r parental care.

But some other creatures, by nature less prolific, have found an
entirely different solution of the problem. They practise parental care
and they secure survival with greatly economised reproduction. This is a
trend of evolution particularly characteristic of the higher animals. So
much so that Herbert Spencer formulated the generalisation that the size
and frequency of the animal family is inverse ratio to the degree of
evolution to which the animal has attained.

Now there are many different methods of parental care which secure the
safety of the young, and one of these is called viviparity. The young
ones are not liberated from the parent until they are relatively well
advanced and more or less able to look after themselves. This gives the
young a good send-off in life, and their chances of death are greatly
reduced. In other words, the animals that have varied in the direction
of economised reproduction may keep their foothold in the struggle for
existence if they have varied at the same time in the direction of
parental care. In other cases it may have worked the other way round.

In the interesting archaic animal called _Peripatus_, which has to face
a modern world too severe for it, one of the methods of meeting the
environing difficulties is the retention of the offspring for many
months within the mother, so that it is born a fully-formed creature.
There are only a few offspring at a time, and, although there are
exceptional cases like the summer green-flies, which are very prolific
though viviparous, the general rule is that viviparity is associated
with a very small family. The case of flowering plants stands by itself,
for although they illustrate a kind of viviparity, the seed being
embryos, an individual plant may have a large number of flowers and
therefore a huge family.

Viviparity naturally finds its best illustrations among terrestrial
animals, where the risks to the young life are many, and it finds its
climax among mammals.

Now it is an interesting fact that the three lowest mammals, the
Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
the Marsupials, on the next grade, bring forth their young, as it were,
prematurely, and in most cases stow them away in an external pouch;
while all the others--the Placentals--show a more prolonged ante-natal
life and an intimate partnership between the mother and the unborn
young.

Sec. 4

There is another way of l