Mammals furnish a crowning instance of a trend of evolution which
expresses itself at many levels--the tendency to bring forth the young
at a well-advanced stage and to an increase of parental care associated
with a decrease in the number of offspring. There is a British starfish
called _Luidia_ which has two hundred millions of eggs in a year, and
there are said to be several millions of eggs in conger-eels and some
other fishes. These illustrate the spawning method of solving the
problem of survival. Some animals are naturally prolific, and the number
of eggs which they sow broadcast in the waters allows for enormous
infantile mortality and obviates any necessity for 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.
§ 4
There is another way of looking at the sublime process of evolution. It
has implied a mastery of all the possible haunts of life; it has been a
progressive conquest of the environment.
1. It is highly probable that living organisms found their foothold in
the stimulating conditions of the shore of the sea--the shallow water,
brightly illumined, seaweed-growing shelf fringing the Continents. This
littoral zone was a propitious environment where sea and fresh water,
earth and air all meet, where there is stimulating change, abundant
oxygenation and a copious supply of nutritive material in what the
streams bring down and in the rich seaweed vegetation.
[Illustration: THE HOATZIN INHABITS BRITISH GUIANA
The newly hatched bird has claws on its thumb and first finger and so is
enabled to climb on the branches of trees with great dexterity until
such time as the wings are strong enough to sustain it in flight.]
[Illustration: _Photograph, from the British Museum (Natural History),
of a drawing by Mr. E. Wilson._