Januaey 29, 1897.] . 



SCIENCE. 



.169 



rangement and classification forms occur- 

 ring in the paracme, just as Agassiz's law of 

 palingenesis can be used to explain the re- 

 lations of the links in the chain of being 

 forming the epacme of groups. 



The cycle of the ontogeny is, therefore, 

 the individual expression and abbreviated 

 recapitulation of the cycle that occurs in 

 the phylogeny of the same stock, and, while 

 the embryonic, nepionic and neanic stages 

 give us, in abbreviated shape, the record of 

 the epacme, the gerontic stages give, in a 

 similar manner, the history of the paracmo. 



The difference between the nature of the 

 two records is, however, necessarily as 

 great as between the beginnings and the 

 endings of existence. The successive stages 

 of the individual are derived from the past, 

 and simply point backwards along the 

 track traversed by the phylum ; the suc- 

 cessive changes of the gerontic stage on 

 the other point to the future, and are pro- 

 phetic of what is to come in the decline of 

 the type. The retrogressive decline of the 

 individual and of its type are along par- 

 allel lines and the two are in direct correla- 

 tion, so that the former becomes an abbrevi- 

 ated index of the latter. 



One of the most useful results of these 

 studies has been the method of work de- 

 veloped, the mode of study by series. To 

 follow it out successfully one must trace 

 the terms of series from the first, or most 

 primitive, grade to the last, through per- 

 haps long periods of time and, if upon the 

 same level, through many gradations of 

 structure. 



The histologist and embryologist picks 

 out a convenient form here and there for 

 thorough investigation, but does not seem 

 as yet to see the importance of the point of 

 view here insisted upon, viz., that the only 

 method of getting at the correlations of 

 ontogeny and phylogeny is by following 

 out the history of representative series of 

 genetically connected embryos, and the 



same is true of the experimentalist. While, 

 consequently, their results have been in the 

 highest degree instructive and progressive 

 along other lines of research, they throw no 

 very strong light on the laws of evolution, 

 and the best modern works on embryology, 

 zoology and experimentation neglect the 

 only proper and efficient mode of studying 

 one very important side of their subject. 



One of the results of this mode of study 

 has been the discovery of the law of ac- 

 celeration in the inheritance of characters, 

 or tachygenesis. Thus it has been found 

 that characteristics are inherited in succes- 

 sive species or forms in a given stock at 

 earlier and earlier stages in the ontogeny of 

 each member of the series. These charac- 

 teristics, as a rule, disappear from the ontog- 

 eny altogether in the terminal, or last-oc- 

 curring, members of a series, and terminal 

 forms thus become very distinct in their de- 

 velopment. This law I habitually illustrate 

 as the crawling, walking, hopping, skipping 

 and jumping law. 



Another result of this mode of study is 

 the discovery that, in most genetic series, 

 primitive forms exhibit much greater in- 

 difference to geologic changes, persist 

 with comparatively unchanged structures 

 through longer periods of time than those 

 that occur at the acme of groups, and par- 

 acmatic forms, if widely distributed, are 

 apt to be particularly short lived, and are 

 very often narrowly localized in origin and 

 duration. Primitive forms are also less 

 changeable in their ontogeny ; the adult dif- 

 fers less from either the young or the old 

 than in acmatic forms. The same is true 

 of phylogerontic forms ; their old age and 

 youth are less distinct as stages from each 

 other than in acmatic forms. Primitive 

 forms are less affected by gerontic changes 

 in their ontogeny, i. e., they have shorter 

 old-age stages than acmatic forms. Par- 

 acmatic forms have much longer old-age or 

 gerontic stages than acmatic forms. 



