DEVELOPMENT 437 



complex function to complex individuals with organs 

 of simple function : the addition as we ascend being not 

 function, but parts to discharge those functions ; and the 

 advantage gained, not another thing done, but the same 

 thing done better. Advance 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 impor- 

 tant feature in this specialization is the tendency to 

 concentrate the nervous energy toward the head (ceph- 

 alizatiori). It increases as we pass from lower to higher, 

 animals. 



As a rule, fixed species are inferior to the free, water 

 species to land species, fresh-water animals to marine, 

 arctic forms to tropical, and the herbivorous to the car- 

 nivorous. 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 

 the size, structure, and rank of animals. Vertebrates 

 not only grow to a greater size, but also live longer than 

 invertebrates. Whales and elephants are the longest- 

 lived ; and falcons, ravens, parrots, and geese, alligators 

 and turtles, and sharks and pikes are said to live a cen- 

 tury. The life of quadrupeds generally reaches its limit 

 when the molar teeth are worn down : those of the sheep 

 last about 1 5 years ; of the ox, 20 ; of the horse, 40 ; of 



