dtl 
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 Clymeninnz and Goniatitinz 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 Ammonitine 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 Arietidee or 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: Zypes are evolved more 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- 
