SECT. XIV THE EURYPTERID^ 251 



therefore, we ask why these animals died out, in spite 

 of their having acquired the habit of free swimming, 

 we can only suggest that the very perfection of their 

 specialisation may have been fatal to them ; the line 

 of their development ended in a cul-de-sac. They 

 were not plastic enough to adapt themselves to some 

 great change or other which took place in their sur- 

 roundings, such as the perfection of the protective 

 arrangements of their prey, and consequently died 

 out. The theory which deduces the Arachnida from 

 them through the scorpions seems to us to be 

 very improbable in the face of this extraordinary 

 specialisation. But to this important and interesting 

 discussion we shall return in a final section dealing 

 with the other division of the Arthropoda, viz. the 

 Tracheata. 



