If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa.
The result is, on the one hand, a clearing away of much fantastic phylogeny, on the other, an enormous reduction of the supposed gaps between groups.
The term morphology, which was introduced into science by Goethe (1817), designates, in the first place, the study of the form and composition of the body and of the parts of which the body may consist; secondly, the relations of the parts of the same body; thirdly, the comparison of the bodies or parts of the bodies of plants of different kinds; fourthly, the study of the development of the body and of its parts (ontogeny); fifthly, the investigation of the historical origin and descent of the body and its parts (phylogeny); and, lastly, the consideration of the relation of the parts of the body to their various functions, a study that is known as organography.
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.
In this, as in all morphological inquiries, two lines of investigation have to be followed, the phylogenetic and the ontogenetic. Beginning with its phylogeny, it appears, so far as present knowledge goes, that the differentiation of the shoot of the sporophyte into stem and leaf first occurred in the Pteridophyta; and, in accordance with the views of Bower (Origin of a Land..
The phylogeny of the various floral leaves, for instance, was generally traced as follows: foliage-leaf, bract, sepal, petal, stamen and carpel (sporophylls)in accordance with what Goethe termed ascending metamorphosis.
Accepting this view of the phylogeny of the leaf, the perianthleaves (sepals and petals) and the foliage-leaves may be regarded as modified or metamorphosed sporophylls; that is, as leaves which are adapted to functions other than the bearing of spores.
If we admit that the larva has, in the phylogeny of insects, gradually diverged from the imago, and if we recollect that in the ontogeny the larva has always to become the imago (and of course still does so) notwithstanding the increased difficulty of the transformation, we cannot but recognize that a period of helplessness in which the transformation may take place is to be expected.
No indications beyond those furnished by comparative anatomy help us to unravel the phylogeny of the Collembola.
Osborn, "Phylogeny of the Rhinoceroses of Europe," op. cit.
(Berlin, 1899); and the classification and phylogeny are considered by E.
C. Seward and others, and has led to important discoveries on the nature of extinct groups of plants and also on the phylogeny of existing groups.
Parker ("On the Cranial Osteology, Classification and Phylogeny of the Dinornithidae," Tr.
The great philosophical impulse was that given by Darwin in 1859 through his demonstration of the theory of descent, which gave tremendous zest to the search for pedigrees (phylogeny) of the existing and extinct types of animal and plant life.
His speculations on phylogeny, or the descent of invertebrates and vertebrates, were, however, most fantastic and bore no relation to palaeontological evidence.
Nevertheless, the tracing of phylogeny, or direct lines of descent, suddenly began to attract far more interest than the naming and description of species.
This law, that in the stages of growth of individual development (ontogeny), an animal repeats the stages of its ancestral evolution (phylogeny) was, as we have stated, anticipated by d'Orbigny.
In an epoch-making essay, On the Parallelism between the Different stages of Life in the individual and those in the entire group of the Molluscous Order Tetrabranchiata (1866), and in a number of subsequent memoirs, among which Genesis of the Arietidae (1889) and Phylogeny of Characteristic (1894) should be mentioned, he laid the foundations, by methods of the most exact analysis, for all future recapitulation work of invertebrate palaeontologists.
Attention has been especially directed to the investigation of the most primitive forms in each group, and accordingly we can now form much more definite conceptions of the phylogeny and evolution of the various classes.
A, ap, c, c.o, c.t, d.v, e, ep, g Whatever may be the conclusion as to the position of Dinophilus and Histriodrilus, it seems only reasonable to suppose that Polygordius and Protodrilus, so far from representing a stage in the phylogeny of the Annelida before setae were developed, have lost the setae, which are already in a reduced state in Saccocirrus.
The position of Angiosperms as the highest plant-group is unassailable, but of the point or points of their origin from the general stem of the plant kingdom, and of the path Phylogeny or paths of their evolution, we can as yet say little.
Within each class the flower-characters as the essential feature of Angiosperms supply the clue to phylogeny, but the uncertainty regarding the construction of the primitive angiospermous flower gives a fundamental point of divergence in attempts to construct progressive sequences of the families.
Throughout the whole, the researches made since 1860 have not only added a great throng of new species, genera and families, but have thrown a flood of light upon questions of their phylogeny, systematic arrangement, horizontal and bathymetric distribution, organization, habits of life and economic importance.
Pediculatum - can be matched by Algae such as Oocardium, Hydrurus, and some Diatoms. It is clear then that the bacteria are very possibly a heterogeneous group, and in the present state of our knowledge their phylogeny must be considered as very doubtful.
On the data afforded by comparative anatomy and embryology in attempting to reconstruct the probable phylogeny of the class.
A feature of interest in connexion with the phylogeny of cycads is the presence of long hairs clothing the scale-leaves, and forming a cap on the summit of the stem-apex or attached to the bases of petioles; on some fossil cycadean plants these outgrowths have the form of scales, and are identical in structure with the ramenta (paleae) of the majority of ferns.
What is known at present, while it does not indicate the phylogeny of the Lycopodiales, at least shows that such living orders as Lycopodiaceae and Selaginellaceae cannot be regarded as forming a linear series.
The available evidence does not suffice Phylogeny to solve this question, although certain indications exist.
Cope, "On the Phylogeny of the Vertebrata," Proc. Amer.
Iii., 1899 Morgan, " Embryology and Phylogeny of the Pycnogonids," Biol.
For these reasons no attempt has been made to arrange the orders in larger divisions, since such a division as that of the ligulate and eligulate forms, while convenient for practical purposes, may not express the phylogeny of the group. The Psilotaceae, formerly included in the Lycopodiales, have been described separately owing to their resemblance to the Sphenophyllales.
C. Jeffrey, " The Comparative Anatomy and Phylogeny of the Coniferales, part i.