646 THE AMERICAN NATURALIST. [VoL. XXXII. 
For forty years past, since the time of Darwin, the idea that 
these early forms were more rapidly evolved, and that they were 
more plastic than forms now existing, has: constantly cropped 
out in the writings of our more thoughtful and studious paleon- 
tologists and biologists. 
Darwin, in his Origin of Species, as quoted by Walcott with 
approval, remarked that it is indisputable that, before the Low- 
est Cambrian stratum was deposited, long periods elapsed, as 
long as, or probably far longer than, the whole interval from 
the Cambrian age to the present day ; and that “during these 
vast periods the world swarmed with living creatures.” Darwin 
then adds: “It is, however, probable, as Sir William Thompson 
insists, that the world at a very early period was subjected to 
more rapid and violent changes in its physical conditions than 
those now occurring ; and such changes would have tended to 
induce changes at a corresponding rate in the organism which 
then existed.” 
Professor Hyatt,! from his exhaustive studies on the 
Nautiloidea and Ammonoidea, concludes : 
These groups originated suddenly and spread out with great rapidity, 
and in some cases, as in the Arietidz 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 large numbers of all the distinct 
types of invertebrates in the Paleozoic, and of all the greater number o 
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 subsequently. The variations or differences may take 
place quickly in the fundamental structural characteristics, and even the 
embryo may become different when in the earliest period, but subsequently 
only more superficial structures become subject to great variations.” 
If this applies to the evolution of these cephalopods in the 
Mesozoic, how much more rapidly and efficaciously did the 
principle operate in the Precambrian period, after the initial 
1 Phylogeny of an Acquired Characteristic. Proc. Amer. Phil. Soc., vol. xxxii, 
p. 371. 2 Geological Biology, p. 322. 
