1876.] 379 [Hyatt. 



• 



leanum are much more prominent than in Humplriesianum and ma- 

 crocephalum; and the changes introduced are very distinct in these 

 from what they are in the distorted forms of Brongniartii and others; 

 again, the Pettos-like form is retained in the full grown and even old 

 specimens of the lower forms of the coronatum and subleve series, 

 whereas it is only a characteristic of the development of the young 

 in other series. 



In fact, the manifestations are exceedingly complicated, and pre- 

 vent the application of the three stages to the solution of the affini- 

 ties and to the classification of the genus as a whole, except in a very 

 general way. Thus it may be said that all the lower members of the 

 genus, Steph. nodosum and subcoronaium and contraction are similar 

 to Pettos, and that the higher, such as planulum, macroceplialum, 

 Brongniartii, etc., exhibit during the adult period senile charac- 

 teristics corresponding to the senile characteristics of the individual.. 

 But this can only be asserted as we have seen with considerable 

 qualification until each genetic series is considered by itself, then in- 

 deed an exact correspondence comes to light between the senility 

 of an ancestor and the senility of the descendant or congeneric 

 species. 



This statement exhibits completely the difference between geratol- 

 ogy 1 and embryology. With the former it is possible to indicate only 

 what must have been the dying forms of the particular genetic series 

 to which the individual belonged, whereas with the assistance of em- 

 bryology and the history of the younger stages we can with equal 

 probability point out an unknown ancestral form for all the series of 

 a group. The right use of both the correspondences of embryol- 

 ogy and of geratology gives the means of mapping out with considera- 

 ble probability both the past and future of groups from the study of 

 even a limited number of individuals. 



The laws of heredity secure the constant inheritance of the adult 

 characteristics of the parents at earlier and earlier periods in succes- 

 sive descendants, until the permanent characteristics of an adult an- 

 cestor, or what remains of them, becomes embryological. This tend- 

 ency to constantly reproduce similar characteristics in successive 



1 From yripas -aros, old age, and \6yos. I have adopted this new term with con- 

 siderable hesitation and doubt, and have only done so under the pressure of neces- 

 sity. In no other way can I better convey my conviction that there is a traceable 

 correspondence between all manifestations of decline in the individual and in the 

 group to which tbe individual belongs, which may, like embryology, be used induct- 

 ively in reasoning upon the probable affinities of animals. 



