210 COMPARATIVE ZOOLOGY. 
tion of the forms of life. For the variation extends to 
the whole being, even to every organ and mental char- 
acteristic as well as to form and color. It is very slight 
from generation to generation; but it can be accumulated 
by choosing from a large number of individuals those 
which possess any given variation in a marked degree, 
and breeding from these. Nature does this by the very 
gradual process of “natural selection ;”’ Man hastens it, so 
to speak, by selecting extreme varieties. lence we have 
in our day remarkable specimens of Poultry, Cattle, and 
Dogs, differing widely from the wild races. 
Sometimes we notice that children resemble, not their 
parents, but their grandparents or remoter ancestors. This 
tendency to revert to an ancestral type is called atavism. 
Occasionally stripes appear on the legs and shoulders of 
the Horse, in imitation of the aboriginal Horse, which was 
striped like the Zebra. Sheep have a tendency to revert 
to dark colors. 
The laws governing inheritance are unknown. No one 
can say why one peculiarity is transmitted from father to 
son, and not another; or why it appears in one member 
of the family, and not in all. Among the many causes 
which tend to modify animals after birth, are the quality 
and quantity of food, amount of temperature and light, 
pressure of the atmosphere, nature of the soil or water, 
habits of fellow-animals, ete. 
Occasionally animals occur, widely different in struct- 
ure, having a very close external resemblance. Barna- 
cles were long mistaken for Shells, Polyzoans for Polyps, 
and Lamprey-eels for Worms. Such forms are homo- 
morphic. Members of one group often put on the out- 
ward appearance of allied species in the same locality: 
this is called mzmicry. “They appear like actors or 
masqueraders dressed up and painted for amusement, or 
like swindlers endeavoring to pass themselves off for weil- 
