Geol.— Vol. I.] SMITH— PLACENTICER AS. 229 



and Hoflites ancestors were the same for both species, and 

 the differentiation could not have taken place before the 

 Lower Cretaceous ; the difference is due to unequal accel- 

 eration of characters. 



The development of Placenticeras gives us an unusually 

 fine illustration of the law of acceleration, or tachygenesis, 

 with its two corollaries, unequal acceleration and retard- 

 ation. In species from the Paleozoic or early Mesozoic we 

 get better correlations of growth stages with ancestral gen- 

 era, for with them there is little unequal acceleration, 

 and almost no retardation apparent. But they are usually 

 so poorly preserved that this sort of work is impossible with 

 them. In Cretaceous species the preservation is usually 

 better and the young specimens may be taken out from the 

 old in good state of preservation, and the comparison with 

 supposed ancestral genera facilitated. But in these later 

 genera, so far removed in time from their origin, the devel- 

 opment is so much more complex, on account of unequal 

 acceleration, and in some cases on account of retardation, 

 that correlation with ancestral genera is no longer a simple 

 problem, although all the more fascinating because of its 

 difficulty ; it calls to its aid all the resources of biology and 

 geology. 



Stanford University, 

 California, 



March, 1900. 



(4) August II, 1900. 



