1876.] 377 [Hyatt. 



than any other form. This fact is very significant when we observe 

 how completely it appears to be the genetic source or origin from 

 which spring all the other forms of the group. 



If this were an isolated result I should be slow to attach much im- 

 portance to it, but I am constantly confronted in these researches by 

 the fact, not only that the simplest or most embryonic forms, those 

 standing nearest to the source or roots of a group, are the most pro- 

 lific; but often that those among their direct descendants which retain 

 this simple structure are the longest lived, most enduring and least 

 changeable of all others. Compare for instance, the slight differences 

 existing between Steph. subleve in the Athleta-bed, and Steph. subcor- 

 onatum or Blagdeni in the Humphriesianus-bed, with those between 

 the same species and macrocephalum or platystomum or Sauzei] also 

 the longer existence of this series with that of the other and more 

 changeable series. 



Not only are the changes observable in the whole series from sub- 

 coronatum to subleve less marked, but tliis necessarily correllates with 

 the changes in the course of individual development and growth 

 which are also less marked ; there is less force used up in the produc 

 tion of new characteristics in the ancestral forms, and therefore a 

 greater capacity for propagation and resistance to the modifying 

 effects of changing conditions of climate and habitat. The forms of 

 subcoronaium, which exhibit no marks of senility even when very 

 large, lead directly into the true Blagdeni forms. On the other hand, 

 those which change much in old age exhibit intermediate stages, in 

 which the abdomen becomes rounded and more elevated, and the 

 ribs similar to those of Deslongchampsu. Though not able to trace 

 this connection so fully as the others, there seems to be reasonable 

 ground for joining plicatissimum with Deslongchampsu, since both of 

 these exhibit similar characteristics. 



In following the coronalum series from Blagdeni, we are struck by 

 the gradations which gradually lead the observer from the immature 

 form of Blagdeni to the flat-sided, untuberculated form of planulum, 

 with its elevated abdomen. This, as I have previously pointed out, is 

 a direct repetition of the retrogressive old age characteristics of the 

 individual, as shown in Humphriesianum, and in some specimens of 

 subcoronatum. 



The individuals of one series, the macrocephalum series, show old 

 ao-e onlv in the elevation and narrowing of the abdomen. There is 

 here but a slight retrogression, so far as the individual is concerned. 



