In his individual development, man does in some measure climb up his own
genealogical tree. Stages in the development of the body during its nine
months of ante-natal life are closely similar to stages in the
development of the anthropoid embryo. Babies born in times of famine or
siege are sometimes, as it were, imperfectly finished, and sometimes
have what may be described as monkeyish features and ways. A visit to an
institution for the care of children who show arrested, defective, or
disturbed development leaves one sadly impressed with the risk of
slipping down the rungs of the steep ladder of evolution; and even in
adults the occurrence of serious nervous disturbance, such as
"shell-shock," is sometimes marked by relapses to animal ways. It is a
familiar fact that a normal baby reveals the past in its surprising
power of grip, and the careful experiments of Dr. Louis Robinson showed
that an infant three weeks old could support its own weight for over two
minutes, holding on to a horizontal bar. "In many cases no sign of
distress is evinced and no cry uttered, until the grasp begins to give
way." This persistent grasp probably points back to the time when the
baby had to cling to its arboreal mother. The human tail is represented
in the adult by a fusion of four or five vertebræ forming the "coccyx"
at the end of the backbone, and is normally concealed beneath the
flesh, but in the embryo the tail projects freely and is movable. Up to
the sixth month of the ante-natal sleep the body is covered, all but the
palms and soles, with longish hair (the lanugo), which usually
disappears before birth. This is a stage in the normal development,
which is reasonably interpreted as a recapitulation of a stage in the
racial evolution. We draw this inference when we find that the unborn
offspring of an almost hairless whale has an abundant representation of
hairs; we must draw a similar inference in the case of man.
It must be noticed that there are two serious errors in the careless
statement often made that man in his development is at one time like a
little fish, at a later stage like a little reptile, at a later stage
like a little primitive mammal, and eventually like a little monkey. The
first error here is that the comparison should be made with
_embryo_-fish, _embryo_-reptile, _embryo_-mammal, and so on. It is in
the making of the embryos that the great resemblance lies. When the
human embryo shows the laying down of the essential vertebrate
characters, such as brain and spinal cord, then it is closely comparable
to the embryo of a lower vertebrate at a similar stage. When, at a
subsequent stage, its heart, for instance, is about to become a
four-chambered mammalian heart, it is closely comparable to the heart
of, let us say, a turtle, which never becomes more than three-chambered.
The point is that in the making of the organs of the body, say brain and
kidneys, the embryo of man pursues a path closely corresponding to the
path followed by the embryos of other backboned animals lower in the
scale, but at successive stages it parts company with these, with the
lowest first and so on in succession. A human embryo is never like a
little reptile, but the developing organs pass through stages which very
closely resemble the corresponding stages in lower types which are in a
general way ancestral.
The second error is that every kind of animal, man included, has from
the first a certain individuality, with peculiar characteristics which
are all its own. This is expressed by the somewhat difficult word
_specificity_, which just means that every species is itself and no
other. So in the development of the human embryo, while there are close
resemblances to the embryos of apes, monkeys, other mammals, and even,
at earlier stages still, to the embryos of reptile and fish, it has to
be admitted that we are dealing from first to last with a human embryo
with peculiarities of its own.
[Illustration: "DARWIN'S POINT" ON HUMAN EAR (MARKED D.P.)
It corresponds to the tip (T) of the ear of an ordinary mammal, as shown
in the hare's ear below. In the young orang the part corresponding to
Darwin's point is still at the tip of the ear.]
[Illustration: _Photo: J. Russell & Sons._