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.
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.
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.
Thus an abortive supernumerary finger may not cause much, if any, inconvenience to the possessor, but nevertheless it must be regarded as a type of disease, which, trivial as it may appear, has a profound meaning in phylogeny and ontogeny.
On the data afforded by comparative anatomy and embryology in attempting to reconstruct the probable phylogeny of the class.
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.
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.
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.
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.
But there is reason to believe that they have been differentiated quite independently in various groups, such as the Marchantiaceae, the Jungermanniaceae, and the mosses proper; consequently their phylogeny is not the same, they are polyphyletic, and therefore they are not completely homologous, but are parallel developments.