small balance. But the amount of expenditure is proportional to the
mass and activity of the muscular and nervous systems. And the
mammal is, and from the beginning had to be, an exceedingly active,
energetic, and nervous animal. The income has increased, but the
expenses have far outrun the increase. The mammal can devote but
little to reproduction.

Moreover, it requires a large amount of material to form a mammalian
egg, such as that of the monotreme. It requires indefinitely more
nutriment to build a mammal than a worm, for the former is not only
larger and more perfect at birth; it is also vastly more
complicated. The embryonic journey has, so to speak, lengthened out
immensely. One monotreme egg represents more economy and saving than
a thousand eggs of a worm. Moreover, where the individuals are
longer lived and the generations follow one another at longer
intervals, the number of favorable variations and the possibility of
conformity to environment through these is greatly lessened. In such
a group it is of the utmost importance that every egg should
develop; the destruction of a single one is a real and important
loss to the species. It is not enough to produce such an egg; it
must be most scrupulously guarded. Even the egg of the platypus is
deposited in a nest in a hole in the bank, and the female Echidna
carries the egg in a marsupial pouch until it develops.

Notice further that among certain species of fish, amphibia, and
reptiles, the females carry the eggs in the body until the embryos
or young are fairly developed. Viviparous forms are unknown by
birds, probably because this mode of development is incompatible
with flight, their dominant characteristic. Putting these facts
together, what more probable than that certain primitive egg-laying