222 PROCEKDINGS OF THE AMERICAN ACADEMY. 



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 2:)roper 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 acceleration in the inheritance of characters, or tachyo'enesis. 

 Thus it has been found that characteristics are inherited in successive 

 species or forms in a given stock at earlier and earlier stages in the onto- 

 geny of each member of the series. These characteristics, as a rule, 

 disappear from the ontogeny altogether in the terminal, or last occurring, 

 members of a series, and terminal forms thus become very distinct in 

 their development. This law I habitually illustrate as the crawling, 

 walking, hopping, skipping, and jumping law. 



Auotlier result of this mode of study is the discovery that, in most 

 genetic series, primitive forms exhibit much greater iiulifference to 

 geologic changes, persist with comparatively unchanged structures 

 through longer periods of time than those that occur at the acme of 

 groups, and paracmatic forms, if widely distributed, are apt to be par- 

 ticularly short lived, and are very often narrowly localized in origin and 

 duration. Primitive forms are also less changeable in their ontogeny ; 

 the adult differs 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 from each other as stages than in acmatic forms. 

 Primitive forms are less affected by gerontic changes in their ontogeny, 

 that is, they have shorter old-age stages, than acmatic forms. Parac- 

 matic forms have much longer old-age or gerontic stages than acmatic 

 forms. 



Lastly, it has been found that at the beginning of the evolution of any 

 stock the progress was not only very rapid, but the departures in struc- 

 tures much more marked between the diverging lines of different species, 

 genera, or families, and so on, than those that subsequently occurred in 

 any one of these. This rapidity of expansion is also marvellously sudden 

 in every series near its point of origin, and it is equally so in the whole 

 animal kingdom, which appears with the larger proportion of all its 

 principal divisions in the earliest known fossil-bearing rocks. Each series 

 or type appears to have had a more or less free field, and its first steps 

 in evolution were obviously not affected by natural selection. Subse- 

 quently, in the acme of the same series or type, the departures became 

 less marked, and the divergences took place in less important structures; 

 in other words, as stated above, the evolution is slower. 



