MORPHOLOGICAL DIFFERENTIATION. 341 



When \vc observe the number of the different species of a 

 genus, recorded in the rocks of each period in which the genus 

 occurs, we find that the greater number of species of the 

 genus, as well as the greater number of genera of the family, 

 are recorded from the initial geological period, which in this 

 case is the Ordovician ; and the genera and the species gradu- 

 ally decrease in number for each following period until the 

 close of the Paleozoic, with the exception of the genera Nau- 

 tilus and Trochoceras, whose expansion appears to be later 

 and its life-period longer. Even in these cases, however, the 

 law is relatively the same. 



This, again, is expressive of the general law before stated, 

 that the chief expansion of any type of organisms takes place at 

 a relatively early period in its life-history. This law was ob- 

 served in the case of the Brachiopods, and is observed here 

 among the Cephalopods. There are some modifications or 

 exceptions to it, which the facts regarding other groups 

 suggest ; but the general law is suf^ciently well attested to 

 be defined in these general terms. 



Hyatt's Formulation of the Law of Rapid Expansion of Differ- 

 entiation at the Point of Origin of a New Type of Organism. — 

 Hyatt has given expression to this law in an article on 

 "Genera of Fossil Cephalopods."* The generalization is 

 based upon a very exhaustive study of the Cephalopods. 

 He had access to the collections in the Agassiz Museum of 

 Natural History, which was the most complete in this 

 country; and he also visited all the museums in this 

 country and in Europe where Cephalopods are found, and 

 made particular examination of every species he could learn 

 of throughout the scientific world. Speaking of the Nau- 

 tiloidea and Ammonoidea both, he wrote: "These groups 

 originate suddenly and spread out with great rapidity, and in 

 some cases, as in the ArietidcE of the Lower Lias, are traceable 

 to an origin in one well-defined species, which occurs in close 

 proximity to the whole group in the lowest bed of the same 

 formation. These facts, and the acknowledged sudden ap- 

 pearance of large numbers of all the distinct types of In- 



* Published in 1883, in the Proceedings of the Boston Society of Natural His- 

 tory. 



