an evolutio bigemina was characteristic, for the articulates the
evolutio gemina, for the molluscs the evolutio contorta, and for
the radiates the evolutio radiata. Here we meet for the first time
the idea that for the correct solution of the questions of relation-
ship of animals, and therefore a basis for a natural classification,
comparative embryology is indispensable; an idea which in recent
years has proved exceedingly fruitful.
Cell Theory. Of fundamental importance for the further
growth of comparative anatomy and embryology was the proof
that all organisms, as well as their embryonic forms, were com-
posed of the same elements, the cells. This knowledge is the
quintessence of the cell theory, which during the third decade of
the last century was advanced by Schleiden and Schwann, and
which two decades later was completely remodelled by the proto-
plasm theory of Max Schultze. In the cell theory a simple prin-
18 GENERAL PRINCIPLES OF ZOOLOGY.
ciple of organization was found for all living creatures, for highly
and for lowly organized plants and animals, and the wide realm of
histology was laid open for scientific treatment.
KEFORM OF THE SYSTEM.
Foundation of Modern Zoology. With the establishment of
comparative anatomy and embryology and the application of these
to classification, and with the development of the cell theory and
of histology, which is connected with it, we may say that the
foundation of zoology was laid. Wonderful advances were made
in vertebrate anatomy by the classic researches of Owen, Johannes
Miiller, Rathke, Gegenbaur, and others; our conceptions of organ-
ization have been completely altered by the work of Dujardin,
Max Schultze, Haeckel, and others, who have proved the unicellu-
larity of the lowest animals. The germ -layer theory was further
elaborated by Remak and Kolliker; and applied to the invertebrate
animals by Kowalewsky, Haeckel, and Huxley. It is beyond the
limits of this brief historical summary to go into what has been
accomplished in regard to the other branches of the animal king-
dom; it must here be sufficient to mention the most important
changes which the Cuvierian system has undergone under the
influence of increasing knowledge.
The Division of the Radiata. Of the four types of Cuvier the
branch Radiata was undoubtedly the one of whose representatives
he had the least knowledge; it was therefore the least natural,
since it comprised, besides the radially symmetrical coelenterates
and echinoderms, other forms, which, like the worms, were
bilaterally symmetrical, or, like many infusorians, were asym-
metrical. Thus it came about that most reforms have here found
their point of attack.
C. Th. von Siebold was the originator of the first important
reform. He limited the type Radiata, or, as ho termed them, the
Zoophytes, to those animals with radially symmetrical structure
(Echinoderms and the Plant-animals) ; separating all the others,
he formed of the unicellular organisms the branch of ' primitive
animals' or Protozoa; the higher organized animals he grouped
together as worms or Vermes; at the same time he transferred a
part of the Articulata, the annelids, to the worm group, and pro-
posed for the other articulates, crabs, millipedes, spiders, and
insects, the term Arthropoda.
Leuckart, about the same time (1848), divided the branch
HISTORY OF ZOOLOGY. 19
Eadiata into two branches differing greatly in structure. The
lower forms, in which no special body-cavity is present, the
interior of the body consisting only of a system of cavities serving
for digestion, the alimentary canal, he called the Ccelentera
(essentially the Zoophyta of the older zoologists) ; to the rest, in
which the alimentary canal and the body-cavity occur as two
separate cavities, he gave the name Echinoderma.
The Present System. Thus there resulted seven classes:
i Protozoa, Coelentera, Echinoderma, Vermes, Arthropoda, Mol-
lusca, and Vertebrata. Still this arrangement does not meet the
requirements of a natural system and hence is more or less unsat-
isfactory. Some zoologists are returning to the Cuvierian classifi-
cation to the extent of uniting the segmented worms with the
arthropods in a group Articulate. Upon the ground of important
anatomical and embryological characters the Brachiopoda, the
Bryozoa, and the Tunic^ita have been separated from the Mollusca;
they form the subject of diverse opinions. The relationships of
the first two groups have not yet been settled : of the Tunicata we
know indeed that they are related to the Vertebrata, but the
diiferences are such that they cannot be included in that group.
The only way out of the difficulty is to unite vertebrates, tunicates,
and some other forms in a larger division, Chordata. The Vermes,
too, must be divided, as will appear in the second part of this
volume.
HISTORY OF THE THEORY OF EVOLUTION.
Importance of the Subject. Before closing the historical intro-
duction we must consider the historical development of a question
whose importance might, on a superficial examination, be under-
rated, but which from a small beginning has grown into a problem
completely dominating zoological research, and has occupied not
only zoologists, but all interested in science generally. This is the
question of the logical value of the systematic conceptions species,
genus, family, etc.
The Nature of Species. In nature we find only separate
animals : how comes it that we classify them into larger and smaller
groups ? Are the single species, genera, and the other divisions
which the systematist distinguishes, fixed quantities, as it were
fundamental conceptions of nature, or a Creator's thoughts, which
find expression in the single forms ? Or are they abstractions
which man has brought into nature for the purpose of making it
20 GENERAL PRINCIPLES OF ZOOLOO Y.
comprehensible to his mental capabilities ? Are the specific and
generic names only expressions which have become necessary, from
the nature of our mental capacity, for the gradation of relation-
ship in nature, which in and for themselves are not immutable,
and hence can undergo a gradual change ? Practically speaking,
the problem reads: are species constant or changeable? What is
true for species must necessarily be true for all other categories of
the system, all of which in the ultimate analysis rest upon the
conception of species.
Ray's Conception of Species. One of the first to consider the
conception of species was Linnaeus's predecessor, the Englishman
John Ray. In the attempt to define what should be understood
as a species he encountered difficulties. In practice, animals which
differ little in structure and appearance from one another are
ascribed to the same species; this practical procedure cannot be
carried out theoretically; for there are males and females within
the same species which differ more from one another than do the
representatives of different species. Thus John Ray reached the
genetic definition when he said: for plants there is no more
certain criterion of specific unity than their origin from the seeds
of specifically or individually like plants; that is to say, generalized
for all organisms : to one and the same species belong individuals
which spring from similar ancestors.
The Cataclysm Theory. 1 With Ray's definition an entirely
uncontrollable element was brought into the conception of species,
since no systematist usually knew anything, nor indeed could he
know anything, as to whether the representatives of the species
placed before him sprang from similar parents. It was therefore
only natural that the conception of species put on a religious garb,
since by resting upon theological ideas it found a firmer support.
Linnaeus said: "Tot sunt species quot ab initio creavit infinitum
Ens"; with this he built up a conception of species upon the
tradition cf the Mosaic history of creation, a procedure quite
unjustified upon grounds of natural science, since it drew one of
its iundamental ideas from transcendental conceptions, not from
the experience of natural science. Linnaeus's definition showed
itself untenable, as soon as paleontology began to make accessible
a vast quantity of extinct animals deposited as fossils. With an
odd fancy, the fossils, being inconvenient for study, were for a
long time regarded as outside the pale of scientific research. They
might be sports of nature, it was said, or remains of the Flood, or
of the influence of the stars upon the earth, or products of an aura
HISTORY OF ZOOLOGY. 21
seiniiialis, a fertilizing breath, which, if it fell upon organic bodies,
led to the formation of animals and plants, but if it strayed upon
inorganic materials gave rise to fossils. The foundation of
scientific paleontology by Cuvier put an end to such empty specu-
lations. Cuvier proved beyond a doubt that these fossils were the
remains of animals of a previous time. Just as the formation of
the earth's crust by successive overlying layers made possible the
recognition of different periods in the earth's history, so paleon-
tology taught how to recognize also the different periods in the
vegetable and animal world of life on our globe. Each geological
age was characterized by a special world of animals quite peculiar
to it; and these animal worlds differed the more from the present,
the older the period of the earth to which they belonged. All
these generalizations led Cuvier to his cataclysm theory, that a
great revolution brought each period of the earth's history to an
end, destroying all life, and that upon the newly formed virgin
earth a new organic world of immutable species sprang up.
Objections to the Cataclysm Theory. By the supposition of
numerous acts of creation the Linnean conception of species
seemed to be rescued, though, to be sure, by summoning to its aid
hypotheses which had neither foundation in science nor justifica-
tion in theology. The logical results of Cuvier's cataclysm theory
were conceptions of a Creator who built up an animal world only
for the purpose of destroying it after a time as a troublesome toy;
it has therefore at no time found warm supporters, at least among
geologists, for whom it was intended. Of the prominent zoologists
there is only to be mentioned Louis Agassiz, who till the end of
his life remained faithful to this theory.
Under these conditions it is readily understood how thinking
naturalists, who felt the necessity of explaining the character of
organic nature simply and by a natural law capable of general
application, began to doubt the fixity of species, and were led to
the theory of change of form, the Theory of Descent, or Evolution.
Darwin's Predecessors. Even in Cuvier's time there prevailed
a strong current in favor of this theory. It found expression in
England in the writings of Erasmus Darwin (grandfather of the
renowned Charles Darwin); in Germany in the works of Goethe,
Oken, and the disciples of the ' natural philosophical' school; in
France the genealogical theory was developed particularly by
Buffon, Geoffroy St. Hilaire, and Lamarck. Its completest ex-
pression was found in Lamarck's " Philosophic zoologique " (1809) ;
its arguments will be considered in the following paragraphs.
22 GENERAL PRINCIPLES OF ZOOLOGY.
Lamarck (Jean Baptiste de Monet, Chevalier de Lamarck,
born in Picardy, 1744, died, Professor at the Jardin des Plantes,
1829) taught that on the earth at first organisms of the simplest
structure arose in the natural way through spontaneous generation
from non-living matter. From these simplest living creatures have
developed, by gradual changes in the course of an immeasurably
vast space of time, the present species of plants and animals,
without any break in the continuity of life upon oitr globe; the
terminal point of this series is man; the other animals are the
descendants of those forms from which man has developed.
Lamarck, in accordance with the then prevailing conceptions,
regarded the animal kingdom as a single series grading from the.
lowest primitive animal up to man. Among the causes which may
influence th6 change and perfecting of organisms, Lamarck
emphasized particularly use and disuse; the giraffe has obtained a
long neck because by a special condition of life he was compelled
to stretch, in order to browse the leaves on high trees; conversely,
the eyes of animals which live in the dark have degenerated from
lack of use into functionless structures. The direct influence of
the external world must be unimportant; the changes in the sur-
roundings (Geoffrey St. Hilaire's le monde ambient) must for the
most part act indirectly upon animals by altering the conditions
for the use of organs.
Evolution vs. Creation. Lamarck's ingenious work remained
almost unnoticed by his contemporaries. On the other hand there
arose a violent controversy between the defenders and the
opponents of the evolution theory when [1830] Geoffroy St. Hilaire
in a debate in the Academy at Paris defended against Cuvier the
thesis of a near relationship of the vertebrates and the insects, and
set up the proposition that the latter were " vertebrates running
on their backs." The conflict ended in the complete overthrow of
the theory of evolution; the defeat was so complete that the
problem vanished for a long time from scientific discussion, and
the theory of the fixity of species again became dominant. This
error was occasioned by many causes. Above all, the theory of
Geoffroy St. Hilaire and Lamarck was rather a clever conception
than founded 011 abundant facts; besides, it had in it as a funda-
mental error the doctrine of the serial arrangement of the animal
world. * Opposed to this stood Cuvier's great authority and his
extensive knowledge, the latter making it easy for him to show
that the animal kingdom was made up of separate co-ordinated
groups, the types.
HISTORY OF ZOOLOGY. 23
Lyell. In the same year in which Cuvier obtained his victory
over Geoffroy St. Hilaire, his theory of the succession of numerous
animal worlds upon the globe received its first destructive blow.
Cuvier's cataclysm theory had two sides, a geological and a
biological. Cuvier denied the continuity of the various terrestrial
periods, as well as the continuity of the fauna and flora belonging
to them. In 1830-32 appeared the "Principles of Geology" by
Lyell, an epoch-making work, which, in the realm of geology,
completely set aside the cataclysm theory. Lyell proved that the
supposition of violent revolutions on the earth was not necessary
in order to explain the changes of the earth's surface and the
superposition of its strata; that rather the constantly acting
forces, elevations and depressions, the erosive action of water, be
it as ebb and flow of the tide, as rain, snow, or ice, or as the flow
of rivers and brooks rushing as torrents towards the sea, are suffi-
cient to furnish a complete explanation. Very gradually in the
course of a vast space of time the earth's surface has changed, and
passed from one period into the next, and still at the present day
the constant process of change is going on. The continuity in the
geological history of the earth, here postulated for the first time,
has since then become one of the fundamental axioms of Geology;
on the other hand the discontinuity of living creatures, although
the geological support of this was frail, was for a long time
regarded as correct.
Darwin. It is the great merit of Charles Darwin that he took
up the theory of descent anew after it had rested a decade, and
later brought it into general recognition. With this began the
most important period in the history of zoology, a period in which
the science not only made such an advance as never before, but
also began to obtain a permanent influence upon the general views
of men.
Charles Darwin was born at Shrewsbury, Eng., in 1809. After
studying at the universities of Edinburgh and Cambridge, he joined
as naturalist the English war-ship " Beagle." In its voyage from
1831 to 36 around the globe, Darwin recognized the peculiar
character of island faunas, particularly of the Galapagos Islands,
and the remarkable geological succession of edentates in South
America; these facts formed for him the germ of his epoch-making
theory. Further results of this journey were his beautiful mono-
graph on the Cirripedia, and the classic investigation of coral-reefs.
After his return to England Darwin lived, entirely devoted to
scientific work, chiefly in the hamlet of Down, county Kent, up
24 GENERAL PRINCIPLES OF ZOO LOOT.
to the time of his death in 1882. He was incessantly busy in
developing his conception of the origin of species, and in collect-
ing for this a constantly increasing array of facts. The first
written notes, the fundamental ideas of which he communicated
to friends, particularly the geologist Lyell and the botanist
Hooker, were made in 1844, but the author was not persuaded to
give them publicity. Not until 1858 did Darwin decide to make
his first contribution to science. In this year he received an essay
sent by the traveller Wallace, which in its most important points
coincided with his own views. At the same time with Wallace's
manuscript an abstract of Darwin's theory was published. In the
next year (1859) appeared the most important of his writings,
" On the Origin of Species by means of Natural Selection/' and
in rapid succession a splendid series of works, the fruit of many
years, of preparatory labors. For the history of the theory the
most important of these are: (1) "Upon the Variation of Plants
and Animals under Domestication/' two volumes, which chiefly
contain a collection of material for proofs; (2) on "The Descent
of Man," a work which gives the application of the theory to man.
No scientific work of this century has attracted so much atten-
tion in the zoological, we may even say in the whole educated
world, as "The Origin of Species." It was generally received as
something entirely new, so completely had the scientific tradition
been lost. In professional circles it was stoutly combated by one
faction, with another it found well-wishing but hesitating accept-
ance. Only a few men placed themselves from the beginning in
a decided manner on the side of the great British investigator.
There was a lively scientific battle, which ended in a brilliant vic-
tory for the theory of evolution. At the present time all our
scientific thoughts are so permeated with the idea of evolution that
we can scarcely speak of any considerable opposition to it.
Post-Darwinian Writers. Among the men who have most
influenced this rapid advance is to be mentioned, besides A. K.
Wallace, the co-founder of Darwinism, above all Ernst Haeckel,
who in his "General Morphology" and his "Natural History of
Creation" has done much towards the extension of the theory.
Among other energetic defenders of the theory in Germany should
be mentioned Fritz Miiller, Carl Vogt, Weismann, Moritz Wagner,
and Nageli, even if they have taken special standpoints in refer-
ence to the causes conditioning the changes of form. Among the
English naturalists are to be named particularly Huxley, Hooker,
and Lyell. In America Gray, Cope, "and Hyatt were early sup-
HISTORY OF ZOOLOGY. 25
porters. Darwinism was long in obtaining an entrance into
France.
DARWIN'S THEORY OF THE ORIGIN OF SPECIES.
Before Darwin wrote the idea of fixity of species prevailed
among systematists. It was recognized that all the individuals of
a species are not alike, and that more or less pronounced variability
occurs, so that it was possible to distinguish races and varieties
within the species, but it was believed that the variations never
transcended specific bounds.
The Problem Stated. Darwin begins with a criticism of the
term species. Is the conception of species on the one side and
that of race and variety on the other something entirely different ?
Are there special criteria for determining beyond the possibility
of a doubt whether in a definite case we have to do with a variety
of a species or with a different species ? or do the conceptions in
nature pass into one another ? Are species varieties which have
become constant, and precisely in the same manner are varieties
species in the process of formation ?
Morphological Characters. A. Distinction between Species and
Variety. For the settlement of this fundamental question morpho-
logical and physiological characters can be considered. In the
practice of the systematists usually the morphological characters
prevail exclusively; for that reason they will be here considered
first. If, among a great number of forms similar to one another,
two groups can be adduced which differ considerably from one
another, if the difference between them be obliterated by no inter-
mediate forms, and if in several successive generations they
remain constant, then the systematist speaks of a ' good species ' ;
on the other hand he speaks of varieties of the same species when
the differences are slight and inconstant, and when they lose their
importance through the existence of intermediate forms. A
definite application of this rule discloses great incongruities, many
animal and vegetable groups being regarded by one set of systema-
tists as good species, by another only as 'sports/ i.e., as varieties
of the same species. The differences between the ' sports ' of our
domestic animals are in many instances so considerable that
formerly they were regarded not only as sufficient for the founda-
tion of good species, but even of genera and families. In the
fantail pigeon the number of tail-feathers, formerly only 12-14,
has increased to 30-42 (fig. Ic) ; among the other races of pigeons
26 GENERAL PRINCIPLES OF ZOOLOGY.
enormous variations are found in the size of the beak and feet in
comparison with the rest of the body (figs. IA, IB); even the
skeleton itself participates in this variation, as is shown by the fact
FIG. IA. English, carrier-pigeon. (After Darwin.)
FIG. IB. English tumbler-pigeon. (After Darwin.)
that the total number of vertebrae varies from 38 (in the carrier-
pigeon) to 43 (in the pouter), the number of sacral vertebra from
14 to 11.
B. Variation within the Species. Now in respect to the occur-
rence of transitional forms and the constancy of differences, there
is within one and the same < good species ' the greatest conceivable
difference. In many very variable species the extremes are united
HISTORY OF ZOOLOGY.
27
"by many transitions; in other cases sharply circumscribed groups
of forms, or races, can be distinguished within the same species.
In the race, the peculiar characteristics are inherited from genera-
Fio. lc. English fantail pigeon. (After Darwin.)
tion to generation with the same constancy as in good species.
This is shown in the human races, and in many pure, cultivated
races of domesticated animals.
Physiological Characters. A. Crossing of Species and Varie-
ties. A critical examination leads to the conclusion that Mor-
phology is indeed useful for grouping animals into species and
varieties, but that it leaves us completely in the lurch when it is
Called upon to show the distinctions between what should be called
a species and what a variety. Therefore there remains open to
the systematist only one resource, i.e., to summon Physiology
to his aid. This has been done, and it has disclosed considerable
distinctions in reproduction. We should expect a priori that the
individuals of different species would not reproduce with each
other; on the other hand under normal conditions the individuals
of one and the same species, even though they are of different
varieties or races, should be entirely fertile. One must beware of
arguing in a circle in proof of these two propositions; it would be
an argument in a circle if an experimenter should regard two
animals as representatives of one species only because they proved
to be fertile together, while under their former relations he
assigned them to different species. Bather the question for him
must read: does physiological experiment lead to the same
28 GENERAL PRINCIPLES OF ZOOLOGY.
systematic distinctions as does the common systematic experience,
viz., the depreciation of constancy and the divergence of distin-
guishing characters ?
B. The Intercrossing of Species. This field is as yet far from
being sufficiently investigated experimentally; yet some general
propositions can be set up: (1) that not a few so-called 'good
species' can be crossed with one another; (2) that in general the