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observed in the Paleozoic ; they are all close coiled and have, as 

 observed by M. Barrande, small umbilical perforations. This same 

 statement applies also to the Ammonoidea ; when near their point 

 of origin in the Silurian their forms are very quickly evolved, but 

 are much less quickly evolved after this period. The smaller gen- 

 etic groups in the Paleozoic are distinguished by differences between 

 the sutures, which are decided indications of structural distinctions. 

 Thus the groups of Clymeninnae and Goniatitinse differ widely in 

 their sutures and position of siphuncle, and smaller groups have also 

 decided structural differences. In later times the families and, in 

 fact, the whole of the Ammonitinse are more alike. There are 

 many genetic series in the Jura which can be distinguished by the 

 minor details of the ornaments and outlines of the sutures, the dif- 

 ferences being less structurally than in the Paleozoic. In other 

 words, the field of variation is structurally decidedly narrower in 

 the Mesozoic than in Paleozoic, whether we consider the Nautiloidea 

 or Ammonoidea. 



I have observed the same phenomena repeated in each period 

 and in the mode of appearance of the genera and families in 

 lesser divisions of geologic time. Groups originate suddenly and 

 spread out with great rapidity and often, as in the Arietidae 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 appearance of the greater part of all the distinct types of 

 Invertebrata and Vertebrata in the Paleozoic speak strongly for the 

 quicker evolution of forms in that time and indicate a general law 

 of evolution. This has, in former publications, been formulated 

 as follows : Types are evolved ?nore quickly and there are greater 

 structural differences between genetic groups of the same stock while 

 still near the point of origin than appear subsequently. The varia- 

 tions or differences take place quickly in fundamental structural 

 characteristic, and even the embryos may become different when in 

 the earliest period of evolution, but subsequently only more superficial 

 structures become subject to great variations. 



This law applies only to the epacme or rise and acme, not to the 

 paracme or decline of the same genetic groups or stocks. These 

 last will be shown further on to reverse this law of progressive evo- 

 lution. 



The degraded uncoiled forms of the Nautiloidea and Ammonoi- 



