1903.] PACKARD—CLASSIFICATION OF ARTHROPODA. 155 
are in a degree artificial or ideal conceptions. And so it was with 
the evolution of mammals from theromorphous reptiles, and of birds 
such as the Archzopteryx from reptiles. With our present knowl- 
edge we can trace an almost exact parallel between the tachygenic 
origin, by change in the medium, inducing changes in habits and 
the functions, of flying in sectsfrom Synapterous forms, that of the 
Arachnida from the Merostomes, of Amphibia from Ganoid fishes, 
of reptiles possibly from Amphibia like the Labyrinthodonts, of 
birds from dinosaurian reptiles, and of mammals from theromorph 
reptiles (unless the Amphibians, as some contend, were the source 
of mammalian life). 
The exciting causes of the differentiation of classes, as well as 
orders, families and genera, were geological and topographic 
changes, enforced migration and consequent isolation, adaptation 
to a new medium, to new conditions of life, such as a change from 
marine to fresh water, from fresh water to land, and in the case of 
pterodactyls, birds and insects, from a terrestrial life to one spent 
partly in the air. 
The early Paleozoic ages as well as the Precambrian were periods 
of the rapid evolution of phyla, and of class and ordinal types, as 
shown by Hyatt, the writer, and others. Indeed, it would seem as 
if the evolution and differentiation of varieties and species suc- 
ceeded rather than preceded the formation of genera and higher 
groups. It may be questioned whether the natural selectionists 
could make any progress in evolution, so to speak, by beginning 
with merely simple variations, although after the higher or more 
general groups were originated, and this was by far the most diffi- 
cult and important step, specific variations set in very rapidly, as 
early as Cambrian times. Few, except paleontologists, appear to 
appreciate the rapidity with which evolution in Precambrian and 
Cambrian times must have operated among the plastic forms which 
here and there crowded the early paleozoic seas. 
Phylum III. Meropopa. This group is proposed to include 
the classes of Pauropoda, Diplofoda and Symphyla. 
Prosogoneate myriopods, in which the body is in the typical 
forms cylindrical, the ‘trunk-segments variable in number, but 
usually numerous, and each segment ‘‘ double’’ —z. ¢., united bya 
dorsal plate, which was originally two plates which had been 
fused together (Heathcote), unless we adopt the views of Kenyon 
that the alternate plates disappeared, the remaining plates overgrow- 
