Page:The New International Encyclopædia 1st ed. v. 15.djvu/285

and the larval condition of many parasites may be explained in this manner. Retardation of acceleration results often in regressive evolution. Thesc laws of acceleration and retardation ex- jilain disturbing factors in the application of embryogcnic methods to elucidation of the past histories of the races of modern animals, and enable us to undcr.stand why the ontogeny of a living animal cannot always be depended upon to furnish a synopsis of the sequence of events that have occurred in the geological history of its phylum.

Ctetology {KTTiTit, something acquired). The question of the inheritance of acquired cluiracters, one of the cardinal princi])lcs of evolution, has been denied by some zoiilogists, and allirnied by most paleontologists. Hyatt, after tracing the genetic relations of varieties and species of fos- sil cephalopods through geologic time, came to the conclusion "that there is no class of characteris- tics which may be described as non-inheritable," and he has proved beyond an.y doubt the inher- itance of one particular characteristic, namely the so-called 'impressed zone,' of the nautiloids, due to the adoption by the animal of a crawling mode of life, with its" eonsecpieut inlliience upon the form of the shell. This impressed zone ap- peared lirst in tile adult stages of early Paleo- zoic nautiloids. and through inheritance and ac- celeration it became fixed in successively earlier stages of growth in the succeeding nautiloids of the later Paleozoic.

Gerons and Piiylogebons. When an organism has passed the acme of its development and be- gins to get old it sometimes acquires at that late date pc;uliar characteristics that seem to in- dicate loss of energy. The shells of mollusks and brachiopods thicken and the ornamentation de- creases in prominence, etc. Among fossil cephalo- pods the last whorl often shows a tendency to uncoil. By acceleration these gerontic characters appear earlier in the ontogeny of succeeding spe- cies, and finally the normal closel.v coiled shell is found only in the verj' youngest stages of the phylogerontic members of the race.

Ad.pt.tion and Specialization. The manner in which many types of modern organisms have become adapted to their particular modes of exist- ence is often well illustrated by their fossil an- cestors, in which may be observed a gradual change in organization from earlier less special- ized and less adapted types. One of the best known cases of gradual adaptation is that of the horse, whose one-toed hoof has become adapted from an original five-toed Eocene ancestor of gen- eralized type. ( See HoBSE, Fossil. ) Some cases of iidaptation to particular modes of existence reached a higlicr degree of perfection during past times than any known at the present day. There was a winged reptile, Ornithostoma or Pteranodon of the Cretaceous, in comparison with which the best of modern birds are mere tyros. This crea- ture, with a body weighing not more than thirty pounds, had wings that spread twenty feet from tip to tip, and a skeleton so delicate that the bones are almost paperlike in structure. The cause of extinction of such a creature, which would seem to have been almost beyond the in- fluences which are known to have caused the ex- tinction of other races of animals, is as yet a mystery.

Extinction of Organisms, One of the most impressive phenomena brought to the attention of the student of paleontology is the extinction or disappearance of species, families, and even whole orders, as well as faunas, of fossil animals and plants. In some cases tlic rate of disappearance is gradual, the group diiiiinishing in ini])ortaiice before final extinction occurs; in other cases ex- tinction is sudden, as if due to .some catastrophe. E.xtinctions of the latter type are usually asso- ciated with important changes in the sediments or with unconformities, and occur at levels which have been used to mark the limits of geological systems or formations. In many cases such ex- tinction is only apparent, and is due to migration

Eras Paleozoic Mesozoic Cenozoic DominsntTvpes

Invertebrates Fish RtPTILES Mammals Periods

Sponges

Graptolites

Hydroid Corals

Corals

Crinoids

Cystoids

Blastoids

Echinoids

Worms

Polyzoa

Brachiopoda

Pelecypoda

Nautiloidea

Ammonoidea

Trilobftes

Phyllocarida

Decapoda

Merostomata

Insecta

Amphibia

Reptiles

Mammals

Distribution of Important Groups of Fossil. .MMALe. of the fauna into some distant or unknown re- gion. In some cases the extinction is very real, entire groups of animals having been,, as it were, suddenly annihilated. Some types of organisms after having enjoved a more or less extended pe- riod of life have slowly died out and have become extinct, a|i])arciitly uninlluenccd by any physio- graphic catastrophe, after having passed through a maximum period of evolution and a subsiTpicnt phylogerontic decline. They liave grown old and died aiqiarentlj' through lack of growth force.