No. 381.1] A HALF-CENTURY OF EVOLUTION. 647 
steps in the divergence of types from the unicellular Protozoa 
took place? The same law of fact obtains with the insects, 
the eight holometabolous orders having, so far as the evidence 
goes, originated at nearly the same geological date, near or soon 
after the close of the Paleozoic era. Williams also shows from 
a study of the variations of Atrypa reticularis that this species 
in its specific characters shows a greater degree of variability 
of plasticity in the earlier than in the later stages of its history. 
We thus conclude that after the simplest protoplasmic organ- 
isms originated, the greatest difficulties in organic development, 
t.e., the origination of the founders of the different classes, were, 
so to speak, met and overcome in Precambrian times. The 
period was one of the rapid evolution of types. As Williams? 
has well remarked : 
The chief expansion of any type of organism takes place at a relatively 
early period in its life-history. Since then, as with the evolution of the con- 
tinent itself, the farther progressive differentiation of marine invertebrate 
forms has, since the close of the Precambrian, been a matter of detail. 
As well stated by Brooks, since the first establishment of 
the Cambrian bottom fauna, “evolution has resulted in the 
elaboration and divergent specialization of the types of structure 
which were already established, rather than in the production 
of new types.” 
In accepting the general truth of this statement and its 
application to the marine or Cambrian types, it may, however, 
be modified to some extent. For during the late Paleozoic 
was witnessed the evolution of the three tracheate, land-inhabit- 
ing, air-breathing classes of Arachnida, Myriopoda, and insects, 
and of the air-breathing vertebrates, with limbs and lungs, com- 
prising the four classes of amphibians, reptiles, birds, and 
mammals, 
2. The Appalachian Revolution and its Biological Results. — 
Unless we except the great changes in physical geography 
which took place at the end of the Tertiary period, when the 
mountain chains of each continent assumed the proportions we 
now see, the Appalachian revolution, or the mountain building 
and continent making at the close of the Paleozoic age, was 
1 Loc. cit, p. 347+ 
