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1903.] PACKARD—CLASSIFICATION OF ARTHROPODA. 153 
the Crustacea were comparatively scanty in numbers, and repre- 
sented by primitive types showing no trace of trilobite characters. 
The Chief Factors in the Evolution of Classes.—Assuming that 
the Arachnida, represented by the most typical form, the scorpion, 
have evolved from the class of merostomes, in the way suggested 
by Pocock, the entire process or phenomenon has the most direct 
and instructive bearings on the method of evolution of one class 
from another. 
In the first place, the single group of scorpions—say a single gen- 
eric form—appears to have arisen from some genus of Eurypterida, 
allied to Eurypterus, and by divergent evolution the great class of 
Arachnida, with its eight orders, appears to have originated by one 
step after another from a single type, not necessarily an individual, 
but many, all the members of the genus being modified by similar 
causes, in the same manner and at the same time. 
The modification of a marine Eurypteroid form, perhaps living in 
a shallow, land-locked basin, perhaps finally becoming brackish, into 
a terrestrial scorpion, was due to changes in the environment, in 
the topography; this reacted on the Eurypterid and resulted in 
change of habits, and consequent adaptation to brackish, and per- 
haps to fresh, water, and finally to land. With little doubt, all the 
forms inhabiting the area underwent the same kind of modification 
and similar adaptation to a new medium, the same changes of func- 
tion resulting in the disuse of organs adapted for marine existence 
and the evolution of structures adapting the animal for terrestrial 
life. 
The changes by which the connecting links (Palzeophonus) be- 
came transformed into a genuine scorpion, the ancestor and founder 
of the Arachnida, were the following : 
1. The loss by disuse of the abdominal swimming appendages 
‘(except the pectines), and the ingrowth and reduction by disuse of 
the expodites, the gills attached to them being carried in, forming 
eventually the book-lungs of the scorpion, each with its spiracular 
opening, adapted for aérial respiration. 
2. The four hinder pairs of cephalothoracic appendages became 
slenderer after the animal had left the water and adopted a life on 
land, under stones or the bark of trees, etc.,and the single stout 
claw of the original Paleeophonus became by use, in climbing trees, 
etc., two-clawed, like those of all Arachnida and insects. 
PROC. AMER. PHILOS. 80C. XLII. 173. K. PRINTED JUNE 11, 1908. 
