220 
COMPARATIVE ZOOLOGY. 
their young, i. e., 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. e., 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 we 
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 
