Let us now recapitulate the ancestral chain of man, as it is set forth
in the accompanying diagram (p. 55), which represents our present
knowledge of our descent. For simplicity's sake the many side-issues
or branches which lead to groups not in the main line of our descent
have been left out, or have been indicated merely. Many of the stages
are of course hypothetical, arrived at by the study of comparative
anatomy and ontogeny; but an example for each of them has been taken
from those living or fossil creatures which seem to be their nearest
representatives.
1. The most remote ancestors of all living organisms were living beings
of the simplest imaginable kind, organisms without organs, like
the still existing _Monera_. Each consisted of a simple granule of
protoplasm, a structureless mass of albuminous matter or plasson, like
the recent Chromaceæ and Bacteriæ. The morphological value of these
beings is not yet that of a cell, but that of a cytode, or cell without
a nucleus. Cytoplasm and nucleus were still undifferentiated.
I assume that the first Monera owe their existence to spontaneous
creation out of so-called anorganic combinations, consisting of carbon,
hydrogen, oxygen, and nitrogen. An explanation of this hypothesis I
have given in my 'Generelle Morphologie.'
The Monera probably arose early in the Laurentian period. The oldest
are the Phytomonera, with vegetable metabolism. They possessed the
power (characteristic of plants) of forming albumin by synthesis from
carbon, water, and ammonia. From some of these plasma-forming Monera
arose the plasmophagous Zoomonera with animal metabolism, living
directly upon the produce of their plasmodomous or plasma-forming
sisters. This is the first instance of the great principle of division
of labour.
2. The second stage is that of the _simple and single cell_, a bit
of protoplasm with a nucleus. Such unicellular organisms are still
very common. The _Amoebæ_ are their simplest representatives. The
morphological value of such beings is the same as that of the egg
of any animal. The naked egg cells of the sponges creep about in an
amoeboid fashion, scarcely distinguishable from Amoeba. The same
remark applies to the egg-cell of man himself in its early stages
before it is enclosed in a membrane. The first unicellular organisms
arose from Monera through differentiation of the inner nucleus from the
outer protoplasm.
3. Repeated division of the unicellular organism produces the
_Synamoebium_, or community of Amoebæ, provided the divisional
products, or new generations of the original cell, do not scatter,
but remain together. The existence of such a _Coenobium_, a number
of equal and only loosely-connected cells, as a separate stage in the
ancestral history of animals, is made highly probable by the fact that
the eggs of all animals undergo after fertilization such a process of
repeated self-division, or 'cleavage,' until the single egg cell is
transformed into a heap of cells closely packed together, not unlike a
mulberry (_morula_)--hence _morula_ stage in ontogeny.
4. The morula of most animals further changes into a _Blastula_, a
hollow ball filled with fluid, the wall being formed by a single layer
of cells, the blastoderm or germinal layer. This modification is
brought about by the action of the cells--they conveying nourishing
fluid into the interior of the whole cell colony and thereby
being themselves forced towards the surface. The Blastula of most
Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ,
or hair-like processes, the vibrating motion of which causes the whole
organism to rotate and advance in the water. Living representatives of
such Blastæads, namely, globular gelatinous colonies of cells enclosing
a cavity, are Volvox and Magosphæra.
5. The Blastula of most animals assumes a new larval form called
_Gastrula_, in which the essential characteristics are that a portion
of the blastoderm by invagination converts the Blastula into a cup
with double walls, enclosing a new cavity, the primitive gut. This
invagination or bulging-in obliterates the original inner cavity of
the Blastula. The outer layer of the Gastrula is the ectoderm, the
inner the endoderm; both pass into each other at the blastoporus, or
opening of the gut cavity. The Gastrula is a stage in the embryonic
development of the various great groups of animals, and some such
primitive form as ancestral to all Metazoa is thus indicated. This
hypothetical _Gastræa_ is still very essentially represented by the
lower Coelenterates--_e.g._, Olynthus, Hydra.
6. The sixth stage--that of the _Platodes_, or flat-worms--is very
hypothetical. They are bilateral gastræads, with a flattened oblong
body, furnished with ciliæ, with a primitive nervous system, simple
sensory and reproductive organs, but still without appendages, body
cavity, vent, and blood-vessels. The nearest living representatives of
such creatures are the acoelous Turbellarians--_e.g._, Convoluta, a
free-swimming, ciliated creature.
7. The next higher stage is represented by such low animals as the
_Gastrotricha_--_e.g._, Chætonotus among the Rotatoria, which differ
from the rhabdocoelous Turbellarians chiefly by the formation of
a vent and the beginnings of a coelom, or cavity, between gut and
body wall. The addition of a primitive vascular system and a pair of
nephridia, or excretory organs, is first met with in the _Nemertines_.
8. These, together with the _Enteropneusta_ (Balanoglossus), are
comprised under the name of Frontonia, or Rhynchelminthes, and form the
highest group of the Vermalia.
The Enteropneusta especially fix our attention, because they alone,
although essentially 'worms,' exhibit certain characteristics which
make it possible to bridge over the gulf which still separates the
Invertebrata from the vertebrate phylum. The anterior portion of the
gut is transformed into a breathing apparatus--hence Gegenbaur's
term of Enteropneusta, or Gut-breathers. Moreover, Balanoglossus and
Cephalodiscus possess another modification of the gut--namely, a
peculiar diverticulum, which, in the present state of our knowledge,
may be looked upon as the forerunner of the chorda dorsalis.
9. Stage of _Prochordonia_, as indicated by the larval form, called
Chordula, which is common to the Tunicata and all the Vertebrata.
These two groups possess three most important features: (_a_) A chorda
dorsalis, a stiff rod lying in the long axis of the body, dorsally from
the gut and below the central nervous system. This latter, for the
first time in the animal kingdom, appears in the shape of a spinal
cord. (_b_) The use of the anterior portion of the gut for respiratory
purposes. (_c_) The larval development of the Tunicata is essentially
the same as that of the Vertebrata in its early stages. Only the
free-swimming Copelata or Appendicularia among the Tunicates retain
most of these features. The others, which become sessile--namely, the
Ascidiæ, or sea-squirts--degenerate and specialize away from the main
line.
10. Stage of the _Acrania_, represented by Amphioxus. The early
development of this little marine creature agrees closely with
that of the Tunicates; but one important feature is added to its
organization--namely, metamerism, segmentally arranged mesoderm.
Amphioxus still possesses neither skull nor vertebræ, neither ribs
nor jaws, and no limbs. But it is a member of the Vertebrata if we
define these as follows: Bilateral symmetrical animals with segmentally
arranged mesoderm, with a chorda dorsalis between the tubular nervous
system and the gut, and with respiratory organs which arise from the
anterior portion of the gut. We do not assume that Amphioxus stands
in the direct ancestral line; it is probably much specialized, partly
degenerated, and represents a side-branch; but it is, nevertheless,
the only creature, hitherto known, which satisfactorily connects the
Vertebrata with their invertebrate ancestors. Many other efforts have
been made to solve the mystery of the origin of the Vertebrata--all
less satisfactory than the present suggestion, or even absolutely
futile. This remark applies especially to the attempts to derive them
from either Articulata or Echinoderms. The other great and highly
developed phylum, the Mollusca, is quite out of the question. We have
to go back to a level at which all these principal phyla meet, and
there we find the Vermalia, the lower of which alone permit connection
in an upward direction with the higher phyla.