ACCELERATION AND RETARDATIoN. In scone cases the correspondence between the two classes of stages mentioned above is incomplete, throogh action of 'tachygenesis,' or 'acceleration of d opment,' which has been defined by Hyatt as fol ”Ail variati.ms in pro gressive series tend to appear f.r-t in the adolescent or adult stages of growth, :Ind then 10 be inherited in successive descendants at earlier and earlier stages according to the law of ac celeration, until they either beemne embryonic, or are crowded out of the organization, and replaced in the development by of later Example,: are seen in the spiny lary.e of the trilobites Aeidaspis and Arges, which differ greatly from the protaspi: stages 4 If ot ner trilobites. Betardation of development is the re verse of acceleration and is due to the later stages dropping out of the ontogeny : in other wi animals in which this operate: grow old quickly. are atTorded some am monites which have sutures of goniatitie and eeratilic type. These ammonites are derived from Jurassic ancestors having complex sutur.
they never attained their normal development : they stopped growing in their youth. ''zin.i'ar cases have been noted among the brachiopods, and the larval condition of many parasites may be explained in this manner. l:etardation of acceleration results often in regressive evolution. These laws of acceleration and retardation ex plain disturbing factors in the application of embryogenic methods to elucidation of the past histories of the races of modern animals, and enable us to understand 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.
something acquired). The question of the inheritance of acquired characters, one of the cardinal principles of evolution, has been denied by some zoUlogists, and affirmed by most paleontologists. Ilyatt, 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 any 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 consequent influence upon the form of the shell. This impressed zone ap peared first in the adult stages of early Paleo zoic nautiloids, and through inheritance and ae celeration it became fixed in successively earlier stages of growth in the succeeding nautiloids of the later Paleozoic.