PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
IV. MAMMALS.
The System of Mammals according to Linnæus and
Blainville.—Three Sub-classes of Mammals (Ornithodelphia,
Didelphia, Monodelphia).—Ornithodelphia, or Monotrema.—Beaked
Animals (Ornithostoma).—Didelphia, or Marsupials.—Herbivorous
and Carnivorous Marsupials.—Monodelphia, or Placentalia
(Placental Animals).—Meaning of the Placenta.—Tuft
Placentalia.—Girdle Placentalia.—Disc
Placentalia.—Non-deciduates, or Indeciduata.—Hoofed
Animals.—Single and Double-hoofed Animals.—Whales.—Toothless
Animals.—Deciduates, or Animals with
Decidua.—Semi-apes.—Gnawing Animals.—Pseudo-hoofed
Animals.—Insectivora.—Beasts of Prey.—Bats.—Apes.
There are only a few points in the classification of organisms upon
which naturalists have always agreed. One of these few undisputed points
is the privileged position of the class of Mammals at the head of the
animal kingdom. The reason of this privilege consists partly in the
special interest, also in the various uses and the many pleasures, which
Mammals, more than all other animals, offer to man, and partly in the
circumstance that man himself is a member of this class. For however
differently in other respects man’s position in nature and in the system
of animals may have been regarded, yet no naturalist has ever doubted
that man, at least from a purely morphological point of view, belongs
to the class of Mammals. From this there directly follows the
exceedingly important inference that man, by consanguinity also, is a
member of this class of animals, and has historically developed out of
long since extinct forms of Mammals. This circumstance alone justifies
us here in turning our especial attention to the history and the
pedigree of Mammals. Let us, therefore, for this purpose first examine
the groups of this class of animals.
Older naturalists, especially considering the formation of the jaw and
feet, divided the class of Mammals into a series of from eight to
sixteen orders. The lowest stage of the series was occupied by the
whales, which seemed to differ most from man, who stands at the highest
stage, by their fish-like form of body. Thus Linnæus distinguished the
following eight orders: (1) Cetæ (whales); (2) Belluæ (hippopotami and
horses); (3) Pecora (ruminating animals); (4) Glires (gnawing animals
and rhinoceroses); (5) Bestiæ (insectivora, marsupials, and various
others); (6) Feræ (beasts of prey); (7) Bruta (toothless animals and
elephants); (8) Primates (bats, semi-apes, apes, and men). Cuvier’s
classification, which became the standard of most subsequent zoologists,
did not rise much above that of Linnæus. Cuvier distinguished the
following eight orders: (1) Cetacea (whales); (2) Ruminantia (ruminating
animals); (3) Pachyderma (hoofed animals, with the exclusion of
ruminating animals); (4) Edentata (animals poor in teeth); (5) Rodentia
(gnawing animals); (6) Carnassia (marsupials, beasts of prey,
insectivora, and bats); (7) Quadrumana (semi-apes and apes); (8) Bimana
(man).
The most important advance in the classification of Mammals was made as
early as 1816 by the eminent anatomist Blainville, who has already been
mentioned, and who first clearly recognised the three natural main
groups or sub-classes of Mammals, and distinguished them according to
the formation of their generative organs as _Ornithodelphia_,
_Didelphia_, and _Monodelphia_. As this division is now justly
considered by all scientific zoologists to be the best, on account of
solid foundation on the history of development, let us here keep to it
also.
The first sub-class consists of the _Cloacal Animals_, or _Breastless
animals_, also called _Forked animals_ (Monotrema, or Ornithodelphia).
This class is now represented only by two species of living mammals,
both of which are confined to Australia and the neighbouring island of
Van Diemen’s land, namely, the well-known Water Duck-bill
(Ornithorhynchus paradoxus) with the beak of a bird, and the less known
Beaked Mole (Echidna hystrix), resembling a hedgehog. Both of these
curious animals, which are classed in the order of _Beaked Animals_
(Ornithostoma), are evidently the last surviving remnants of an animal
group formerly rich in forms, which alone represented the Mammalia in
the secondary epoch, and out of which the second sub-class, the
Didelphia, developed later, probably in the Jurassic period.
Unfortunately, we as yet do not know with certainty of any fossil
remains of this most ancient primary group of Mammals, which we will
call _Primary Mammals_ (Promammalia). Yet they possibly comprise the
oldest of all the fossil Mammalia known, namely, the Microlestes
antiquus, of which animals, however, we as yet only know some few small
molar teeth. These have been found in the uppermost strata of the
Trias, in the Keuper, first in Germany (at Degerloch, near Stuttgart,
in 1847), later also in England (at Frome), in 1858. Similar teeth have
lately been found also in the North American Trias, and have been
described as Dromatherium sylvestre. These remarkable teeth, from the
characteristic form of which we can conclude that they belonged to an
insectivorous mammal, are the only remains of mammals as yet found in
the older secondary strata, namely, in the Trias. It is possible,
however, that besides these many of the other mammalian teeth found in
the Jura and Chalk systems, which are still generally ascribed to
Marsupials, in reality belong to Cloacal Animals. This cannot be decided
with certainty owing to the absence of the characteristic soft parts. In
any case, numerous Monotrema, with well-developed teeth and cloaca, must
have preceded the advent of Marsupial animals.
The designation, “_Cloacal animals_” (Monotrema), has been given to the
Ornithodelphia on account of the cloaca which distinguishes them from
all other Mammals; but which on the other hand makes them agree with
Birds, Reptiles, and Amphibia, in fact, with the lower Vertebrata. The
formation of the cloaca consists in the last portion of the intestinal
canal receiving the mouth of the urogenital apparatus, that is, the
united urinary and genital organs, whereas in all other Mammals
(Didelphia as well Monodelphia) these organs have an opening distinct
from that of the rectum. However, in these latter also the cloaca
formation exists during the first period of their embryonal life, and
the separation of the two openings takes place only at a later date (in
man about the twelfth week of development). The Cloacal animals have
also been called “_Forked animals_,” because the collar-bones, by means
of the breast bone, have become united into one piece, similar to the
well-known fork-bone, or merry-thought, in birds. In all other Mammals
the two collar-bones remain separated in front and do not fuse with the
breast bone. Moreover, the coracoid bones are much more strongly
developed in the Cloacal animals than in the other Mammalia, and are
connected with the breast bone.
In many other characteristics also—especially in the formation of their
internal genital organs, their auricular labyrinth, and their
brain—Beaked animals are more closely allied to the other Vertebrata
than to Mammals, so that some naturalists have been inclined to separate
them from the latter as a special class. However, like all other
Mammals, they bring forth living young ones, which for a time are
nourished with milk from the mother. But whereas in all other Mammals
the milk issues through nipples, or teats, from the mammary glands,
teats are completely wanting in beaked animals, and the milk comes
simply out of a flat, sieve-like, perforated patch of the skin. Hence
they may also be called _Breastless_ or _Teatless animals_ (Amasta).
The curious formation of the beak in the two still living Beaked
animals, which is connected with the suppression of the teeth, must
evidently not be looked upon as an essential feature of the whole
sub-class of Cloacal animals, but as an accidental character of
adaptation distinguishing the last remnant of the class as much from the
extinct main group, as the formation of a similar toothless snout
distinguishes many toothless animals (for instance, the ant-eater) from
the other placental animals. The unknown, extinct Primary Mammals, or
Promammalia—which lived during the Trias period, and of which the two
still living orders of Beaked animals represent but a single
degenerated branch developed on one side—probably possessed a very
highly developed jaw like the marsupial animals, which developed from
them.
_Marsupial_, or _Pouched Animals_ (Didelphia, or Marsupialia), the
second of the three sub-classes of Mammals, form in every respect—both
as regards their anatomy and embryology, as well as their genealogy and
history—the transition between the other sub-classes—the Cloacal and
Placental Animals. Numerous representatives of this group still exist,
especially the well-known kangaroos, pouched rats, and pouched dogs; but
on the whole this sub-class, like the preceding one, is evidently
approaching its complete extinction, and the living members of the class
are the last surviving remnants of a large group rich in forms, which
represented the Mammalia during the more recent secondary and the
earlier tertiary periods. The Marsupial Animals probably developed
towards the middle of the Mesolithic epoch (during the Jura) out of a
branch of the Cloacal Animals, and in the beginning of the Tertiary
epoch again, the group of Placental Animals arose out of the Marsupials,
and the latter then succumbed to the former in the struggle for life.
All the fossil remains of Mammals known to us from the Secondary epoch,
belong either exclusively to Marsupials, or partly perhaps to Cloacal
animals. At that time Marsupials seem to have been distributed over the
whole earth; even in Europe (France and England), well-preserved fossil
remains of them have been found. On the other hand, the last off-shoots
of the sub-class now living are confined to a very narrow tract of
distribution, namely, to Australia, the Australasian, and a small part
of the Asiatic Archipelago. There are also a few species still living
in America, but at the present day not a single marsupial animal lives
on the continent of Asia, Africa, or Europe.
The name of pouched animals is given to the class on account of the
purse-shaped pouch (marsupium) existing in most instances on the
abdominal side of the female animals, in which the mother carries about
her young for a considerable time after their birth. This pouch is
supported by two characteristic marsupial bones, also existing in
Cloacal animals, but not in Placental animals. The young Marsupial
animal is born in a much more imperfect form than the young Placental
animal, and only attains the same degree of development which the latter
possesses directly at its birth, after it has developed in the pouch for
some time. In the case of the giant kangaroo, which attains the height
of a man, the newly born young one, which has been carried in the
maternal womb not much longer than five weeks, is not more than an inch
in length, and only attains its essential development subsequently, in
the pouch of the mother, where it remains about nine months attached to
the nipple of the mammary gland.
The different divisions generally distinguished as families in the
sub-class of Marsupial animals, deserve in reality the rank of
independent orders, for they differ from one another in manifold
differentiations of the jaw and limbs, in much the same manner, although
not so sharply, as the various orders of Placental animals. In part they
perfectly agree with the latter. It is evident that adaptation to
similar conditions of life has effected entirely coincident or analogous
transformations of the original fundamental form in the two sub-classes
of Marsupials. According to this, about eight orders of Marsupial
animals may be distinguished, the one half of the main group or legion
of which are herbivorous, the other half carnivorous. The oldest fossil
remains of the two legions (if the previously mentioned Microlestes and
the Dromatherium are not included) occur in the Jurassic strata, namely,
in the slates of Stonesfield, near Oxford. The slates belong to the
Bath, or the Lower Oolite formation—strata which lie directly above the
Lias, the oldest Jura formation. (Compare p. 15.) It is true that the
remains of Marsupials found in the slates of Stonesfield, as well as
those which were found later in the Purbeck strata, consist only of
lower jaws. (Compare p. 29.) But fortunately the lower jaw is just one
of the most characteristic parts of the skeleton of Marsupials. For it
is distinguished by a hook-shaped process of the lower corner of the jaw
turning downwards and backwards, which neither occurs in Placental nor
in the (still living) Cloacal animals, and from the existence of this
process on the lower jaws from Stonesfield, we may infer that they
belonged to Marsupials.
SYSTEMATIC SURVEY OF CLOACAL AND MARSUPIAL MAMMALIA.
I. _First Sub-class of Mammalia:_
_Forked or Cloacal Animals (Monotrema, or Ornithodelphia)._
Mammals with Cloaca, without Placenta, with Marsupial Bones.
------------------------------------------------------------------------------------
I. }
=Primary Mammals= } Unknown extinct Mammalia from the { (Microlestes?)
+Promammalia+ } Trias Period { (Dromatherium?)
II. } 1. Aquatic beaked 1. Ornithorhynchida { 1. Ornithorhynchus
=Beaked Animals= } animals { paradoxus
+Ornithostoma+ } 2. Terrestrial 2. Echidnida { 2. Echidna hystrix
} beaked animals {
------------------------------------------------------------------------------------
II. _Second Sub-class of Mammalia:_
_Pouched or Marsupial Animals (Marsupialia, or Didelphia)._
Mammals without Cloaca, without Placenta, with Marsupial Bones.
------------------------------------------------------------------------------------
_Legions_ | _Orders_ | _Systematic Name_ | _Families of the_
_of_ | _of_ | _of_ | _Marsupialia._
_Marsupialia._ | _Marsupialia._ | _the Orders._ |
------------------------------------------------------------------------------------
{ 1. Hoofed 1. Barypoda { 1. Stereognathida
{ Marsupial animals { 2. Nototherida
III. { { 3. Diprotodontia
=Herbivorous= {
=Marsupial= { 2. Kangaroo 2. Macropoda { 4. Plagiaulacida
=Animals= { Marsupial animals { 5. Halmaturida
{ (Leaping pouched { 6. Dendrolagida
+Marsupialia+ { animals)
+Botanophaga+ {
{ 3. Root-eating 3. Rhizophaga {
{ Marsupial animals {
{ (Gnawing pouched { 7. Phascolomyida
{ animals) {
{
{ 4. Fruit eating 4. Carpophaga {
{ Marsupial animals { 8. Phascolarctida
{ (Climbing pouched { 9. Phalangistida
{ animals) { 10. Petaurida
{ 5. Insectivorous 5. Cantharophaga { 11. Thylacotherida
{ Marsupial animals { 12. Spalacotherida
{ (Primæval pouched { 13. Myrmecobida
{ animals) { 14. Peramelida
IV. {
=Carnivorous= { 6. Marsupial animals 6. Edentula {
=Marsupial= { poor in teeth {
=Animals= { (Pouched animals { 15. Tarsipedina
{ with trunks) {
{
{ 7. Rapacious marsupial 7. Creophaga { 16. Dasyurida
{ animals { 17. Thylacinida
+Marsupialia+ { (Rapacious pouched { 18. Thylacoleonida
+Zoophaga+ { animals)
{
{ 8. Ape-footed 8. Pedimana {
{ Marsupial animals { 19. Chironectida
{ (Pouched animals { 20. Didelphyida
{ with hands)
SYSTEMATIC SURVEY OF PLACENTAL ANIMALS.
III. _Third Sub-class of Mammalia:_
_Placentalia, or Monodelphia (Placental Animals)._
Mammals without Cloaca, with Placenta, without Marsupial Bones.
-----------------------------------------------------------------------------------
_Legions of_ | _Orders of_ | _Sub-orders of_ |_Systematic Name_
_the_ | _the_ | _the_ | _of_
_Placental Animals._|_Placental Animals._|_Placental Animals._ | _the Sub-orders._
-----------------------------------------------------------------------------------
III. 1. INDECIDUA. _Placental Animals without Decidua._
-----------------------------------------------------------------------------------
V. { I. Single-hoofed { 1. Tapirs 1. Tapiromorpha
=Hoofed Animals= { _Perissodactyla_ { 2. Horses 2. Solidungula
+Ungulata+ {
{ II. Double-hoofed { 3. Pigs 3. Choeromorpha
{ _Artiodactyla_ { 4. Ruminating 4. Ruminantia
{ III. Herbivorous {
VI. { Whales { 5. Sea cows 5. Sirenia
=Whales= { _Phycoceta_ {
+Cetacea+ {
{ IV. Carnivorous { 6. Whales 6. Autoceta
{ Whales { 7. Zeuglodonta 7. Zeugloceta
{ _Sarcoceta_ {
VII. { V. Digging Animals { 8. Ant-eaters 8. Vermilinguia
=Animals= { _Effodientia_ { 9. Armadilloes 9. Cingulata
=poor in teeth= {
+Edentata+ { VI. Sloths { 10. Giant Sloths 10. Gravigrada
{ _Bradypoda_ { 11. Dwarf Sloths 11. Tardigrada
-----------------------------------------------------------------------------------
III. 2. DECIDUATA. _Placental Animals with Decidua._
-----------------------------------------------------------------------------------
VIII. { VII. Rapacious {12. Rapacious land 12. Carnivora
=Placental Animals= { Animals { animals
+Zonoplacentalia+ { _Carnaria_ {13. Rapacious sea 13. Pinnipedia
{ { animals
{
{ VIII. False-hoofed {14. Hyrax 14. Lamnungia
{ Animals {15. Toxodonts 15. Toxodontia
{ _Chelophora_ {16. Dinotheria 16. Gonyognatha
{ {17. Elephants 17. Proboscidea
XI. { IX. Semi-apes {18. Fingered animals 18. Leptodactyla
=Disc Placental= { _Prosimiæ_ {19. Flying lemur 19. Ptenopleura
=Animals= { {20. Long-footed 20. Macrotarsi
+Discoplacentalia+ { {21. Short-footed 21. Brachytarsi
{
{ X. Gnawing Animals {22. Squirrel species 22. Sciuromorpha
{ _Rodentia_ {23. Mouse species 23. Myomorpha
{ {24. Porcupine species 24. Hystrichomorpha
{ {25. Hare species 25. Lagomorpha
{
{ XI. Insect-eating {26. With a cœcum 26. Menotyphla
{ Animals {27. Without a 27. Lipotyphla
{ _Insectivora_ { cœcum
{
{ XII. Flying Animals{28. Flying foxes 28. Pterocynes
{ _Chiroptera_ {29. Bats 29. Nycterides
{
{ XIII. Apes { 30. Clawed apes 30. Arctopitheci
{ _Simiæ_ { 31. Flat-nosed 31. Platyrrhinæ
{ { 32. Narrow-nosed 32. Catarrhinæ
----------------------------------------------------------------
=Man=
+Homines+
Elephants |
_Proboscidea_ | Bats
| | _Nycterides_
Rock Conies | | | Marine animals of prey
_Lamnungia_ | Narrow-nosed | _Pinnipedia_
| | _Catarrhinæ_ | |
| | | Flying foxes |
\------v-----/ Flat-nosed | _Pterocynes_ |
Pseudo-hoofed _Platyrrhinæ_ | =Flying Animals= |
_Chelophora_ | | +Chiroptera+ |
| | | | |
| \----v---/ | Land animals of prey
| =Apes= | _Carnivora_
=Gnawing Animals= +Simiæ+ | =Animals of Prey=
+Rodentia+ | | +Carnaria+
| Fingered animals | Lemurs | |
| _Leptodactyla_ | _Brachytarsi_ | |
True | | | | \-------v---------/
whales | | | | Insect eaters
_Sarcoceta_ \-----v----/ \---v----/ _Insectivora_
| | | |
| \------------------v---------------------/
Sea cows Semi-apes
_Sirenia_ _Prosimiæ_
=Whales= =Deciduous Animals=
+Cetacea+ +Deciduata+
| Poor in teeth |
=Hoofed Animals= _Edentata_ |
+Ungulata+ | |
| | |
\-------v--------/ |
=Indeciduous= |
+Indeciduata+ |
| |
\-------------v-------------/
=Placental Animals=
+Placentalia+
Herbivorous marsupials | Carnivorous marsupials
_Marsupialia botanophaga_ | _Marsupialia zoophaga_
| | |
\--------------v-----------------/
=Marsupial=
+Marsupialia+
Beaked animals |
_Ornithostoma_ |
| |
\---------v--------/
Primary mammals
_Promammalia_
=Cloacal Animals=
+Monotrema+
Of _Herbivorous marsupials_ (Botanophaga), only two fossils are as yet
known from the Jura, namely, the Stereognathus ooliticus, from the
slates of Stonesfield (Lower Oolite), and the Plagiaulax Becklesii, from
the middle Purbeck strata (Upper Oolite). But in Australia there are
gigantic fossil remains of extinct herbivorous Marsupials from the
diluvial period (Diprotodon and Nototherium) which were far larger than
the largest of the still living Marsupials. The Diprotodon Australis,
whose skull alone is three feet long, exceeded even the river-horse, or
Hippopotamus, in size and upon the whole resembled it in the unwieldy
and clumsy form of body. This extinct group, which probably corresponded
with the gigantic placental hoofed animals of the present day—the
hippopotami and rhinoceroses—may be called Hoofed Marsupials (Barypoda).
Closely allied to them is the order of kangaroos, or Leaping Marsupials
(Macropoda), which all have seen in zoological gardens. In their
shortened fore legs, their very lengthened hind legs, and very strong
tail, which serves as a jumping pole, they correspond with the leaping
mice in the class of Rodents. Their jaw, however, resembles that of
horses, and their complex stomach that of Ruminants. A third order of
Herbivorous Marsupials corresponds in its jaws to Rodents, and in its
subterranean mode of life, especially, to digging mice. Hence they may
be termed Rodent Marsupials, or root-eating pouched animals
(Rhizophaga). They are now represented only by the Australian wombat
(Phascolomys). A fourth and last order of Herbivorous Marsupials is
formed by the climbing or Fruit-eating Marsupials (Carpophaga), whose
mode of life and structure resembles partly that of squirrels, partly
that of apes (Phalangista, Phascolarctus).
The second legion of Marsupials, the _Carnivorous Marsupials_
(Zoophaga), is likewise divided into four main groups or orders. The
most ancient of these is that of the primæval, or Insectivorous
Marsupials (Cantharophaga). It probably includes the primary forms of
the whole legion, and possibly also those of the whole sub-class. At
least, all the lower jaws from Stonesfield (with the exception of the
Stereognathus) belong to Insectivorous Marsupials, and the still living
Myrmecobius is their nearest relative. But some of those oolitic
Primæval Marsupials possessed a larger number of teeth than all the
other known mammals, for each half of the lower jaw of the
Thylacotherium contained sixteen teeth (three incisors, one canine
tooth, six pseudo, and six genuine molars). If the upper jaw, which is
unknown, had as many teeth, then the Thylacotherium had no less than
sixty-four teeth, just double the number possessed by man. The Primæval
Marsupials correspond, on the whole, with the Insectivora among
Placental animals, which order includes hedgehogs, moles, and
shrew-mice. A second order, which has probably developed out of a branch
of the last, consists of the Snouted, or Toothless Marsupials
(Edentula), which resemble the Toothless animals, or Edentata, among the
Placental animals by their tube-shaped snout, their degenerated jaws,
and their corresponding mode of life. On the other hand, the mode of
life and formation of the jaws of Rapacious marsupials (Creophaga)
correspond with those of the genuine Beasts of Prey, or Carnivora, among
Placental animals. This order includes the pouched marten (Dasyurus) and
the pouched wolf (Thylacinus) in Australia. Although the latter attains
to the size of a wolf, it is but a dwarf in comparison with the extinct
Australian pouched lions (Thylacoleo) which were at least as large as a
lion, and possessed huge canine teeth more than two inches in length.
Finally, the eighth and last order is formed by the marsupials with
hands, or the Ape-footed Pouched animals (Pedimana), which live both in
Australia and America. They are frequently kept in zoological gardens,
especially the different species of the genus Didelphys, and are known
by the name of pouched rats, bush rats, or opossums. The thumb on their
hinder feet is opposable to the four other toes, as in a hand, and by
this they are directly allied to the Semi-apes, or Prosimia, among
Placental animals. It is possible that these latter are really next akin
to the marsupials with hands, and that they have developed out of their
long since extinct ancestors.
It is very difficult to discover the genealogy of Marsupials, and this
more especially because we are but very imperfectly acquainted with the
whole sub-class; and the Marsupials of the present day are evidently
only the last remnants of a group that was at one time rich in forms. It
is possible that Marsupials with hands, those with snouts, as well as
rapacious Marsupials, developed as three diverging branches out of the
common primary group of Primæval Marsupials. In a similar manner, on the
other hand, the rodent, leaping, and hoofed Marsupials have perhaps
arisen as three diverging branches out of the common herbivorous primary
group, that is, out of the Climbing Marsupials. Climbing and Primæval
Marsupials might, however, be two diverging branches of the common
primary forms of all Marsupials, that is, of the _Primary Marsupials_
(Prodidelphia), which originated during the older secondary period out
of Cloacal animals.
The third and last sub-class of mammals comprises the _Placental
animals_, or _Placentals_ (Monodelphia, or Placentalia). It is by far
the most important, comprehensive, and most perfect of the three
sub-classes; for the class includes all the known mammalia, with the
exception of Marsupials and Beaked animals. Man also belongs to this
sub-class, and has developed out of its lower members.
Placental animals, as their name indicates, are distinguished from all
other mammals, more especially by the formation of a so called
_placenta_. This is a very peculiar and remarkable organ, which plays
an exceedingly important part in nourishing the young one developing in
the maternal body. The placenta (also called after-birth) is a soft,
spongy, red body, which differs very much in form and size, but which
consists for the most part of an intricate network of veins and blood
vessels. Its importance lies in the exchange of substance between the
nutritive blood of the maternal womb, or uterus, and the body of the
germ, or embryo. (See vol. i. p. 298.) This very important organ is
developed neither in marsupials nor in beaked animals. But placental
animals are also distinguished from these two sub-classes by many other
peculiarities, thus more especially by the absence of marsupial bones,
by the higher development of the internal sexual organs, and by the more
perfect development of the brain, especially of the so-called callous
body or beam (_corpus callosum_), which, as the intermediate commissure,
or transverse bridge, connects the two hemispheres of the large brain
with each other. Placental animals also do not possess the peculiar
hooked process of the lower jaw which characterizes Marsupials. The
following classification (p. 246) of the most important characteristics
of the three sub-classes will best explain how Marsupials, in these
anatomical respects, stand midway between Cloacal and Placental animals.
Placental animals are more variously differentiated and perfected, and
this, moreover, in a far higher degree, than Marsupials, and they have,
on this account, long since been arranged into a number of orders,
differing principally in the formation of the jaws and feet. But what is
even of more importance than these, is the different development of the
placenta, and the manner of its connection with the maternal uterus.
For in the three lower orders of Placental animals, in Hoofed animals,
Whales, and Toothless animals, the peculiar spongy membrane, which is
called the _deciduous membrane_, or _decidua_, and which connects the
maternal and the fœtal portions of the placenta, does _not_ become
developed. This takes place exclusively in the seven higher orders of
Placental animals, and we may, therefore, according to Huxley, class
them in the main group of _Deciduata_, or animals with _decidua_. They
are contrasted with the three first-mentioned legions of indeciduous
animals, or _Indeciduata_.
---------------------------+-------------------+-------------------+-------------------
| _Cloacal Animals_ | _Pouched Animals_ |_Placental Animals_
_Three Sub-Classes_ | MONOTREMA | MARSUPIALIA | PLACENTALIA
_of_ | _or_ | _or_ | _or_
_Mammals._ | ORNITHODELPHIA | DIDELPHIA | MONODELPHIA
---------------------------+-------------------+-------------------+-------------------
1. Cloaca formation | Constant | Embryonal | Embryonal
| | |
2. Nipples of the pectoral | Wanting | Existing | Existing
glands, or milk | | |
warts | | |
| | |
3. Fore collar bones, | United | Not united | Not united
or clavicles, grown | | |
together in the middle, | | |
with the breast bone, | | |
and forming a forked | | |
bone | | |
| | |
4. Marsupial bones | Existing | Existing | Wanting
| | |
5. _Corpus callosum_ of | Feebly | Feebly | Strongly developed
the brain | developed | developed |
| | |
6. Placenta | Wanting | Wanting | Existing
---------------------------+-------------------+-------------------+-------------------
But in the various orders of Placental animals the placenta differs not
only in important internal differences of structure, which are connected
with the absence or the presence of a decidua, but also in the external
form of the placenta itself. In the Indeciduata it consists, in most
cases, of numerous, single, scattered bunches or tufts of vessels, and
hence this group may be called _tufted placental animals_
(Villiplacentalia). In the Deciduata, however, the single tufts of
vessels are united into a cake, which appears in two different forms. In
the one case it surrounds the embryo in the form of a closed band or
ring, so that only the two poles of the oval egg bladder are free of
tufts; this is the case in animals of prey (Carnaria) and the
pseudo-hoofed animals (Chelophora), which may consequently be comprised
as _girdled-placental animals_ (Zonoplacentalia). In the other
Deciduata, to which man also belongs, the placenta is a simple round
disc, and we therefore call them _disc-placentals_ (Discoplacentalia).
This group includes the five orders of Semi-apes, Gnawing animals,
Insectivora, Bats, and Apes, from the latter of which, in the zoological
system, man cannot be separated.
It may be considered as quite certain, from reasons based upon their
comparative anatomy and their history of development, that Placental
animals first developed out of Marsupials, and that this very important
development—the first origin of the placenta—probably took place in the
beginning of the tertiary epoch, during the eocene period. But one of
the most difficult questions in the genealogy of animals is the
important consideration whether all Placental animals have arisen out of
one or out of several distinct branches of Marsupials; in other words,
whether the origin of the placenta occurred but once, or several times.
When, in my General Morphology, I for the first time endeavoured to
establish the pedigree of Mammals, I here, as in most cases, preferred
the monophyletic, or one-rooted, to the polyphyletic, or many-rooted,
hypothesis of descent. I assumed that all Placental animals were derived
from a single form of Marsupial animal, which, for the first time, began
to form a placenta. In this case the Villiplacentals, Zonoplacentals,
and Discoplacentals would perhaps have to be considered as three
diverging branches of the common primary form of Placentals, or it might
also be conceived that the two latter, the Deciduata, had developed only
at a later period out of the Indeciduata, which on their part had arisen
directly out of the Marsupials. However, there are also important
reasons for the alternative; namely, that several groups of Placentals,
differing from the beginning, arose out of several distinct groups of
Marsupials, so that the placenta itself was formed several times
independently. This opinion is maintained by Huxley, the most eminent
English zoologist, and by many others. In this case the Indeciduata and
the Deciduata would perhaps have to be considered as two completely
distinct groups; then the order of Hoofed animals, as the primary group
of the Indeciduata, might be supposed to have originated out of the
Marsupial hoofed animals (Barypoda). Among the Deciduata, on the other
hand, the order of Semi-apes, as the common primary form of the other
orders, might possibly have arisen out of Handed Marsupials (Pedimana).
But it is also conceivable that the Deciduata themselves have arisen out
of several different orders of Marsupials, Animals of Prey out of
Rapacious Marsupials, Gnawing animals out of Gnawing Marsupials,
Semi-apes out of Handed Marsupials, etc. As we do not at present possess
sufficient empiric material to solve this most difficult question, we
must leave it and turn our attention to the history of the different
orders of Placental animals, whose pedigree can often be very accurately
established in detail.
We must, as already remarked, consider the order of _Hoofed animals_
(Ungulata) as the primary group of the Indeciduata, or Tuft-placentals;
the two other orders, Whales and Toothless animals, developed out of
them, as two diverging groups, probably only at a later period, by
adaptation to very different modes of life. But it is also possible that
the animals poor in teeth (Edentata) may be of quite a different origin.
Hoofed animals are in many respects among the most important and the
most interesting Mammals. They distinctly show that a true understanding
of the natural relationship of animals can never be revealed to us
merely by the study of living forms, but in all cases only by an equal
consideration of their extinct and fossil blood-relations and ancestors.
If, as is usually done, only the living Hoofed animals are taken into
consideration, it seems quite natural to divide them into three entirely
distinct orders, namely: (1) Horses, or _Single-hoofed animals_
(Solidungula, or Equina); (2) Ruminating animals, or _Double-hoofed_
(Bisulca, or Ruminantia); and (3) Thick-skinned, or _Many-hoofed_
(Multungula, or Pachyderma). But as soon as the extinct Hoofed animals
of the tertiary period are taken into consideration—of which animals we
possess very numerous and important remains—it is seen that this
division, but more especially the limitation of the Thick-skinned
animals, is completely artificial, and that these three groups are
merely top branches lopped from the pedigree of Hoofed animals, which
are most closely connected by extinct intermediate forms. The one half
of the Thick-skinned animals—rhinoceroses, tapirs, and
palæotheria—manifest the closest relationships to horses, and have like
them odd-toed feet; whereas the other half of the Thick-skinned
animals—pigs, hippopotami, and anoplotheria—on account of their
double-toed feet are much more closely allied to ruminating animals than
to the former. Hence we must, in the first place, among Hoofed animals
distinguish the two orders of Paired-hoofs and Odd-hoofs, as two natural
groups, which developed as diverging branches out of the old tertiary
primary group of Primary Hoofed animals, or Prochela.
The order of _Odd-hoofed animals_ (Perissodactyla) comprises those
Ungulata in which the middle (or third) toe of the foot is much more
strongly developed than the others, so that it forms the actual centre
of the hoof. This order includes the very ancient, common, primary group
of all Hoofed animals, that is, the _Primary-hoofed animals_ (Prochela),
which are found in a fossil state in the oldest Eocene strata
(Lophiodon, Coryphodon, Pliolophus). Directly allied to this group is
that branch which is the actual primary form of the Odd-hoofed animals,
namely, the _Palæotheria_, fossils of which occur in the upper Eocene
and lower Miocene. Out of the Palæotheria, at a later period, the
rhinoceroses (Nasicornia) and rhinoceros-horses (Elasmotherida) on the
one hand, and the tapirs, lama-tapirs, and primæval horses, on the
other, developed as two diverging branches. The long since extinct
primæval horses, or Anchitheria, formed the transition from the
Palæotheria and tapirs to the Miocene horses, or hipparions, which are
closely allied to the genuine living horses.
The second main group of Hoofed animals, the order of _Pair-hoofed
animals_ (Artiodactyla), comprises those hoofed animals in which the
middle (third) and fourth toe of the foot are almost equally developed,
so that the space between the two forms the central line of the entire
foot. The order is divided into two sub-orders—the Pig-shaped and the
Cud-chewing, or Ruminating. The _Pig-shaped_ (Chœromorpha) comprise in
the first place the other branch of Primary-Hoofed-animals, the
_Anoplotheria_, which we consider as the common primary form of all
Pair-hoofed animals, or Artiodactyla (Dichobune, etc.). Out of the
Anoplotheria arose, as two diverging branches, the primæval swine, or
Anthracotheria, on the one hand, forming the transition to swine and
river-horses, and the Xiphodonta on the other hand, forming the
transition to Ruminating animals. The oldest _Ruminating animals_
(Ruminantia) are the Primæval Stags, or Dremotheria, out of which,
possibly, the stag-shaped (Elaphia), the hollow-horned (Cavicornia), and
camels (Tylopoda), have developed as three diverging branches. Yet these
latter are, in many respects, more allied to the Odd-hoofs than to the
genuine Pair-hoofs. The accompanying systematic survey on p. 252, will
show how the numerous families of Hoofed animals are grouped, in
correspondence with this genealogical hypothesis.
SYSTEMATIC SURVEY
_Of the Sections and Families of Hoofed Animals, or Ungulata._
(N.B. Those families that are extinct are marked with an asterisk.)
----------------+--------------------------------+------------------------+-------------------
_Orders_ | | |
_of_ | _Sections_ | _Families_ | _Systematic Name_
_Hoofed_ | _of_ | _of_ | _of_
_animals._ | _Hoofed Animals._ | _Hoofed Animals._ | _the Families._
----------------+----------------------------=---+------------------------+-------------------
I. { I. Primary Hoofed { 1. Lophiodonta 1. Lophiodontia*
=Odd-toed= { Animals.* { 2. Pliolophida 2. Pliolophida*
=Hoofed= { _Prochela_
=Animals= { { 3. Primary 3. Palæotherida*
{ { Odd-hoofs
+Ungulata+ { II. Tapir-shaped { 4. Lama-tapirs 4. Macrauchenida*
{ _Tapiromorpha_ { 5. Tapirs 5. Tapirida
+Perissodactyla+ { { 6. Rhinoceroses 6. Nasicornia
{ { 7. Rhinoceros-horses 7. Elasmotherida*
{
{ III. Single-hoofs { 8. Primæval 8. Anchitherida*
{ _Solidungula_ { horses
{ { 9. Horses 9. Equina
{ { 10. Primary 10. Anoplotherida*
{ { Pair-hoofs
{ { 11. Primæval 11. Anthracotherida*
{ IV. Pig-shaped { pigs
{ _Chœromorpha_ { 12. Pigs 12. Setigera
{ { 13. River horses 13. Obesa
{ { 14. Primæval 14. Xiphodontia*
{ { ruminants
{
{ { { {15. Primæval 15. Dremotherida*
{ { { { deer
II. { { A. Stag-shaped { a. {16. Pseudo 16. Tragulida
=Pair-toed= { { _Elaphia_ { { musk deer
=Hoofed= { { {
=Animals= { { { b. {17. Musk deer 17. Moschida
{ { { {18. Deer 18. Cervina
+Ungulata+ { { {
{ V. { { c. {19. Primæval 19. Sivatherida*
+Artiodactyla+ { Ruminating { { { giraffes
{ animals { { {20. Giraffes 20. Devexa
{ _Ruminantia_ {
{ { { {21. Primæval 21. Antilocaprina*
{ { { d. { gazelles
{ { { {22. Gazelles 22. Antilopina
{ { {
{ { B. Hollow-horned { {23. Goats 23. Caprina
{ { _Caricornia_ { e. {24. Sheep 24. Ovina
{ { { {25. Oxen 25. Bovina
{ {
{ { C. Pad-footed { 26. Lamas 26. Auchenida
{ { _Tylopoda_ { 27. Camels 27. Camelida
----------------------------------------------------------------
Oxen Giraffes
| |
| Sheep Deer |
| | | |
| | | |
\--v---/ Goats \---v---/ Musk deer Horses
| | | | _Equi_
| | | | |
\---v-----/ Antelopes | | Camels |
| | \----v-----/ and Lamas Intermediate horses
| | Deer-shaped _Tylopoda_ _Hippariones_
\-----v------/ _Elaphia_ | |
Hollow-horned | | |
_Cavicornia_ | | |
| | | |
| | | Primæval horses
\-------v--------/ | _Anchitherida_
Primæval deer | |
_Dremotherida_ | |
| | =Single Hoofers=
| | +Solidungula+
\--v------------------/ |
=Ruminating Animals= |
+Ruminantia+ |
Sea-oxen | Tapirs |
_Sirenia_ | _Tapirida_ |
| River-horses | | Lama-tapirs |
| _Obesa_ | | _Macrauchenida_|
| | | | | |
| | Pigs | | | |
\----v-----/ _Setigera_ | \--------------v--/ |
| | | | |
| | | Rhinoceros-horses | |
| | | _Elasmotherida_ | |
| | | | | |
\------v-------/ | | | |
Primæval pigs | Rhinoceruses | | |
_Anthracotherida_ | _Nasicornia_ | \---v-----/
| | | | |
| Primæval ruminants | | |
| _Xiphodontia_ \---v---/ |
| | | |
| | | |
\------v------/ \--------v--------/
=Primary Pair-hoofs= =Primary Odd-hoofs=
+Anoplotherida+ +Palæotherida+
| |
| |
\------------v-------------/
_Prochela_
Primary-hoofed-animals
(_Lophiodontia_ and _Pliophida_)
|
|
(Hoofed marsupials? _Barypoda?_)
It is probable that the remarkable legion of _Whales_ (Cetacea)
originated out of Hoofed animals, which accustomed themselves
exclusively to an aquatic life, and thereby became transformed into the
shape of fish. Although these animals seem externally very like many
genuine Fish, yet they are, as even Aristotle perceived, genuine
Mammals. By their whole internal structure—in so far as it has not
become changed by adaptation to an aquatic life—they, of all known
Mammals, are most closely allied to Hoofed animals, and more especially
agree with them in the absence of the decidua and in the tufted
placenta. Even at the present day the river-horse (Hippopotamus)
constitutes a kind of transition form to the Sea Cows (Sirenia), and
from this it seems most probable that the extinct primary forms of the
Cetacea are most closely allied to the Sea Cows of the present day, and
that they developed out of Pair-hoofed animals, which were related to
the hippopotamus. Out of the order of _Herbivorous whales_
(Phycoceta)—to which the sea cows belong, and which accordingly, very
probably, contain the primary forms of the legion—the other order of
_Carnivorous whales_ (Sarcoceta) appears to have developed at a later
period. But Huxley thinks that these latter were of quite a different
origin, and that they arose out of the Carnaria through the Seals. Among
the Sarcoceta, the extinct gigantic Zeuglodonta (Zeugloceta)—whose
fossil skeletons some time ago excited great interest, it being thought
that they were “sea serpents”—are probably only a peculiarly developed
lateral branch of genuine whales (Autoceta), which comprise, besides the
colossal whalebone whales, the cachalot or spermaceti whales, dolphins,
narwhals, porpoises, etc.
The third legion of the Indeciduata, or Sparsi-placentalia, comprises
the strange group of the animals _poor in teeth_ (Edentata); it is
composed of the two orders of burrowers and sloths. The order of
_Burrowers_ (Effodientia) consists of the two sub-orders of _ant eaters_
(Vermilinguia), to which the scaled animals also belong, and the _girdle
animals_ (Cingulata), which were formerly represented by the gigantic
Glyptodons. The order of _Sloths_ (Tardigrada) consists of the two
sub-orders of the small, still living _dwarf sloths_ (Bradypoda), and
of the extinct unwieldy _giant sloths_ (Gravigrada). The enormous fossil
remains of these colossal herbivora suggest that the whole legion is
becoming extinct, and that the Edentata of the present day are but a
poor remnant of the mighty order of the diluvial period. The close
relations between the still living South American Edentata and the
extinct gigantic forms which are found beside the latter on the same
part of the globe, made such an impression upon Darwin on his first
visit to South America, that they even then suggested to him the
fundamental idea of the Theory of Descent. (See above, vol. i. p. 134.)
But it is precisely the genealogy of this legion which is most
difficult. The Edentata are perhaps nothing but a peculiarly developed
lateral branch of the Ungulata; but it may also be that their root lies
in quite another direction.
We now leave the first main group of Placental animals, the Indeciduata,
and turn to the second main group, namely, the Deciduata, or animals
with decidua, which are distinguished from the former by possessing a
deciduous membrane, or decidua, during their embryonal life. We here
meet with a very remarkable small group of animals, for the most part
extinct, and which probably were the old tertiary (or eocene) ancestors
of man. These are the Semi-apes, or Lemurs (Prosimiæ); these curious
animals are probably the but little changed descendants of the primæval
group of Placentalia which we have to consider as the common primary
form of all Deciduata. They have hitherto been classed together in the
same order with Apes which Blumenbach called Quadrumana (four-handed).
However, I regard them as entirely distinct from these, not merely
because they differ from all Apes, much more than do the most different
Apes from one another, but also because they comprise most interesting
transitional forms leading to the other orders of Deciduata. I conclude
from this that the few still living Semi-apes, which moreover differ
very much among one another, are the last surviving remnants of a
primary group now almost extinct, but which was at one time rich in
forms, and out of which all the other Deciduata (possibly with the
single exception of Beasts of Prey, and Pseudo-hoofed animals) have
developed as diverging branches. The old primary group of Semi-apes has
probably developed out of Handed or Ape-footed Marsupials (Pedimana),
which are surprisingly like them in the transformation of their hinder
feet into grasping hands. The primæval primary forms themselves (which
probably originated in the eocene period) are of course long since
extinct, as are also the greater portion of the transition-forms between
them and all the other orders of Deciduata. However, individual remnants
of the latter are preserved among the Semi-apes of the present day.
Among these, the remarkable Finger-animal of Madagascar (Chiromys
madagascariensis) constitutes the remnant of the group of the
Leptodactyla and the transition to Rodents. The strange flying lemur in
the South Sea and Sunda islands (Galeopithecus), the only remnant of the
group of Pteropleura, forms a perfect intermediate stage between
Semi-apes and Bats. The long-footed Semi-apes (Tarsius, Otolicnus)
constitute the last remnant of that primary branch (Macrotarsi) out of
which the Insectivora developed. The short-footed forms (Brachytarsi)
are the medium of connection between them and genuine Apes. The
Short-footed Semi-apes comprise the long-tailed Lemur, the short-tailed
Lichanotus, and the Stenops, the latter of which seems to be very
closely allied to the probable ancestors of man among the Semi-apes. The
short-footed as well as the long-footed Prosimiæ live widely distributed
over the islands of southern Asia and Africa, more especially in
Madagascar; some live also on the continent of Africa. No Semi-ape,
either living or in a fossil state, has as yet been found in America.
They all lead a solitary, nocturnal kind of life, and climb about on
trees. (Compare vol. i. p. 361.)
Among the six remaining orders of Deciduata, all of which are probably
derived from long since extinct Semi-apes, the order of _Gnawing
animals_ (Rodentia), which is rich in forms, has remained at the lowest
stage. Among these the _squirrel-like_ animals (Sciuromorpha) stand
nearest akin to the Pedimanous Marsupials. Out of this primary group the
_mouse-like_ animals (Myomorpha) and the _porcupine-like_ animals
(Hystricomorpha) developed probably as two diverging branches, the
former of which are directly connected with the squirrel-like animals,
by the eocene Myoxida, the latter by the eocene Psammoryctida. The
fourth sub-order, the _hare-like_ animals (Lagomorpha), probably
developed only at a later period out of one of the other three
sub-orders.
Very closely allied to the Rodentia is the remarkable order of
_Pseudo-hoofed animals_ (Chelophora). Of these there now live but two
genera, indigenous to Asia and Africa, namely, Elephants (Elephas), and
Rock Conies (Hyrax). Both have hitherto generally been classed among
real Hoofed animals, or Ungulata, with which they agree in the formation
of the feet. But an identical transformation of nails or claws into
hoofs occurs also in genuine Rodentia and in certain hoofed Rodentia
(Subungulata) which live exclusively in South America. Beside smaller
forms (for example, guinea pigs and gold hares) the Subungulata also
include the largest of all Rodentia, namely, the Capybara Rats, which
are about four feet in length. The Rock Conies, which are externally
very nearly akin to Rodents, especially to the hoofed Rodents, were
formerly classed among Rodentia by some celebrated zoologists, as an
especial sub-class (Lamnungia). Elephants, on the other hand, when not
classed among Hoofed animals, were generally considered as the
representatives of a special order which were called Trunked animals
(Proboscidea). But the formation of the placentas of Elephants and of
Hyrax agree in a remarkable manner, and are entirely distinct from those
of Hoofed animals. These latter never possess a decidua, whereas
Elephants and Hyrax are genuine Deciduata. Their placenta is indeed not
of the form of a disc, but of a girdle, as in the case of Animals of
Prey; it is very possible that the girdle-shaped placenta is but a
secondary development of the discoplacenta. Thus, then, it might be
thought that the Pseudo-hoofed animals have developed out of a branch of
the Rodentia, and in a similar manner perhaps the Carnivora out of a
branch of the Insectivora. At all events, Elephants and Hyrax in many
respects, especially in the formation of important skeletal parts, of
the limbs, etc., are more closely allied to the Rodentia, and more
especially to hoofed Rodentia, than to genuine Hoofed animals. Moreover
several extinct forms, especially the remarkable South American
Arrow-toothed animals (Toxodontia), stand in many respects midway
between Elephants and Rodentia. That the still living Elephants and
Hyrax are but the last survivors of a group of Pseudo-hoofed animals,
which was once rich in forms, is proved not only by the very numerous
fossil species of Elephants and Mastodon (some of which are even larger,
others also much smaller than the Elephants of the present day), but
also by the remarkable miocene _Dinotheria_ (Gonyognatha), between which
and their next kindred, the Elephants, there must be a long series of
unknown connecting intermediate forms. Taking all things into
consideration, the most probable hypothesis which can be established at
present as to the origin and the relationship of Elephants, Dinotheria,
Toxodon, and Hyrax is, that they are the last survivors of a group of
Pseudo-hoofed animals rich in forms, which developed out of the
Rodentia, and probably out of relatives of the Subungulata.
The order of _Insect Eaters_ (Insectivora) is a very ancient group, and
is next akin to the common extinct primary form of the Deciduata, as
well as to the Semi-apes of the present day. It has probably developed
out of Semi-apes which were closely allied to the Long-footed Lemurs
(Macrotarsi) of the present day. It is separated into two orders,
Menotyphla and Lipotyphla; the Menotyphla are probably the older of the
two, and are distinguished from the Lipotyphla by possessing an
intestinal cœcum, or typhlon. The Menotyphla include the climbing
Tupajas of the Sunda Isles, and the leaping Macroscelides of Africa. The
Lipotyphla are represented in our country by shrew mice, moles, and
hedgehogs. The Insectivora, in the formation of their jaws and their
mode of life, are nearly akin to Carnivora, but are, on the other hand,
by their discoplacentas and by their large seminal vesicles, allied to
Rodents.
It is probable that the order of _Rapacious animals_ (Carnaria)
developed out of a long since extinct branch of Insectivora, at the
beginning of the Eocene period. It is a natural group, very rich in
forms, but still of very uniform organization. The Rapacious animals are
sometimes also called Girdle-placentals (Zonoplacentals), although the
Pseudo-hoofed animals (Chelophora), in the same way, also deserve this
designation. But as the latter, in other respects, are more closely
allied to the Rodentia than to Carnaria, we have already discussed them
in connection with the former. Animals of prey are divided into two,
externally very different, but internally very closely related,
sub-orders, namely, Land animals of prey and Marine animals of prey. The
_Land animals of prey_ (Carnivora) comprise bears, dogs, cats, etc.,
whose pedigree can be approximately guessed at by means of many extinct
intermediate forms. The _Marine animals of prey_, or _Seals_
(Pinnipedia), comprise sea bears, sea dogs, sea lions, and walruses.
Although marine animals of prey appear externally very unlike land
animals of prey, yet by their internal structure, their jaw and their
peculiar girdle-shaped placenta, they are very nearly akin to them, and
have evidently originated out of a branch of them, probably out of a
kind of weasel (Mustelina). Even at the present day the fish otters
(Lutra), and still more so the sea otters (Enhydris), present a direct
form of transition to Seals, and clearly show how the bodies of land
Carnivora are transformed into the shape of a Seal, by adaptation to an
aquatic life, and how the steering fins of marine rapacious animals have
arisen out of the legs of the former. The latter consequently stand in
the same relation to the former as do the Whales to Hoofed animals
among the Indeciduata. In the same way as the river-horse at present
stands midway between the extreme branches of oxen and sea oxen, the sea
otter still forms a surviving intermediate stage between the widely
separated branches of dogs and sea dogs. In both cases the complete
transformation of the external form, consequent upon adaptation to
entirely different conditions of life, has not been able to efface the
solid foundation of the inherited internal peculiarities.
According to Huxley’s opinion, which has already been quoted, only the
Herbivorous Whales (Sirenia) are derived from Hoofed animals; on the
other hand, the Carnivorous Cetacea (Sarcoceta) are derived from the
marine animals of prey; the Zeuglodonts would form a transition between
the two latter. But in this case it would be difficult to understand the
close anatomical relations which exist between the Herbivorous and
Carnivorous Cetacea. The strange peculiarities in the internal and
external structure which so strikingly distinguish the two groups from
all other mammals would then have to be regarded only as _analogies_
(caused by the same kinds of adaptation), not as _homologies_
(transmitted from a common primary form). The latter, however, strikes
me as being by far the more probable, and hence I have left all the
Cetacea among the Indeciduata as one group of kindred origin.
The remarkable order of _Flying Mammals_, or _Bats_ (Chiroptera), stands
near to the Carnaria as well as to the Insectivora. It has become
strikingly transformed by adaptation to a flying mode of life, just as
marine animals of prey have become modified by adaptation to a swimming
mode of life. This order probably also originated out of the Semi-apes,
with which it is even at present closely allied, through the flying
lemurs (Galeopithecus). Of the two orders of flying animals, the
insect-eating forms, or _flying mice_ (Nycterides), probably developed
out of those eating fruits, or _flying foxes_ (Pterocynes); for the
latter are, in many ways, more closely allied to Semi-apes than are the
former.
We have now still to discuss the genuine Apes (Simiæ) as the last order
of Mammals; but as, according to the zoological system, the human race
belongs to this order, and as it undoubtedly developed historically out
of a branch of this order, we shall devote a special chapter to a more
careful examination of its pedigree and history.