J. D. Dana on Cephalization. 171 
Medusa; and so on. Now it may be urged, against the system 
of classification proposed, that size sometimes means one thing, 
and sometimes the reverse, and there is here manifest indefinite- 
hess and a chance for indefinite assumptions. Or, the charge 
may be made with more point, and much less truth, as follows: 
you ; 
throughout the Animal Kingdom; and when, as nobody can fail 
soon to do, you meet with examples where facts contradict your 
theory, you get over the difficulty by assuming gratuitously that 
size is there due only to what you call ‘ vegetative enlargement.’ 
As I cannot find that you have anywhere laid down any defi- 
nite rules by which this vegetative enlargement is to be distin- 
guished from the normal enlargement, the distinction appears to 
be an empirical one.” 
Now great size is not correlated with superiority in Crustacea 
more than in the rest of the Animal Kingdom, and this I 
tly illustrate in my first paper on the subject; for I 
goish in its movements, or stupid in its 
Senses, there is evidence in this that size is a mark of degrada- 
tion, But I have shown, further, that where size is a mark o 
low grade, the low grade is also manifested in a multitude of 
? . 
characteristics of an inferior group, I have rested mainly upon 
the others for proving the inferiority of the group. 
Should be a mark of high grade, and also why in other cases & 
mark of low grade. I may add one or two comparisons in eluci- 
dation of this point. We all know that if a steam-engine of the 
Size and strength for 100 horse-power has a working-force of 100 
horse-power, it is an engine of respectable grade. But if, while 
thus large in its cylinder, beam, and other parts, it were furnished 
With the means of generating a force-system, as we may Call it, 
‘ 1 horse-power, it would be a very feeble and worthless piece — 
of machinery. Suppose, for closer parallelism with animal life, 
the engine to reach its size by a method of growth; and that — 
* From a recent letter of a critic. — 
