Hyatt.] 238 [December 16, 



of the third and succeeding stages of the young, while the latter re- 

 sult from the natural but inevitable loss of growth-force in the adult 

 shell and its parts. 



This growth seems to me to be due to the favorable nature of the 

 physical surroundings, primarily producing characteristic changes 

 which become perpetuated and increased by inheritance within the 

 group. We can recognize this in the constantly increasing size of 

 the shell, complication and development of the new parts, as has been 

 shown by Prof. Cope in his "Method of Creation of Organic Types." 

 Though he do3S not attribute so much to the influence of the physical 

 surroundings as has been done here, the results of my investigations 

 are, as they have been heretofore, very similar to his. 



The law or general expression for the mode of inheritance by 

 which this is accomplished is the same for all characteristics, whether 

 of form or structure ; namely, that of acceleration. By this I mean 

 the constant tendency of every individual to inherit the characteris- 

 tics of its parents at earlier periods than those in which they have 

 appeared in the parents themselves. I know of no exception to this 

 law, whether the characteristics are due to a healthy adult condition 

 or to old age ; whether they precede or succeed the supposed period 

 of reproduction. This I have already treated of fully in previous 

 publications, and need only refer to the old age parallel forms, 

 presently to be treated of, in order to make it clear to every zoolo- 

 gist that senile characteristics must be inherited or these series of se- 

 nile parallel forms could have no existence. 



This constant tendency to reproduce the ancestral characteristics 

 at earlier and earlier stages accounts for the reduction of the princi- 

 pal characteristics of the Nautiloids and Goniatites to an embryonic 

 condition in the young of the Jurassic Ammonites. 



It also accounts for the inheritance of the more and more involved 

 form in each of the subordinate series. This becomes apparent when 

 the parallel forms of any series are traced from the primary discoidal 

 or open umbilicated through the intermediate forms to the most 

 completely involved. 



We find in all cases the more discoidal or primary with all its 

 characteristics, whatever they may be, repeated at earlier stages in 

 each species, until at last in some of the most involved, all percepti- 

 ble traces of its existence are lost. Then and only then can the series 

 be said to die a natural death. When this form appears I have never 

 found another. The reason for this is that in all cases the disappear- 



