220 
COMPARATIVE ZOOLOGY. 
their young, i. e. 9 they are Mammals. All Ruminant 
hoofed beasts have horns and cloven-feet. If the hoofs 
are even, the horns are even, as in the Ox; if odd, as in 
the Rhinoceros, the horns are odd, i. single, or two 
placed one behind the other. Recent creatures with feath¬ 
ers always have beaks. Pigeons with short beaks have 
small feet; and those with long beaks, large feet. The 
long limbs of the Hound are associated with a long head. 
A white spot in the forehead of a Horse generally goes 
with white feet. Hairless Dogs are deficient in teeth. 
Long wings usually accompany long tail-feathers. White 
Cats with blue eyes are usually deaf. A Sheep with nu¬ 
merous horns is likely to have long, coarse wool. Homol¬ 
ogous parts tend to vary in the same manner; if one is 
diseased, another is more likely to sympathize with it than 
one not homologous. This association of parts is called 
correlation of growth . 
6. Individuality . 
It seems at first sight very easy to define an individual 
animal. A single Fish, or Cow, or Snail, or Lobster is 
plainly an individual; and the half of one such animal is 
plainly not one. But when we consider animals in colo¬ 
nies, like Corals, it is not so easy to say whether the indi¬ 
vidual is the colony or the single Polyp. Is the tree the 
individual, or the bud % If we say the former—the colony 
-—what shall we say to the free buds of a Hydroid colony, 
living independent lives, and scattered over square miles 
of ocean ? Are they parts of one individual ? If w r e 
choose the latter as our standard, we are in equal difficul¬ 
ty; for we must then call an individual the bud of the 
Portuguese man - of - war, reduced to a mere bladder or 
feeler, and incapable of leading an independent life. We 
thus find it necessary to distinguish at least two kinds 
of individuals —physiological individuals , applying that 
