218 COMPARATIVE ZOOLOGY. 
animal may be said to be more perfect in proportion as 
its relations to the external world are more varied, pre- 
cise, and fitting. Complexity of organization, variety, and 
amount of power are secondary to the degree in which 
the whole organism is adapted to the circumstances which 
surround it, and to the work which it has to do. Ascent 
in the animal scale is not a passage from animals with 
simple organs to animals with complex organs, but from 
simple individuals with organs of complex function to 
complex individuals with organs of simple function: the 
addition as we ascend being not function, but of parts to 
discharge those functions; and the advantage gained, not 
another thing done, but the same thing done better. Ad- 
vance in rank is exhibited, not by the possession of more 
life (for some animalcules are ten times more lively than 
the busiest Man), but by the setting apart of more organs 
for special purposes. The higher the animal, the greater 
the number of parts combining to perform each function. 
The power is increased by this division of labor. The 
most important feature in this specialization is the tend- 
ency to concentrate the nervous energy toward the head 
(cephalization). It increases as we pass from the Oyster 
up to Man. 
As a rule, fixed species are inferior to the free, water 
species to land species, fresh-water animals to marine, arc- 
tic forms to tropical, and the herbivorous to the carniv- 
orous. Precocity is a sign of inferiority: compare the 
chicks of the Hen and the Robin, a Colt with a Kitten, 
the comparatively well-developed Caterpillar with the 
footless grub of the Bee. Among Invertebrates, the male 
is frequently inferior, not only in size, but also in grade 
of organization. Animals having a wide range as to cli- 
mate, altitude, or depth are commonly inferior to those 
more restricted: Man is a notable exception. 
There is some relation between the duration of life and 
