Lepidocarpon affords a striking instance of homoplastic modification, for there i s no reason to suppose that the Lycopods were on the line of descent of any existing Spermophyta.
The pair tramped further to the next closest point where they could see his line of descent to the river far below.
If we consider only the logarithms of numbers, the main line of descent from the original calculation of Briggs and Vlacq is Roe, John Newton, Sherwin, Gardiner; there are then two branches, viz.
Some authors have been so much impressed by the similarity of this extinct family to the Cycads, that they have regarded them as being on the direct line of descent of the latter group; it is more probable, however, that they formed a short divergent phylum, distinct, though not remote, from the Cycadean stock.
The Arachnida form a distinct class or line of descent in the grade Euarthropoda, diverging (perhaps in common at the start with the Crustacea) from primitive Euarthropods, which gave rise also to the separate lines of descent known as the classes Diplopoda, Crustacea, Chilopoda and Hexapoda.
The line of descent of recent cycads is comparatively clear in so far as they have undoubted affinity with Palaeozoic plants which combined cycadean and filicinean features; but opinion is much more divided as to the nature of the phylum from which the conifers are derived.
Osborn, who recognises four genera, Titanotherium, Megacerops, Symborodon and Brontotherium, in the typical section of the family, considers that each of these represents a distinct line of descent from the Palaeosyops-like group. The whole assemblage forms one of the four main sections of the Perissodactyla, namely the Titanotheroidea.
Direct line of descent of the modern horses, and, on account of the length of the nasals and their slits, the same probably holds good for Hippidium.
Dinotherium is a member of the group Proboscidea, of the line of descent of the elephants.
That line of descent can be made out with convincing clearness and with no particular difficulty from epoch to epoch, from the precarium and the patrocinium, through the benefice and commendation, to the fief and vassalage.