214 



THE DOCTRINE OF DESCENT. 



cular pleasure," says Wiirtenberger, " when, after divers 

 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 import- 

 ance 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 



FlG. 19. Ammonites Humphresianus. A form analogous to the Planulata. 



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

 of 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. 

 Frequently a modification of this sort has taken posses- 

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

 new one already appears at the outside, and follows the 



