must conclude that it is in all probability the first part which became
entirely denuded of hair. The gorilla, according to Professor Gervais,
is the only mammal which agrees with man in having the hair thinner on
the back, where it is partly rubbed off, than on the lower surface. Du
Chaillu states that he has "himself come upon fresh traces of a
gorilla's bed on several occasions, and could see that the male had
seated himself with his back against a tree-trunk." He also says: "In
both male and female the hair is found worn off the back; but this is
only found in very old females. This is occasioned, I suppose, by their
resting at night against trees, at whose base they sleep." The gorilla
has only very partially acquired the erect position, and probably sits
but little in the attitude common to man. In man the case is different;
in proportion as his progenitors grew more and more erect, he must have
lain less and less upon his stomach, and more and more upon his back or
sides, and this is seen in the savage man during his lazy hours--who
stretches himself on the ground in the sun, with his back propped, where
possible, by a slight mound or the wall of his hut. The continual
friction of the surface of the back would arrest the growth of hair; for
hair grows where there is normally less friction, and _vice versâ_.
As man became more and more hairless, especially among savage and naked
races, we should conclude that such a modification would be considered a
beauty, and women would select such men in preference to more hairy
individuals. The New Zealand proverb is: "There is no woman for a hairy
man." Sexual selection, then, would play a very important part; and the
difficulty of understanding how man became divested of hair is readily
explained.
Haeckel says: "Even if we knew absolutely nothing of the other phenomena
of development, we should be obliged to believe in the truth of the
theory of descent, solely on the ground of the existence of rudimentary
organs."
REPRODUCTION BY MEANS OF EGGS.
It might be thought there existed a missing link between animals which
lay eggs and those which do not; this, however, is done away with in
many instances--one, for example, is found in our commonest indigenous
snake. The ringed snake lays eggs which require three weeks time to
develop; but when it is kept in captivity, and no sand is strewn in the
cage, it does not lay eggs, but retains them until the young ones are
developed. This only shows how powerfully influences affect the habit of
animals.
DOUBLE-SEXED INDIVIDUALS.
Another difficulty might be supposed to arise between animals which
produce themselves other than by sexual reproduction. This has already
been slightly touched upon; and it has been shown that numerous plants
and animals propagate themselves through their double-sexed organs. It
occurs in a great majority of plants, but only in a minority of animals;
for example, the garden-snail, leeches, earth-worms, and many other
worms. Every garden-snail produces in one part of its sexual gland eggs,
and in another part sperm.
Parthenogenesis offers an interesting form of transition from sexual
reproduction to the non-sexual formation of germ-cells (which most
resembles it). It has been demonstrated to occur in many cases among
insects, especially by Seebold's excellent investigations. Among the
common bees, a male individual (a drone) arises out of the eggs of the
queen, if the eggs have not been fructified; a female (a queen or
working bee), if the egg has been fructified.
Gonochorismus or sexual separation, which characterizes the more
complicated of the two kinds of sexual reproduction, has evidently been
developed from the condition of hermaphroditism at a late period of the
organic history of the world. In this case the female individual in both
animal and plant produces eggs or egg-cells. In animals, the male
individual secretes the fructifying sperm (sperma); in plants, the
corpuscles, which correspond to the sperm.
INHERITANCE.
The remarkable facts of inheritance, extending to the reproduction of
unimportant peculiarities of parts or organs (rudimentary parts)
mentioned above, and the occasional outbreak of ancestral characters
that have been dormant through several generations (some of which I will
mention further on), might be thought perfectly unexplainable; but they
are readily accounted for by the supposition that each part of an
organism contributes its constituent and effective molecules to the germ
and sperm particles. Mr. Sorby made numerous investigations with
relation to the number of molecules in the germinal matter of eggs, and
the spermatic matter supplied by the male. Omitting the alkali, Mr.
Sorby takes the formula, C{72}H{112}N{18}SO{22}, as representing the
composition of albumen. In a 1/2000 of an inch cube, he reckons--
Albumen 18,000,000,000,000 molecules.
Water 992,000,000,000,000 "
--------------------------------
1,010,000,000,000,000 molecules.
Or, in a sphere of the same diameter, 530,000,000,000,000 of the two
components. Taking a single mammalian spermatozoon, having a mean
diameter of 1/6000 of an inch; "it might contain two and a half million
of such gemmules. If these were lost, destroyed, or fully developed at
the rate of one in each second, this number would be exhausted in about
one month; but since a number of spermatozoa appears to be necessary to
produce perfect fertilization, it is quite easy to understand that the
number of gemmules introduced into the ovum may be so great that the
influence of the male parent may be very marked, even after having been,
as regards particular character, apparently dormant for many years." The
germinal vesicle of a mammalian ovum being about 1/1000 of an inch, mean
diameter, might contain five hundred million of gemmules, which, if used
up at the rate of one per second, would last more than seventeen years.
If the whole ovum, about 1/150 in diameter, were all gemmules, the
number would be sufficient to last, at this rate, one per second for
5,600 years! This, however, is not probable; but Mr. Sorby's remarks has
completely removed all doubt as to its physical possibility from the
Darwinian theory; "and they prompt us," says Slack, "to a wonderful
conception of the powers residing in minute quantities of matter."
The laws of inheritance are divisible into two series, conservative and
progressive transmission; the laws of adaptation to direct (active) or
indirect (potential) adaptation.
External causes often influence the reproductive system, especially in
organism propagating in a sexual way. This can be strikingly shown in
artificially produced monstrosities. Monstrosities can be produced by
subjecting the parental organism to certain extraordinary conditions of
life; and curiously enough, such an extraordinary condition of life does
not produce a change of the organism itself, but a change in its
descendants. The new formation exists in the parental organism only as a
possibility (potential); in the descendants it becomes a reality
(actual). Most commonly, monstrosities with very abnormal forms are
sterile, but there are instances where they reproduce their kind and
become a species.[35] Geoffroy St. Hilaire, who perhaps made the deepest
investigations ever conducted into the nature and causes of their
production, first conceived the idea of artificially producing them, and
to this end he began modifications of the physical conditions of the
evolution of the chicken during natural and artificial incubation. He
determined the fact that monsters could be produced in this way, but
scarcely carried his investigation further. This work has been taken up
by M. Dareste, and he has lately published a volume in Paris which
recounts the results of a quarter of a century's experimenting. Eggs, he
states, were submitted to incubation in a vertical instead of a
horizontal position; they were covered with varnish in certain places so
as to stop or modify evaporation and respiration. The evolution of the
chick was rendered slower by a temperature below that of the normal heat
of incubation. Finally, eggs were warmed only at one point, so that
the young animal, during development, was submitted at different
parts to variable temperatures.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
These perturbations resulted in the most curious and unlooked for
deformities in the embryo, some being not alone peculiar to the bird,
but being similar to those which have been recognized in many other
animals, and even in the human species. The data obtained have been
deemed so important that M. Dareste has recently received the Lacaze
prize for physiology from the French Academy of Sciences.
It would be impossible to review even a fraction of the many forms of
monstrosities which M. Dareste has discovered. Those that we give will,
however, suffice to convey an idea of the wonderful variations produced.
Fig. 1 is a chick embryo with the encephalon entirely outside the head,
the heart, liver, and gizzard outside the umbilical opening, right wing
lifted up beside the head, and the development of the left one stopped.
In Fig. 2 the encephalon is herniated and marked with blood spots, the
eye is rudimentary and replaced by a spot of pigment, the upper beak is
shorter than the lower one, while the heart, liver, etc., are all
outside. In Figs. 3 and 4 the head is compressed, eyes well developed,
but in the back instead of in the sides of the head; the body is bent,
abdominal intestines not closed, heart largely developed and herniated.
The literal references to the foregoing are: _am_, amnion; _al_,
allantois; _v_, vitellus; _h_, encephalon; _i_, eye; _c_, heart; _f_,
liver; _g_, gizzard; _ms_, upper, and _mi_, lower member.
The commonest case of monstrosity observed by M. Dareste has been that
of the head protruding from the navel, and the heart or hearts above the
head. This is a most extraordinary and new monster, and, if it persist,
a chicken with its heart on its back, like a hump, may be expected. A
curious fact discovered is the duplicity of the heart at the beginning
of incubation, two hearts, beating separately, being clearly seen.
Another anomaly consists in heads with a frontal swelling, which is
filled by the cerebral hemispheres.
M. Dareste's artificial monsters are all produced from the single germ
or cicatricule (as the white circular spot seen in the yellow of the
egg, and from which the embryo springs, is termed). He has not yet been
able to determine artificially the production of monsters, the origin of
which takes place in a peculiar state of the cicatricule before
incubation. But having submitted to incubation some 10,000 eggs, he has
obtained several remarkable examples of double monstrosities in process
of formation, some representations of which are given herewith. Fig. 5
shows three embryos, all derived from a single cicatricule. Fig. 6
represents three embryos from two cicatricules. On one side of the line
of junction are two imperfectly developed embryos, one having no heart.
The single embryo on the other side is generally normal, but has a heart
on the right side. In Fig. 7 are twins, one well formed, the heart
circulating colorless blood, the other having no heart and a rudimentary
head. Fig. 8 exhibits a double monster with lateral union. The heads are
separate, and there are three upper and three lower members, those of
the latter on the median line belonging equally to each of the pair.
ACQUIRED QUALITIES.
When an organism has been subjected to abnormal conditions in life it
can transmit any peculiarity it may have acquired. This is, however, not
always possible, otherwise descendants of men who have lost their arm or
leg would be born without the corresponding arm or leg--this shows that
some acquired qualities are more easily transmitted than
others--although there are cases, as, for instance, a race of dogs
without tails has been produced by cutting off the tails of both sexes
of the dog, during several generations. "A few years ago," says Haeckel,
"a case occurred on an estate near Jena in which, by the careless
slamming of a stable-door, the tail of a bull was wrenched off, and the
calves begotten by this bull were all born without a tail. This is
certainly an exception; but it is very important to note the fact that
under certain unknown conditions such violent changes are transmitted in
the same manner as many diseases." The transmission of diseases such as
consumption, madness, and albinism form examples. Albinoes are those
individuals who are distinguished by the absence of coloring matter from
their skins; they are of frequent occurrence among men, animals and
plants. Among many animals, such as rabbits and mice, albinoes with
white fur and red eyes are so much liked that they are propagated. This
would be impossible were it not for the law of the transmission of
adaptations. Hornless cattle have descended from a single bull born in
1770 of horned parents, but whose absence of horns was the result of
some unknown cause.
The law of interrupted or latent transmission, as illustrated in
grandchildren who are like the grandparents, but quite unlike the
parents. Animals often resume a form which have not existed for many
generations. One of the most remarkable instances of this kind of
reversion, or "atavism," is the fact that in some horses there sometimes
appear singular dark stripes similar to those of the zebra, quagga, and
other wild species of African horse.
Nutrition directly modifies adaptation, as is well illustrated by
animals which have been bred for domestic or other purposes. If a farmer
is breeding for fine wool he gives much different food to the sheep than
he would if he wished to obtain flesh or an abundance of fat. Even the
bodily form of man is quite different according to its nutrition. Food
containing much nitrogen produces little fat, that containing little
nitrogen produces a great deal of fat. People who by means of Banting's
system, at present so popular, wish to become thin, eat only meat and
eggs--no bread, no potatoes.
Man can breed for milk in cattle, for feathers in pigeons, for colored
flowers in plants, and, in fact, for almost any desirable quality.
GEOLOGICAL RECORD.
_The Geological Record_ (palæontology) furnishes weighty evidence of
man's descent; for the circumstantial evidence derived from this source
is written without the possibility of a mistake, with no chance of
error, on the stratified rocks. It is true that the geological record
must be incomplete, because it can only preserve remains found in
certain favorable localities, and under particular conditions; that this
valuable record must be destroyed by processes of denudation, and
obliterated by processes of metamorphosis, it cannot be doubted. "Beds
of rock of any thickness, crammed full of organic remains, may yet,"
says Huxley, "by the percolation of water through them, or the influence
of subterranean heat (if they descend far enough toward the centre of
the earth), lose all trace of these remains, and present the appearance
of beds of rock formed under conditions in which there was no trace of
living forms. Such metamorphic rocks occur in formations of all ages;
and we know with perfect certainty, when they do appear, that they have
contained organic remains, and that those remains have been absolutely
obliterated." If we look at the geological record, we find:
THE FIRST EPOCH.--_The Archilithic_, or Primordial Epoch, constitutes
the _Age of Skull-less Animals and Sea-weed Forests_, and is made up of
the Laurentian, Cambrian, and Silurian Period.
THE SECOND EPOCH.--_The Palæolithic_, or Primary Epoch, constitutes the
_Age of Fishes and Fern Forests_, and is made up of the Devonian, Coal,
and Permian Period.
THE THIRD EPOCH.--_The Mesolithic_, or Secondary Epoch, constitutes the
_Age of Reptiles and Pine Forests, Coniferæ_, and is made up of the
Triassic, Jurassic, and Chalk Period.
THE FOURTH EPOCH.--_The Cænolithic_, or Tertiary Epoch, constitutes the
_Age of Mammals and Leaf Forests_, and is made up of the Eocene,
Miocene, and Phocene Period.
THE FIFTH EPOCH.--The _Anthropolithic_, or Quaternary Epoch, constitutes
the _Age of Man and Cultivated Forests,_ and is made up of the Glacial
and Postglacial Period, and the Period of Culture.
During the archilithic epoch the inhabitants of our planet, as has been
already stated, consisted of skull-less animals, or aquatic forms. No
remains of terrestrial animals or plants, dated from this period, have
as yet been found.
The archilithic period was longer than the whole long period between the
close of the archilithic and the present time; for if the total
thickness of all sedimentary strata be estimated as about one hundred
and thirty thousand feet, then seventy thousand feet belong to this
epoch. It was during this epoch that the little mass of protoplasm,
which has been so often spoken of, came into existence.
It has been stated above that palæontology is quite deficient. This is
not only true of the record, but of the lack as yet of sufficient
investigations. The greatest fields of investigation in this department
have never been explored. The whole of the petrifactions accurately
known do not probably amount to a hundredth part of those which, by more
elaborate explorations, are yet to be discovered. The most ancient of
all distinctly preserved petrifactions is the Eozoon Canadense, which
was found in the lowest Laurentian strata in the Ottawa formation.
Probably no discovery in palæontology ranks higher than the discovery of
the descendants of the horse. The horse, for example, as far as his
limbs and teeth go, differs far more from extant graminivora than man
differs from the ape. Had not fossil ungulates been found, which
demonstrate the common origin of the horse with didactyles and
multidactyles, some would have deemed the horse a special miraculous
creation. But now the links are complete, and the descent of the horse
is found to follow exactly what the doctrine of evolution could have
predicted.
ONTOGENY.
It has been stated that the palæontological record is quite incomplete,
owing to many facts, some of which have been mentioned; fortunately, the
history of the development of the organic individual, or ontogeny, comes
in to fill up many deficiencies.
Ontogeny is a repetition of the principal forms through which the
respective individuals have passed from the beginning of their tribe,
and its great advantage is that it reveals a field of information which
it was impossible for the rocks to retain; for the petrification of the
ancient ancestors of all the different animal and vegetable species,
which were soft, tender bodies, was not possible.
The annexed plate illustrates the dog, rabbit, and man in their first
stages of development. Illustrations of a fish, an amphibious animal, a
reptile, a bird, or any mammal, could also be given; for all vertebrate
animals of the most different classes, in their early stages of
development, cannot be distinguished, and the nearer the animal
approaches man in the ascending scale, the longer does this similarity
continue to exist--when reptiles and birds are distinctly different from
mammals, the dog and the man are almost identical.
The gill-arches of the fish exist in man, in dogs, in fowls, in
reptiles, and in other vertebrate animals during the first stages of
their development. Man also possesses, in his first stages, a real tail,
as well as his nearest kindred--the tailless apes (orang-outang,
chimpanzee, gorilla), and vertebrate animals in general. The tail, as
has been stated, man still retains, though hidden as a rudiment.
[Illustration: FIG. I.--Human Embryo.--_Ecker._]
[Illustration: FIG. II.--Embryo of Dog.--_Bischoff._]
[Illustration: FIG. III.--Dog Embryo.--_Huxley._]
[Illustration: FIGS. IV, V, and VI.--Embryo of Rabbit in three stages of
development.--_Haeckel._]
[Illustration: FIGS. VII, VIII, and IX.--Embryo of Man in three stages
of development.--_Haeckel._ _v_, fore brain; _z_, twix brain; _m_,
middle brain; _h_, hind brain; _n_, after brain; _r_, spinal marrow;
_e_, nose; _a_, eye; _o_, ear; _k_, gillarches; _g_, heart; _w_,
vertebral column; _f_, fore limbs; _b_, hind limbs; _s_, tail.]
"Man presents in his earliest stages of embryonic growth, a skeleton of
cartilage, like that of the lamprey; also, five origins of the aorta and
five slits on the neck, like the _lamprey_ and the _shark_. Later, he
has but four aortic origins, and a heart now divided into two chambers,
like _bony fishes_; the optic lobes of his brain also having a very
fish-like predominance in size. Three chambers of the heart and three
aortic origins follow, presenting a condition permanent in the
_batrachia_; then two origins with enlarged hemispheres of the brain, as
in _reptiles_. Four heart chambers and one aortic root on each side,
with slight development of the cerebellum, agree with the characters of
the _crocodiles_, and immediately present the special mammalian
conditions, single aortic root, and the full development of the
cerebellum. Later comes that of the cerebrum, also in its higher
mammalian or human traits." At no time in the development of the egg,
save at the start, do the embryos of the various vertebra assume the
_exact_ or _entire_ characteristics of one another, but they assimilate
so closely that it requires the eye of the expert to distinguish them;
and, as has already been stated, the more closely an animal resembles
another, the longer and the more intimately do their embryos resemble
one another; so that, for example, the embryo of the snake and of a
lizard remain like one another longer than do those of a snake and of a
bird; and the embryo of a dog and of a cat remain like one another for a
far longer period than do those of a dog and a bird, or a dog and an
opossum, or even those of a dog and a monkey.
Surely it must be admitted that the short brief history given by the
development of the egg, is far more wonderful than phylogeny or the long
and slow history of the development of the tribe, which has taken
thousands of years. Compare this time with the time required for the
development of the smallest mammals--the harvest mice which develops in
three weeks, or the smallest of all birds, the humming-bird, which quits
the egg on the twelfth day, or with man who passes through the whole
course of his development in forty weeks, or with the rhinoceros who
requires 1-1/2 years, or the elephant who requires ninety weeks. How
insignificant are these various periods to the long period originally
required; yet in these short periods the whole phylogeny is run through
in the ontogeny or the history of the development of the egg.
THE ATTRIBUTES OF MAN.
We must now consider briefly some of the attributes of man, and see if
he really possesses attributes which are in no inferior degree possessed
by animals. Before proceeding directly to the consideration of the
attributes of man, it will be best to show the correlation that exists
between what are called man's vital forces and the physical forces of
nature. To do this let us choose three forms of its manifestation: these
shall be heat evolved within the body; muscular energy or motion; and
lastly, nervous energy or that form of force which, on the one hand,
stimulates a muscle to contract, and on the other appears in forms
called mental. It will not take any extensive argument to demonstrate
that the heat of the body does not differ from heat from any other
source. It is known that the food taken into the body contains potential
energy, which is capable of being in part converted into actual heat by
oxidation; and since we know that the food taken into the body is
oxidized by the oxygen of the air supplied by the lungs, the heat of the
body must be due to the slow oxidation of the carbon, perhaps also
hydrogen, sulphur, and phosphorus in the food. Now since this so-called
vital heat is developed by oxidation, is recognized by the same tests
and applied to the same purposes as any other heat, it is as truly
correlated to the other forces as when it has a purely physical origin.
The amoeboid activity of a white blood corpuscle is stimulated within
certain limits by heat. Hatching of eggs and the germination of seeds
may be likewise hastened or retarded by access or deprivation of heat.
It was considerations such as these which led to the doctrine of
correlation of the vital and physical forces.