopodite of higher Crustacea is not free from difficulty.
The homologies between man and other animals which both schools try to account for; the explanation of the intervals, with apparent want of intermediate forms, which seem to the creationists so absolute a separation between species; the evidence of useless " rudimentary organs," such as in man the external shell of the ear, and the muscle which enables some individuals to twitch their ears, which rudimentary parts the evolutionists claim to be only explicable as relics of an earlier specific condition, - these, which are the main points of the argument on the origin of man, belong to general biology.
But the study that should elucidate the fundamental similarities or homologies between the several classes, and should suggest the relations of the Echinoderma to other phyla, had scarcely begun.
The calycinal theory is not merely an assertion of certain homologies, a few of which might be disputed without affecting the rest: it governs our whole conception of the echinoderms, because it implies their descent from a calyculate ancestor - not a "crinoidphantom," that bogey of the Sarasins, but a form with definite plates subject to a quinqueradiate arrangement, with which its internal organs must likewise have been correlated.
But only slight modifications are required to produce the Tornaria larva of the Enteropneusta and other larvae, including the special type that is inferred from the Dipleurula larval stages of recent forms to have characterized the ancestor of the Echinoderms. We cannot enter here into all the details of comparison between these larval forms; amid much that is hypothetical a few homologies are widely accepted, and the preceding account will show the kind of relation that the Echinoderms bear to other animals, including what are now usually regarded as the ancestors of the Chordata (to which back-boned animals belong), as well as the nature of the evidence that their study has been, or may be, made to yield.
P. Sladen, "Homologies of the Primary Larval Plates in the Test of Brachiate Echinoderms," Quart.
This distinction is, however, not so important as it appears at first sight, for their connexion with the bone is only of a secondary nature, and, although it happens conveniently that in the great majority of cases the division between the bones coincides with the interspace between the third and fourth tooth of the series, still, when it does not, as in the mole, too much weight must not be given to this fact, if it contravenes other reasons for determining the homologies of the teeth.
This is very distinct in the canine teeth of the Carnivora, and is a useful guide in determining the homologies of the teeth of the two jaws.
The great length and slender proportions of the segments give the cone a peculiar character, but the relations of position appear to leave no doubt as to the homologies with the fructification of Sphenophylleae; as regards the sporangiophores, Bowmanites Romeri occupies exactly the middle place between S.
The morphology of Stigmaria has been much discussed; possibly the main axes, which do not agree perfectly either with rhizomes or roots, may best be regarded as comparable with the rhizophores of Selaginellae; they have also been compared with the embryonic stem, or protocorm, of certain species of Lycopodium; the homologies of the appendages with the roots of recent Lycopods appear manifest.
Much further investigation will be needed before the homologies between Cordaitean cones and the fructifications of the higher Cryptogams can be established.
But these names will probably have to be changed when the homologies of these parts are better understood.