in the Development of the System of Animal Life. 247 
of a switch-like tail, as in ordinary quadrupeds, has little bear- 
development of muscular force. But, approaching man in the 
system of life, even this seems to have significance. 
In accordance with the principle and method illustrated, 
animals of a given type differ widely as to. the conditions an 
arrangements for action—muscular, sensorial and psychical— 
in the animal structure. In the low,* there is, usually, large 
or arrangements, and a better head; and the progress in grade, 
under a type, is progress along lines from the former condition 
toward the latter, that is, progress in the strength, perfection 
and dominance of the anterior or cephalic extremity ; in a word, 
first, the chief center of nervous power or energy in an animal is 
* the cephalic extremity ; and, secondly, because form in na- 
ture's species is, with some limitations, an expression of force.* 
Again, I have exemplified, in my memoirs, the corresponding 
fact that progress in cephalization generally attends progress in 
at the time of the passage to the adult stage, and the concurrent 
development anteriorly of limbs, with the perfecting of the head 
1, structure and senses; to a similar abbreviation posteriorly in 
the devel pment of modern gars; to the fact that the higher 
with abdominal locomotive appendages, to an adult stage in 
which the abdomen is greatly dwindled in size, the thorax and 
abdomen are distinct segments and the former alone has loco- 
Fe ble members and these of perfected structure, and the well- 
efined head has highly developed sense-organs and exalted 
ea aed also been illustrated that the geological progress in the 
ciple - world has been progress in accordance with the prin- 
of fi ‘s cephalization, this being manifested in the succession 
°ms under the various types, and also in the correspondence 
embryonic forms in structure, I would refer to my former papers. 
