Hyatt.] 384 [June 7, 



tendency to reproduce the old age characteristics of ancestors, ac- 

 cording to the law of acceleration at earlier and earlier periods in 

 successive descendants, and is the normal form of the decline of 

 genetic series; but besides this there are in some species corres- 

 ponding series of forms, evidently due to the unfavorable nature 

 of their surroundings, which are so quickly produced as to have the 

 effect of simultaneity, as if they sprang at once from one brood. The 

 former may be compared to the normal disease or senile period of a 

 healthy individual, and the latter to the premature old age of an un- 

 healthy or prematurely developed individual. 



In the embryo, therefore, we find permanency and exact heredi- 

 tary similarity; in the later stages of the young and the adult, the 

 novelties of adaptation to new or varied surroundings; and in the old 

 or senile periods, a diseased condition, in which these adaptations or 

 novelties tend to be absorbed or lost, and consequently greater uni- 

 formity is noticeable between the old than between adults. 



This precisely corresponds to the relations of a group composed of 

 several series derived from a common ancestor. At first, near the 

 point of origin, the series are similar organically, then great struc- 

 tural and morphological divergence takes place, and finally, though 

 they remain structually just as remote, similar forms begin to make 

 their appearance in the different series. 



I might go on endlessly with these comparisons, but it suffices to 

 say that the conclusions which I published in 1866, in the Memoirs 

 of this Society, — asserting that the life of an individual, and the life 

 of the genetically connected series to which the individual belonged, 

 could be directly correllated, that a series, like an individual, had 

 only a limited force available for growth, development and propaga- 

 tion, that the three stages of existence in the individual corresponded 

 respectively, the young to the past, the adult to the present, and the 

 senile to the future of the group, whatever it might be to which the 

 individual belonged, — have been confirmed by the minute analysis of 

 the groups of Jurassic Ammonites, and the more minute the analysis 

 the more complete the correspondence. 



Note. — Having used the word force in this essay with a very distinct 

 mean"ng from tint with which I first usel it in 1865, it becomes necessary to 

 define it. Organic force or vital force is, in my view, simply an expression for 

 the force resulting from the combination of chemical elements in an organic 

 form. 



