20 ACCELERATION OF DEVELOPMENT IN FOSSIL CEPHALOPODA 



There are three sorts, or rather ideas, of genetic series, as shown 

 by the accompanying diagrams. No. I, on the diagram, where we have 

 a narrow straight line of connected genera or species, would show 

 straight natural selection, if this were in harmony with the evidence 

 of paleontology, but it is not. 



FIG. 5. 



No. II in the diagram gives the commonly accepted idea of a 

 genetic series. Hyatt's genesis of the Jurassic Ammonites proposes 

 such a genetic line, and derives all the later forms from Psiloceras, 

 which is itself a degenerate. I have always agreed with Steinmann 

 in thinking that this idea vv'as improbable, to say the least. We find 

 early in the Lower Jurassic the Arietidae distributed in Europe, Asia, 

 North and South America, and the Indian Ocean; hence it is unlikely, 

 leaving morphology out of the question, that the rare dwarf Psiloceras 

 of the Mediterranean Region was the parent of this varied progeny. 

 The theory expressed in No. II in the diagram has always reminded 

 the writer very forcibly of the Noachian fable in the history of the 

 human race. 



No. Ill in the diagram shows the conditions as the paleontologist 

 finds them, regardless of any theory. He sees a number of species in 

 a genus, and a number of genera, in a family, all tending in somewhat 

 the same direction, as he traces them upwards through the geologic 

 ages. He finds no complete unbroken series, but a series of steps. 



Is this orthogenesis? In a general way it is, although giving a 

 name to a phenomenon is not giving an explanation. There are only 

 certain lines of variation possible, and when the organism starts out 

 with certain characters it can vary only in more or less definite direc- 

 tions, some of which will coincide in different species, genera, and 

 families. There need not be any mysterious force directing the evolution ; 

 it may be merely the limitations of the characters of the organisms. 



