HOW THE MAMMALS DEVELOPED
When the idea of evolution first began to be much discussed,
especially after the publication of the "Origin of Species," there
were several points which appeared to be more than commonly difficult
of explanation. It did not seem impossible that the various types of
domesticated cattle should have descended from a common ancestor. It
did not seem difficult of comprehension that the dog might once have
been a wolf. Though not quite so credible, it did not seem absurd that
the tigers, lions, and leopards should have once all been alike. The
resemblance between these are strong enough to make the idea seem
conceivable. Though men were willing to concede this much, they
insisted that the great branches of the animal kingdom varied so
widely from each other as to make it certain that each was a separate
creation. It was particularly objected that the mammals differed so
entirely from other animals in several important particulars that a
special divine act was necessary for their appearance. The mammals
have a furry covering entirely different from the clothing of any
other animal in the kingdom, and have warm blood, which is found
nowhere else except among the birds. But particularly their method of
producing their young seemed so entirely different from that of any
other group that here a special creation was deemed absolutely
necessary.
Other young creatures are produced from eggs laid by the parent and
subsequently hatched. The young of the mammals are born alive and
comparatively well developed. In addition, their first food, the milk
of the mother, is so entirely different from the food of any other
creature that this again seemed to involve a separate creation.
Gradually we have come to understand the whole matter of reproduction
very much better. Minute and careful dissections of rabbits, of dogs
and cats, of animals slaughtered for food, with occasional post-mortem
examinations of human beings in various stages of the development of
the young, leave us no longer in doubt concerning the main features of
the process. The better we come to understand it the more clearly it
becomes evident that in the development of the mammals we have no new
procedure, but, as in so many other activities, new developments of an
old process.
There are two entirely different methods by which new animals and
plants may arise. One sees sometimes in the home of a friend a
geranium of particular beauty, the like of which he would be glad to
possess. The accommodating friend cuts a small piece from the
geranium. This is stuck into poor but well-watered ground, develops
roots, and eventually grows into a geranium stalk exactly like the one
from which it came and of which it is in reality only a detached part.
In similar fashion, if one wants a particular kind of apple, he never
trusts to planting an apple seed. Going to the tree of the variety he
desires, he takes from it a small twig provided with a bud and inserts
this bud into a cleft made in the young branch of another apple tree.
The young bud so inserted starts up into a new branch, resembling
almost absolutely, not the tree which feeds it with sap, but the tree
from which the bud was originally taken.
When we wish a particular variety of potato we obtain pieces of the
potato of the kind we desire. Each of these must contain an eye, which
is a bud of the old potato. When the sprout appears the new plant will
be practically identical in character with the plant from which the
potato was taken. This sort of reproduction, in which a piece of the
old parent grows up into the new generation, is called the asexual
method. But one parent is concerned in the process, and the offspring
are as nearly as may be like the parent from which they arose.
The gardener who wishes to obtain new varieties is not content with
this method. If he plant the seed of the potato the outcome will be
most uncertain. His seed must be taken, of course, from the fruit of
the potato, and most of these plants never fruit. Every grower of
large quantities of potatoes will have noticed occasionally, on the
tops of the plant, after the flowers disappear, a globular growth
looking not unlike a small tomato, but with a tendency to become
purplish green in color. This is the fruit of the potato and in it are
the seeds. When these are planted all sorts of potatoes are liable to
start up. Most of them will prove worthless. An occasional seed may
produce an uncommonly fine plant. This new variety may thereafter be
propagated from the tuber, as the potato itself is called, and the new
strain will be kept constant in this way. This method of using the
seed for reproducing the plant is called the sexual method, because
two parents cooeperate in the production of the seed. The pollen came
from one parent and the ovule, which after fertilization swelled up
into the seed, came from another. By this combination of two
individuals new varieties become quite possible. Nature seems to be
more concerned in improving her strain than in maintaining her older
strains. In all of her lowest plants and animals she uses the asexual
method of reproduction. As we go higher in the organic world the
two-parent method becomes increasingly common. When we reach the
higher animals, and most of the higher plants, this plan of double
parenthood, the sexual method, alone is used.
In order that we may the more clearly understand how the mammals
produce their young and nourish them, we shall begin at the lowest
class of the backboned animals and note how the process is there
accomplished. As we pass upward through the kingdom the method
acquires greater complexity. When we finally reach the mammals, what
at first seemed an absolutely new process will prove to be, as is all
of nature's work with which we are thoroughly acquainted, but a
modification and an elaboration of some previously existing process.
Some time ago I was passing the early months of summer by the side of
a lake in northern Pennsylvania. Near my tent, on the edge of the
water, was a wharf from which it was possible to look down into the
shallows about the edge of the lake. In early July the bottom began to
take on a strange appearance. Spots as big as a dinner plate became
evident because they were cleaned of the finer sand or mud which is
common on the bottom. A close examination showed that each of these
circular spots was being occupied and cleaned up by a sunfish. The
pebbles were lifted into the mouth of the fish and driven out again
with force. The water which emerged with the stones seemed to wash
away the dirt, while the pebbles themselves became gradually cleaned
of the green plant life which ordinarily covers them. After the
process was completed each spot was saucer-shaped and free from scum
and mud. Over each of these spots hovered the sunfish which made it,
and round and round the fish swam. The circles thus traversed were so
near each other that every now and then the occupants of two adjoining
nests would meet on the border. The fish which was most nearly on its
own ground would at once attack the other and drive him away. In a few
days the other partner in each family seemed to appear. Now two fishes
swam side by side over each nest, bringing the lower edge of their
bodies comparatively close together. In this position they moved
around over the pebbly bottom. The female was discharging her
multitudinous and very small eggs, so that they dropped to the bottom
of the nest. At the same time the male was expelling what in fish is
known as the milt. In this milt are the sperm cells of the male, each
consisting of a rounded head and a very slender body. These are
attracted by the eggs. Pushing up against them, the head of a sperm
cell, consisting almost entirely of the nucleus of the cell and
carrying the determinants which were to decide one-half of its future
characters, penetrated this egg and fused with its nucleus. This was
filled with the determinants of the characters inherited from the
mother. Of course many of the eggs, of which probably there are a
thousand, must have escaped fertilization. There are doubtless a
thousand sperm cells that went to utter waste for one which found an
egg to fertilize. These eggs nestled in the crevices between the
stones in the warm water of the edge of the lake. Here the sun could
easily penetrate to the bottom and hatch them. The little fish, still
guarded by one hovering parent, swam around in the water long before
the yolk of the egg, containing its large amount of food, had been
absorbed into the tissues of the young fish. This fatty store made the
abdomen of the fish in which it lay protrude enormously. Gradually the
fish grew larger and the yolk grew smaller until all had been
consumed. Soon the fish began to forage for himself and no longer to
demand or care for the company and protection of its parent. The
little sunfish is highly favored among his comrades in having any care
whatever by the parent. In the case of most fishes the female,
swimming slowly over the bottom, deposits her eggs, which are
fertilized by the male, which follows behind her. After the eggs have
thus been laid and quickened no other attention is paid to them by
either of the parents.
Fish are stupid almost beyond the comprehension of those who are not
students of the minds of animals. Frogs and toads are a distinct step
in advance, and hence their mental activities play a larger part in
the process.
In the love-making of the frogs and toads the song has an important
share. In each species the voice is a little different from that of
any other. In our familiar garden toad we have an excellent
illustration of the method common to the entire group. When spring
comes an impulse seems to stir in all the toads of a neighborhood.
Heretofore they have stuck faithfully to dry ground; now they start
off for the water. Whether their impulse is simply to move down hill
or whether they by some means detect the near presence of water, I
cannot say. Certainly a new fountain on a lawn will secure in spring
its prompt and full share of the neighborhood's toads. In any event
the toads of a district congregate in great numbers in any pond or
along the edge of any moderate stream. Within a short time their
flutelike, quivering voice is heard far and wide. That this note has
an attractive power over the female there is no doubt. She herself
makes no effort to imitate, but the song of her mate is persistent and
exceedingly sweet. I have seen a male sit upon a clump of grass and
utter his love call. Before he had been singing for more than half a
minute three females hastened toward him from a distance of perhaps
twenty feet. Each seemed anxious to reach as promptly as possible the
creature whose voice had proved so attractive. When the mating comes,
the female discharges a series of small shotlike eggs which are
encased in a very tenacious mucous. While they are being deposited the
male fertilizes them. No sooner have the eggs, fertilized by the sperm
cells, reached the water than the mucous at once begins to swell. The
result is that eggs appear encased in two slender strings of jelly,
each having a diameter about that of a lead pencil. At intervals of
not more than half an inch the shotlike eggs may be seen. The mother
toad, in laying these eggs, moves about rather restlessly in the
water. By this means she succeeds in wrapping the strings about the
grass and sticks of the pool. This will hold them quite safely even
against a considerable current of water, should the stream rise and
flood the side pools in which the eggs are laid. With this amount of
care, however, the attention of both parents to the young entirely
ceases. They are now abandoned to the chances of a fortune to them
exceedingly unkind. A toad will lay about five hundred eggs. It is
evident that on the average only two of these can attain maturity by
the time the parents have died, for the number of toads does not
materially alter season by season. The connecting string is made up
not of nourishment for the eggs, but of a bitter mucous so unpleasant
to the taste that fish are thus deterred from eating the otherwise
nourishing material. This secures for the young embryo a chance to
mature which in the absence of the jelly it would entirely lack.
Imbedded in this mucous is the embryo itself, surrounded by a small
amount of albumen and containing inside of itself a very considerable
amount of yolk. This gives to the egg a volume possibly a hundred
times that of the egg of the sunfish. Thus, even counting the care the
parent sunfish took of its offspring, which care is very uncommon
among fishes, the toad stands a distinctly better chance in life. The
protection of the bitter mucous and the large amount of yolk
permitting considerably larger development before leaving the egg,
give to the toad a material advantage. When the toad first emerges
from the egg it is amazingly like the fish. It has gills at the side
of its neck and swims by the movement of its tail. Later its limbs
develop, the hind ones coming first, its tail is absorbed, and it is
now a true toad, ready to leave the water.
Altogether a higher state of reproduction is encountered when we reach
the reptiles, which are the next higher class of backboned animals.
Here very distinct developments of the process are discovered. The
turtle, to use the best known illustration, may lay but twenty eggs.
But she will not lay them at random in the water, as do the toads and
the fish. Each egg is wonderfully fattened with yolk. This means that
it is possible for the creature to develop to a far greater extent
before leaving the egg than was possible in the case of the toad.
Accordingly the little turtle, while it begins life not unlike a fish
and goes through the gilled and tailed period, during which it is not
unlike a tadpole, passes beyond this period before leaving the shell
and has already acquired its full turtle characters when first it
steps upon the scene. So big an egg as this would be highly nutritious
and animals would desire it immensely for food. Hence it becomes
necessary for the turtle to securely hide her eggs. In order to do
this, she scoops out a pit in the sand in which she deposits them and
here they develop. If no further provisions were made the eggs of the
turtle would dry completely and never hatch. Accordingly it becomes
necessary for the turtle to enclose each egg in a tough, leathery
membrane, known as the shell. Because the egg is thus encased it is
necessary for it to be fertilized before being laid. Accordingly the
male must place the sperm cells within the body of the female. These
cells swim nearly to the top of the tubes in which they are placed,
and there fertilize the descending eggs. Farther down the canal the
shell is secreted about the now swollen mass of yolk and white,
completing the egg just before it leaves the parent.
If the evolutionist understands properly the line of descent, the
birds and mammals are both the descendants of the reptiles. While
there is less exterior resemblance between a chicken and a turtle than
between a cat and a turtle, the real relationship in the first case is
much closer than in the second. This is perhaps most easily seen in
the scaly legs of both bird and reptile. Another remarkable
resemblance lies in the fact that in both cases the eggs are large,
well stored with nourishment, and protected by a resistant shell.
So few people know the turtle's egg that it will be better to describe
that of the hen, which it largely resembles. Underneath the hard shell
is a tough but flexible membrane which lies against the limey coating,
except at the blunt end, where a separation between the two gives room
for a bubble of air. Inside of this shell and its membrane lies the
white of the egg, which is nourishment for the chick, though not
nearly so rich as the yolk. This, besides the albumen which it
contains, is stored with large quantities of fat. It will be
remembered that upon breaking a hen's egg and dropping it into a bowl,
the yolk holds together because it is enclosed in a delicate sac. As
the yolk falls into the bowl there floats to the top of it a lighter
yellow spot as big as the end of a lead pencil. This is all of the egg
which thus far represents the chick itself. All the rest is
nourishment. This disk already consists of three reasonably
distinguishable layers of cells, which grow rapidly different from
each other. They spread and bend and twist, forming the young chick
and a set of organs which serve for its protection and maintenance
during its embryonic life. Within a few days these accessory organs
will have formed distinctly. Within the upper half of the yolk will be
found the small developing chick, which for the first thirty-six hours
of its development passes through a stage not unlike the fish, or the
earlier steps of the turtle. Within a few days it becomes clearly
evident that this creature is to be a bird, though it is much longer
before it is clearly a chick.
This embryo is so soft that it is almost like curd in thickened milk,
and could be very easily destroyed were it not for a protective device
which Nature has employed. It seems necessary that it should be
protected with the utmost care. The matter will be better understood
if we recall a common experience. Almost everyone has tried to
dissolve some substance in water in a vial. If the bottle be filled
with fluid to the top and corked it is very difficult to shake up the
contents. Even vigorous agitation produces little movement of the
material on the inside. If we wish to shake up the solid with water
the bottle must be left partly empty. The brain of a human being is
protected by just the same device. If it simply lay within the skull
the first fall would mash the gray substance against the side of the
cavity. To prevent this calamity the bony case is made somewhat larger
in capacity than the brain itself, and the space between the two is
filled with a watery fluid. This serves to prevent jars and shocks. In
the hen's egg the same plan is pursued. The embryo lies on the inside
of a bag considerably larger than itself. This sac, called the amnion,
is filled with a watery fluid. With such a protection only the most
severe shock to the egg would sufficiently jar the embryo to do it any
harm. The ordinary experiences of an egg leave it undisturbed.
Every living creature requires a constant supply of food and of
oxygen. The embryo is a living creature, and is no exception to the
rule. It needs an abundant supply of easily assimilated food and of
oxygen. When the hen's egg is first laid the entire contents, with the
exception of the little light-colored disk which floats on the top of
the yolk, form the nourishment. The disk alone is the living organism.
In the earliest stages the embryo receives its food by simple
absorption from the yolk. As the chick increases in complexity the
yolk at first grows swampy, with fluid trickling here and there
through the more solid portions. Thin walls form about these little
streams, thus producing blood vessels which cover the entire surface
of the yolk. These absorb the nourishment and turn it over to the
embryo. As the latter grows in size both the yolk and white diminish.
The embryo soon becomes larger than the remaining yolk and is attached
to it by a cord filled with blood vessels which enter the chick near
the center of its body. The abdominal wall has an opening at this
point. One of the later occurrences in the life of the chick, before
it breaks through the egg, is to have the last remnant of the yolk and
its sac slip to the inside of the abdomen, which then completely
closes over it.
As yet, we have seen no arrangement for furnishing air to the chick.
At the same point at which the blood vessels from the yolk enter the
chick, another set of vessels pass in and out. These are attached to a
large flattened bag which floats above the embryo against the upper
side of the shell. This bag is called the allantois, and serves as a
sort of lung for the developing chick. The shell is porous enough to
allow air to pass through it. The blood vessels of the allantois take
in oxygen and give out carbon dioxide through the porous shell. The
blood thus altered is returned to the chick and serves its life
purposes. One of the reasons why the chicken must turn its eggs in the
nest is that, if the allantois remain too long in contact with the
upper shell of the egg, it will become attached to it and will not
thereafter perform its functions.
The embryo thus enclosed in the egg finds its protection in the fact
that it is encased in a fluid contained in the amnion. It draws its
nourishment from the yolk upon which it lives and the nourishment is
transmitted to it by blood vessels. It draws its oxygen and throws off
its wastes through the instrumentality of the allantois, which covers
it over. Day by day the chick becomes larger, day by day it grows to
look more like what it is to be. By the nineteenth day it appears to
be complete. Its nervous organization is, however, not thoroughly
developed. If removed from the shell the chick still is indisposed to
stand upon its feet or to run about. If allowed to remain in the egg
until the twenty-first day, the chick will be able to push its beak
through the skin enclosing the bubble of air at the blunt end of the
egg and get the first breath into its lungs. Now it gives a faint
peep, breaks the shell of the egg, and steps out into the open air.
I have given this somewhat lengthened description of the development
of the chick because of the light it throws upon the method pursued by
the mammals. The features which have been described in the case of the
chicken's egg could be as fully observed in the case of the turtle or
any of the other reptiles. Mammals are descended from the reptiles of
the Mesozoic, and whatever peculiarities there may be in their method
of producing their young must be derived from the reptiles. If we wish
to know how the earliest mammals produced their young, we can only
judge by the lowliest members of the group that live upon the earth
to-day. The most primitive of these is the so-called Duckmole, of
Australia. This little creature has habits not unlike those of the
muskrat. It burrows in the bank of a stream, and makes a nest at the
end of the burrow, where it lays its eggs. This is one of the very few
warm-blooded, hair-covered animals which still lays eggs. A little
higher in the scale stand the kangaroo and the opossum. These
creatures keep the egg inside of the body until it is hatched. But
this happens in so short a time that the young animal is exceedingly
immature and as yet unable to stand the outside air. Accordingly there
is a double fold of skin on the abdomen of the mother, covering her
breasts. This forms a suitable resting place into which these young
are conveyed as soon as they are born and from which they do not
emerge for many days. The little creature instantly fastens upon the
nipple of the mother, keeping its mouth constantly in this position.
At intervals the muscles of the breast force the milk into the mouth
of the young, which is still too undeveloped to suck for itself. As it
gets older the little opossum or kangaroo emerges from the pouch in
the pleasanter part of the day and in the absence of danger. It
returns to the mother's pocket as soon as it becomes cold or a cry
from its parent warns it of its defenseless position.
These creatures are the lowliest of the class upon the earth. The
great majority of all mammals have elaborated a far finer plan, in
which the young are retained within the body of the parent until they
are quite able to stand the air. The length of this time varies in
different mammals from a few weeks to more than a year. The egg must
be fertilized before it leaves the body of the parent. If it should
fail in this it simply passes out and is wasted. If the fertilizing
cell reaches the egg before it has progressed far down the tube it
begins its development. The embryo forms for itself the sort of head
and tail and gill slits which would have served its fish or its
tadpole ancestor. Its limbs develop as little buds indistinguishable
from similar buds that would have formed fins for the fish or wings
for the bird.
Around the embryo there forms a sac, the amnion filled with a fluid
which serves to protect the young mammals exactly as the growing chick
was protected. Under the forming creature there hangs a small but
empty yolksac. This is an actual remnant, a reminder of the past, when
the eggs of the mammals were also packed with yolk and the growing
embryo secured its nourishment exactly as does the maturing chick. But
a new method has been provided for the mammal, and consequently the
yolksac, though it has not entirely disappeared, has no nutritive
content for the growth of the embryo.
The allantois of the chick now gains a new development and an altered
function. In the case of the chick it floats against the shell of the
egg and absorbs oxygen through the shell. Inside the body of the
mammal this is impossible, because the air is too far away. No shell
is formed about the egg because it is not to be laid. The tube of the
parent's body in which the egg lies becomes thickened at the point of
contact with the egg. It grows spongy and full of blood vessels.
Meanwhile the allantois is also growing spongy. These two tissues are
so closely pressed against each other that the blood vessels of the
transformed allantois mesh in with those of the thickened parent
wall. Thus the blood vessels of the mother are brought into close
contact with those of her offspring. Her blood seeps over into the
transformed allantois which is now called a placenta. From this it is
handed over to the offspring, which thus receives from the mother her
blood, and returns its own used blood for enrichment and purification.
So the allantois of the reptile has become the placenta of the mammal.
In the first instance it served only as an organ of respiration. Now
it has come to supply the embryo with rich blood containing both food
and oxygen derived from the mother. After the offspring is born this
thickened pad breaks loose, and subsequently is also extruded from the
body, forming what is known as the afterbirth.
Thus far we have spoken of the change in the method by which the young
are brought to such a stage of development that they can stand the
outer air. One of the improved differences between the mammals and
other animals lies in the method by which they nourish their young for
some time after birth. The very word mammals signifies an animal who
is in the true sense of the word a mamma. This name for mother is
given to her because of the fact that she possesses what are
technically known as mammary glands, or, in simpler language, breasts.
It would seem as if here we had an entirely new organ. No other
animal gives nourishment to its young in such fashion; all mammals do.
What is the origin of the habit? How did the organ arise?
A part of an animal's body that has the power to gather material from
the blood and pour it out in the shape of fluid is known as a gland.
Sometimes a whole organ does nothing else. Sometimes small glands are
scattered through, or over, the surface of another organ. There are
two kinds of glands in the skin of the mammal. The best known and most
frequently thought of are those which pour out the perspiration. These
have a double function. In the first place they assist in keeping the
temperature of the body uniform. When we are too warm they pour out a
watery fluid over the surface of the body. If the air is dry enough
and our body not too closely protected by clothing, this perspiration
passes off in the form of vapor. All evaporation requires heat, which
in this case is extracted from the body. So soon as the temperature
returns to its normal level the flow of perspiration ceases. The other
function of the sweat glands is to take from the blood some of the
waste matters of the body and pour them out upon the surface. This is
done in order that the body may free itself from substances which, if
they were to accumulate, would have a poisonous effect upon its
action. It is this function of the sweat glands which makes it
necessary for us to bathe the surface of our bodies with water. Dirt,
in the ordinary sense of the word, is not harmful to a sound skin. Our
reason for bathing is really to remove the wastes which we ourselves
have poured upon the surface of the skin. These, if allowed to remain,
soon decompose, like all nitrogenous substances, and become very
offensive. They may then be reabsorbed into the skin and nature's
effort to throw them off has been in vain. These glands, since they
contain waste matter, could not possibly yield food for the young.
They would poison and not nourish. Hence, whatever the breasts may be,
they are not altered sweat glands.
There is another set of organs in the mammalian skin. At the base of
each hair lies an oil gland. The function of these is to pour out a
substance which spreads along each hair and over the surface of the
body. The outside of the skin is always dead, and would easily crack
were it not for the constant secretion of this oil. In winter, when
the blood circulates less freely and these glands consequently pour
out less oil, the supply frequently runs short. If what little is
poured out is too frequently removed by washing, the skin becomes
brittle, and, on bending a joint, the epidermis cracks. The gloss of
the hair is due to the oil thus poured out. This oil becomes one
ingredient in the milk produced by the transformed gland. But there
is another important constituent. When one does unaccustomed manual
work the ordinary result is the formation of a blister. The epidermis,
or scarfskin, becomes detached from the dermis, or true skin, and the
space between the two rapidly fills with the fluid portion of the
blood, known as lymph. The fact that no blood vessels have been broken
in this detachment results in there being no red corpuscles in this
fluid. Wherever a cavity forms in the body lymph is liable to enter
it.
The milk glands of the mammals are modified oil glands. The fluid
which they now pour out is no longer exactly the old oil with the
addition of the lymph. Undoubtedly in the past the first milk was more
like this simple mixture. There seems no doubt that the breasts of
to-day are the enlarged and modified oil glands of earlier mammals. In
one of the most primitive of our mammals the young simply lick certain
bare spots on the surface of the mother's abdomen. As higher forms
arise there develops a smaller or larger mound with a distinct
projection, about which the lips of the offspring can easily fasten.
Lamarck would have said that the suction of the infant had produced
such a mound, and that this had been transmitted to later offspring
until it had grown to be the highly developed organ we now find, for
instance, in the cow. Since, however, we have come to disbelieve in
the transmission of acquired characters, this explanation will no
longer serve. We must content ourselves with saying that, by whatever
accident the nipple arose, the success of it when present determined
its selection by nature and its consequent persistence. With increase
in its function has come increase in the size of the glands. Lower
animals which, like the hog, produce a large number of offspring,
possess a large number also of these glands. With the diminishing
number of young and greater care of them as we rise in the scale has
come also a diminishing number of breasts in the female. Whether those
on the front of the body should persist, or those on the rear, depends
upon other factors in the life of the animal. Hoofed animals, perhaps
because their best weapon is the hoof and they can there best protect
their young, have retained them in the rear of the body. In the group
of animals known as the primates, including monkeys, apes, and man,
the habit of holding the young in the arms for protection has
determined the persistence of the breasts upon the chest rather than
the abdomen.
It is interesting to notice that the habit of the elephant of
protecting its young by means of its tusks has also resulted in a
similar position of the milk glands.
That the primates had once a larger number of offspring is confirmed
by double evidence. Even to-day the number of children at a birth is
often two, sometimes three, rarely four. The day before this was
written came the report of a case of five children at a birth, all of
whom seemed sound and all of whom lived. Still more direct evidence is
found in the fact that occasionally in the human female there are two
pairs of breasts, and very rarely three pairs. These are then disposed
in a double line down the front of the body.
The new plan of caring for the young is one of the priceless heritages
of the higher animals. As we rise in the grade of life the number of
the young produced at one time steadily diminishes, while the care
spent upon them increases. The shad may lay four hundred thousand eggs
and trust them entirely to their fate. The sunfish will lay a
thousand, by no means all of which can be fertilized, but it guards
them somewhat after deposition. The toad lays several hundred, stores
them with a considerable amount of nourishment, and protects them by a
bitter deposit of mucous. The turtle has reduced the number of eggs to
perhaps a score. Each of these is supplied with abundant nourishment,
so that the young may develop to considerable size and activity before
emerging from the egg. This material is enclosed in a firm protective
shell and hidden away from sight by being buried in the ground. In the
mammals comparatively few eggs are produced at one time. These are
fertilized within the body of the parent, are attached to the parent,
and absorb her blood. No shell is needed because nothing will kill the
developing offspring that is not likely to injure the parent. Not only
do the young feed upon the blood of the mother up to the time of
birth, but they are practically dependent upon this same blood after
birth. Though they do not take it directly from the veins, the milk
is, none the less, the transformed blood of the mother. This assures
the young of food as well as of protection. Best of all, the young are
provided with the companionship of the mother. Now for the first time
animals learn by example. Heretofore they have been born with a nearly
undeviating instinct; now intelligence begins to arise. They can
imitate their mother. Heretofore no acquired characters affected the
young. In the mammals, although the young cannot inherit the acquired
habits of the parents, they can get them by imitation, which serves
nearly as well.
There is, however, a more wonderful advantage that comes from the
close attachment between mother and offspring. This intimate
relationship brings about an affection of the mother for her young
heretofore unknown in the animal world. It is somewhat paralleled
among birds, but here the care of the nestling is less intimate, far
less maternal, than the care of the mammal for her young. As the
number of the young grows less and the care taken of them increases,
the intensity of the affection also increases. By the time we get as
high as the dog or the cat this fondness becomes a fierce,
self-sacrificing love. When we come to man, with his high intellectual
powers, with his deeper moral sense, we find a wonderful change. This
love of the mother for her child has grown into the finest emotion
possible to the human heart. It no longer is confined to the dependent
life of the child, but follows the offspring through its entire life,
guiding, guarding, shaping its destiny, handing on to the child the
treasured wisdom of the race. Influenced by the example of the mother,
the father comes to have a love for his children. It is not so strong
as that of the mother, nor so utterly unselfish, but it is still a
noble and exquisite love. Developing in a different direction, the
love of the mother for her children grows as civilization advances,
and spreads over the father of those children as well. Again
reflecting her love, the man finds himself filled with a new feeling
for the woman. It is never as unselfish, as free from desire, as is
her love, but it completely transforms his relation to her. What has
been with him simply desire is ennobled and enriched until it becomes
the finest passion of his life, absolutely transforming him, in
relation to her, from a selfish brute into a tender and life-long
companion. So utterly does the love thus engendered transfigure human
life that when we seek to express the divine nature in human terms,
and these are the only terms we know how to use, the richest
revelation that has come to us is the conception taught by the Master
that "God is Love" and that "as a father pitieth his children, so the
Lord loveth them that fear him."