Hyatt.] 380 [June 7, 



generations of individuals, has been heretofore supposed to be con- 

 fined to the inheritance of adult characteristics, or to those character- 

 istics which make their appearance in the parents previous to the 

 period of reproduction. 



Heretofore it has also been generally assumed that only the active 

 elements of the growth of the parts and their tendency to the more or 

 less powerful performance of certain functions were necessarily inher- 

 ited. It has not, so far as I know, been even hinted at that animals 

 could also inherit a tendency to change in a way that was unfavora- 

 ble to the continued existence of a race or group as a whole. This is, 

 however, not an isolated but a very general fact among the Am- 

 monites. The successive species in almost all large groups sooner or 

 later inherit the old age tendencies of their ancestors so completely, 

 that they manifest these even in their early stages. In other words, 

 they never attain a stage which can be closely compared with the 

 adult stages of the most common or characteristic of the ancestral 

 forms. This is left out. The embryo passes into the young, and the 

 young proceeds by growth to develop parts and organs entirely want- 

 ing in those characteristics which distinguished the similar parts and 

 organs of the adults of the ancestral forms. When we compare these 

 accelerated forms, or forms which have thus skipped some of the 

 previously existing stages of their ancestors, with the senile stages of 

 those same ancestral forms, they present a correspondence of greater 

 or less exactness, according to their affinities, sometimes very per- 

 fect, sometimes very remote. Thus the old age stages of one of the 

 Arietidae do not at all closely resemble those of H umphriesianum', 

 the complete correspondences are limited to genetically connected 

 series or groups, and sometimes only to organs or certain sets of 

 organs which alone show the effects of senility in the individuals and 

 in the group. The fact of the inheritance of old age characteristics, 

 and of the extinction of types as shown in this way, is, however, of 

 general application, and will probably be found in all departments of 

 the animal kingdom. 



Of course it will be readily understood that these statements apply 

 only to the most perfect groups, or those which complete their cycles 

 of forms. It is not intended to assert that every group has an old 

 age, or even that every individual has; on the contrary, there are 

 Borne forms in nearly every large group among the earliest ancestors 

 which manifest senility very slightly, though attaining. a very large 

 size, and there are some groups which show only a partial decline, or 



