140 ANIMAL LIFE 
birds, such as the chicken, the duck, and the auk, as with 
the reptiles, the young animal is hatched with well-devel- 
oped muscular system and sense 
organs, and is capable of running 
about, and, to some extent, of feed- 
ing itself. Birds of this type are 
known as precocial (Fig. 72), while 
the name aliricial (Fig. 73) is ap- 
plied to the more highly organized 
forms, such as the thrushes, doves, 
and song-birds generally. With 
these the young are hatched in a 
wholly helpless condition, with in- 
effective muscles, deficient senses, 
and dependent wholly upon the 
parent. The altricial condition de- 
mands the building of a nest, the 
establishment of a home, and the 
continued care of one or both of 
Fie. 75.—Egg-case of California 
barn-door skate (2aja binocu- 
lata) cut open to show young the parents. 
inside. (Young issues natu- 
wally anoneend of the enKe) The very lowest mammals known, 
the duck-bills (Monotremes) of 
Australia, lay large eggs in a strong shell like those of a 
turtle, and guard them with great jealousy. But with 
almost all mammals the egg is very small and without 
much food-yolk. The egg begins its development within 
the body. It is nourished by the 
blood of the mother, and after birth 
the young is cherished by her, and 
fed by milk secreted by specialized 
glands ofthe skin. All these features 
are adaptations tending toward the 
preservation of the young. In the 
division of mammals next lowest to the Monotremes—the 
kangaroo, opossum, etc.—the young are born in a very im- 
mature state and are at once seized by the mother and 
ian 
Fig. 16.—Egg-case of the cock- 
roach. 
