Hyatt.] 262 [April 4, 



by differences between the sutures which are marked and decided 

 by structural distinctions. Thus the groups of Clymeninnae and 

 Goniatitinae, differ widely in their sutures and position of siphon 

 and smaller groups have also decided structural differences. 



In later times the families and in fact the whole of the Ammo- 

 nitinae are very similar in their sutures. There are, however, 

 many genetic series, in the Jura families, which can be distin- 

 guished by the minor details of the outlines of the sutures, but 

 these distinctions are not so marked as in the Paleozoic, and the 

 form of the whorl in section, and costations and ornaments of the 

 shell are decidedly characteristic. 



In other words the field of variation is structually decidedly 

 narrower, in the Mesozoic than in Paleozoic, whether we con- 

 sider the Nautiloidea or Ammonoidea. 



We have observed the same phenomena repeated in each forma- 

 tion and in the mode of appearance of all the genera and families. 

 These groups originate suddenly and spread out with great 

 rapidity and in some cases 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 larger number of all the distinct types of 

 invertebrata in the Paleozoic, and of the greater number of all 

 existing and fossil types before the expiration of Paleozoic time, 

 speak strongly for the quicker evolution of forms in the Paleozoic 

 and indicate a general law of evolution. This we think can be 

 formulated as follows, types are evolved more quickly and exhibit 

 greater structural differences between genetic groups of the same 

 stock while still near the point of origin, than they do subse- 

 quently. The variations or differences may take j^lace quickly in 

 the fundamental structural characteristics, and even the embryos 

 may become different when in the earliest period, but subsequently 

 only more superficial structures become subject to great variations. 



During this investigation we have been able to add to the 

 facts we have already brought forward in support of the law of 

 acceleration, or as we now prefer to designate it, the law of con- 

 centration of development. All more generalized or lower types 

 have a direct mode of development and the more specialized or 

 complicated progressive types have, when at the acme of their 



