4' 



Acceleration of Development in Fossil 

 Cephalopoda 



JAMES PERRIN SMITH. 



Ideal Recapitulation in Progressive Forms. 



IN THE development of organisms there are two theoretical extremes, 

 the one with simple persistence without modification, the other with 

 complete modification. The former is almost realized in the Protozoa, 

 the latter is approached by the higher vertebrates. All other organisms, 

 in their development, fall somewhere between the two extremes, coming 

 into being in simpler form, and becoming more complex in the course 

 of life. Each starts out on somewhat the same plane of development as 

 its distant ancestors, inheriting potentially all the characters of all its 

 ancestors, tending to take on some characters that its ancestors never 

 had, and to transmit the old and the new to its own posterity. 



Theoretically, each organism ought to recapitulate all its race his- 

 tory, each stage of growth corresponding in character and in size to 

 successive ancestral forms. This is true, in a general way, in some 

 groups, for most later members of genetic series have increased in size 

 with increased complexity of development. 



FIG. 1. 



This is partly true even of the highly specialized Cephalopods, for 

 there is a constant tendency to increase in size from the simple Goniatites 

 of the Devonian to the complex Ammonites of the Jurassic. The increase 

 in size accompanying the addition of ontogenic stages is especially strik- 

 ing in a primitive genetic series of genera near each other in time, and 

 relatively near the beginning of the race, as in the lineage of Goniatites- 

 Gastrioceras-C olumbites. 



