SUPPLEMENT. -AMMONITE DEVELOPMENT. cxcix 



(pp. 133, 288) ; they have been fully stated by Hyatt in his ' Genesis of the Arietidae,' 

 while similar phenomena of development have been proved for Cephalopoda 

 generally by A. Hyatt, for various Ammonites by J. P. Smith, for Pelecypoda by 

 R. T. Jackson, for Brachiopoda by Beecher, by Schuchert, by Cumings and others, 

 and in both zoological and botanical series generally by Jackson. In fact, they arc 

 the phenomena of bioplastology. 



Such phenomena of development show a certain sequence — stages of elabora- 

 tion (anagenesis) are followed by stages of simplification (catagenesis) ; and 

 this applies not only to the ontogeny of the individual and the phylogeny 

 of the race, but also to the ontogeny and phylogeny of the particular features 

 or characters which distinguish one species, one genus, one race, from another. 

 Variation in the characters themselves along their lines of progression or 

 degeneration is a specific, but not a generic, distinction ; variation in the relative 

 development of characters is generic, so that in such associations as A, b, c ; a, B, c ; 

 a, b, C, indicating the characters of three species, the generic distinctions are the rela- 

 tive development expressed as A to a, b to B, c to C respectively. 



Working on such lines as these in regard to Ammonites, Hyatt made for the 

 ArietidaB some six genera, which with better knowledge would be increased perhaps 

 to 8 or 9 ; but in the Hildoceratidse continued development has produced so much 

 complication, that, by work on similar lines, the number of genera must be increased 

 enormously. The fauna has, however, increased far more than proportionately in 

 richness ; in Ammonites, the numerical acme and what may be called the acme of 

 poecilomorphy — variety of form — is reached in the period called Inferior Oolite, so 

 that the multitude of individuals and their great diversity of characters make the 

 task of classification a matter of extreme complexity. 



In Ammonites it may be seen that there are five characters to deal with ; they 

 may be stated as follows : 



(1) Whorl-shape. 



(2) Umbilication. 



(3) Suture-line. 



(4) Test ornament — transverse and longitudinal. 



(5) Radial curve, in which rostration plays so important a part. 



Each of these characters has its definite ontogenetic and phylogenetic history. 

 To each there is a stage of anagenesis and a stage of catagenesis, and between 

 them an acme, or period of prime development. But the acmes of the various 

 characters are by no means coincident ; and it is this diversity of incidence which 

 produces diversity of form. With five characters, to each of which may be given, 

 say, five stages of anagenesis and five of catagenesis, the possible number of 

 different combinations that may be produced thereby is almost innumerable. 



Any approximate coincidence of the majority of acmes of characters does not 



