DEVELOPMENT. 193 
float in the water; but a few build a kind of flat nest in 
the sand or mud, hovering over the eggs until they are 
hatched; while the Acara of the Amazons carries them 
in its mouth. The Amphibians, generally, envelop their 
egos in a gelatinous mass, which they leave to the ele- 
ments; but the female of the Surinam Toad carries hers 
on her back, where they are placed by the male. The 
great Amazon-Turtles lay their eggs in holes two feet 
deep in the sand; while the Alligators simply cover theirs 
with a few leaves and sticks. Nearly all Birds build nests, 
those of the Perchers being most elaborate, as their chicks 
are dependent for a time on the parent.” The young of 
Marsupials, as the Kangaroo, which are born in an ex- 
tremely immature state, are nourished in a pouch ontside 
of the body." But the embryo of all other Mammals is 
developed within the parent to a more perfect condition 
by means of a special organ, the placenta. It is a gen- 
eral law, that animals receiving in the embryo state the 
longest and most constant parental care ultimately attain 
the highest grade of development. 
CHAPTER XX. 
DEVELOPMENT. 
Development is the evolution of a germ into a com- 
plete organism. The study of the changes within the egg 
constitutes the science of Embryology; the transforma- 
tions after the egg-life are called metamorphoses, and in- 
clude growth and repair. 
The process of development is a passage from the gen- 
eral to the special, from the simple to the complex, from 
the homogeneous to the heterogeneous, by a series of dif- 
13 
