214 "^'^^ DOCTRINE OF DESCENT. 



careful comparative studies, I at last detected an inter- 

 esting and simple conformity to law in the variations of 

 the Ammonites. Namely, on the first appearance of a 

 modification which subsequently attains essential im- 

 portance in an entire group, it is only slightly indicated on 

 a portion of the last convolution. Towards more recent 

 deposits, this modification is more and more plainly 

 shown, and then advances, following the spiral course of 

 the shell; that is to say, it gradually takes possession of 



FIG. 19. — Ammonites Humphresianus. A form analogous to the Planulata. 



the central turns also, as we trace the forms to higher 

 strata. This reproduction in younger stages of life of 

 modifications first occurring at a more advanced age, 

 makes but slow progress-, so that we see the older forms 

 repeated with great persistency in the central turns. Fre- 

 quently a modification of this sort has taken possession 

 of only a small part of the convolutions, when a new 

 one already appears at the outside, and follows the 

 first. Thus searching through the strata from below 



