Ctstoidea; Crinoidea.) At a date after the
Cystoidea and Blastoidea had become extinct, and
after the Echiuoidea had passed throu{;h a con-
siderable amomit of evolution, there appeared in
the Triassic the isolated genus Tiarechinus, which
resembles both the Blastoidea and the Kchinoidea,
and whieh thus constitutes a synthetic type l)e-
tween these two classes. The Phjilocarida form
fa generalized group connecting the Entomostraca
and Malacostraca ; the Jlerostomatai, containing
the Eurypterida; and Liniulidie, connect the Crus-
tacea and Araehnida (especially the scorpions) ;
the suborder Condylarthra of Lower Eocene time
contains the ancestors of all the later suborders
of the Ungulata, and also presents characters
resembling those of the Carnivora; and the
Gnetaceae are synthetic between the angiosperms
and gymnosperms. Xearly all races of fossil-
animals that can be traced back through ancestral
forms are found to have their origin in such
groups of generalized types.
Paleontology and Evolution. The causes of variability among species, the meaning and processes of evolution and natural selection, and the relations between evolution and classification are considered in other articles under those particular titles. The bearing of paleontnlogic research upon these subjects and some of the results attained deserve brief notice here, and for further information regarding these lines of research the reader is referred to the papers cited in the bibliography at the end of this article. The following lines of research have been dis- tinguished: Auxology or Bathmologv', the study of growth of organisms; Genesiology, the study of heredity; Ctetology, relating to the origin of acquired characteristics; and Bioplastology, deal- ing with the correlation of the ontogeny and phylogeny. or the stages of development with those of evolution.
Embrtogext of Fossil Organi.sms. Emlny- onic shells of mollusks. brachiopods, and crusta- ceans are sometimes found as minute objects in higlily fossiliferous shales and limestone. Some adult shells retain at their apices the form of the embryonic shell, and others, like the ammonites, have the young shell which hatched from the egg inclosed within the centre of their coiled disks. By examining large numbers of brachio- pod shells of all sizes, Beecher and Clarke, and later Schuchert, were able to arrange the in- dividuals of certain species in series according to size, and to show that they all were derived from an embryonic stage, called the protegulum, of very simple form, resembling the Cambrian genus Paterina. They have shown that members of all the families of brachiopods began their existence as paterina-like shells, and that the distinctive adult shapes have been attained through modifications in the mode of growth of the shell during the stages subsequent to the protegulum stage. Beecher has shown that mem- bers of the principal families of trilobites began their existence as embryonic forms, called the 'protaspis.' which is comparable with the pro- tonauplius stage of the more primitive living Crustacea. Among the corals, several fossil genera, as Favosites, Syringopora, etc. (fide Beecher and Girty). pass through an embryonic stage that resembles another fossil coral Aulopora. The nautiloid and ammonoid cephalopods present the most favorable conditions for the preserva- tion of the embryonic stages of growth, because their shells hold within their centres all the successive stages through which they have passed in their ontogenetic development. ' By breaking open such a shell the developmental stages can be studied from the earliest -protoconch' hatched from the egg and found at the centre of the coil, to the senile or old age stage represented by the last chamber in which the animal lived." See Cephalopoda.
Agassiz's Law of Recapitulation, subsequent- ly termed by Haeckel the law of palingenesis, according to which the stages of development or ontogeny of the individual can be correlated with the stages of evolution or phylogenj' of the race to which the particular individual belongs, has received abundant confirmative evidence from paleontology', and the literature on this and allied branches of research is quite formidable, esjw- cially in its technicality of expression. The fol- lowing scheme of terms adapted from Hyatt has been devised to distinguish the corresponding stages :
 | A table should appear at this position in the text. See Help:Table for formatting instructions. |
Ontogeny, or Development Collaqmal Technical Fte tal Embryonic Baby Neplonic Adolescent Xeanlc Adult Ephebic Senile Gerontic Phylogeny. or Evolution of Race Stages Epacme Pbylembryonic Phylouepionic Phyloneanic Phylephebic Acme Pbylo^rontic Paracme
Fine illustrations of such correlations between ontogenetic and phylogcnetic stages have been fur- nished Ijy Hyatt's study of the Arietid;p. a group of ammonites; by Beecher's demonstration of the phylogeny of the Tcrebratellidic. a family of brachiopods; and by Beecher's studies on the larval forms of trilobites. This palingenetic law is of much value to the stratigraphic paleon- tologist, for it enables him to correlate geological formations of which the faunas consist of wholly unknown specie.s. It also enables him to postu- late the existence in earlier formations of un- known genera which when found will prove to be counterparts of larval or adolescent stages of species already under observation.
Acceleration and Retard.a.tion. In some cases the correspondence between the two classes of stages mentioned above is incomplete, through action of 'tacliygenesis,'or 'acceleration of devel- opment,' which has been defined by Hyatt as fol- lows: "All modifications and variations in pro- gressive series tend to appear first in the adolescent or adult stages of growth, and then to be inherited in successive descendants at earlier and earlier stages according to the law of ac- celeration, untij they either become embryonic, or are crowded out of the organization, and replaced in the development by characteristics of later origin." Examples are seen in the spiny larviB of the trilobites Acidaspis and Argcs, which differ greatly from the smooth protaspis stages of other trilobites. Retardation of development is the re- verse of acceleration and is due to the later stages dropping out of the ontogeny; in other words, animals in which this operates grow old quickly. Examples are afforded by some Cretaceous ain- monites which have sutures of goniatitic and ceratitic type. These ammonites are derived from Jurassic ancestors having complex sutures, but they never attained their normal development; they stopped growing in their youth. Similar cases have been noted among the brachiopods.
