MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups—Natural system—Rules and
difficulties in classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used in
classification—Analogical or adaptive characters—Affinities, general,
complex and radiating—Extinction separates and defines
groups—Morphology, between members of the same class, between parts of
the same individual—Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding
age—Rudimentary organs; their origin explained—Summary.
_Classification._
From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that
they can be classed in groups under groups. This classification is not
arbitrary like the grouping of the stars in constellations. The
existence of groups would have been of simple significance, if one
group had been exclusively fitted to inhabit the land, and another the
water; one to feed on flesh, another on vegetable matter, and so on;
but the case is widely different, for it is notorious how commonly
members of even the same subgroup have different habits. In the second
and fourth chapters, on Variation and on Natural Selection, I have
attempted to show that within each country it is the widely ranging,
the much diffused and common, that is the dominant species, belonging
to the larger genera in each class, which vary most. The varieties, or
incipient species, thus produced, ultimately become converted into new
and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups
which are now large, and which generally include many dominant species,
tend to go on increasing in size. I further attempted to show that from
the varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms, which, in any small area, come
into the closest competition, and by certain facts in naturalisation.
I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the preceding, less divergent and less improved forms.
I request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that
the inevitable result is, that the modified descendants proceeding from
one progenitor become broken up into groups subordinate to groups. In
the diagram each letter on the uppermost line may represent a genus
including several species; and the whole of the genera along this upper
line form together one class, for all are descended from one ancient
parent, and, consequently, have inherited something in common. But the
three genera on the left hand have, on this same principle, much in
common, and form a subfamily, distinct from that containing the next
two genera on the right hand, which diverged from a common parent at
the fifth stage of descent. These five genera have also much in common,
though less than when grouped in subfamilies; and they form a family
distinct from that containing the three genera still further to the
right hand, which diverged at an earlier period. And all these genera,
descended from (A), form an order distinct from the genera descended
from (I). So that we here have many species descended from a single
progenitor grouped into genera; and the genera into subfamilies,
families and orders, all under one great class. The grand fact of the
natural subordination of organic beings in groups under groups, which,
from its familiarity, does not always sufficiently strike us, is in my
judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus
arranged. In this case there is of course no relation to genealogical
succession, and no cause can at present be assigned for their falling
into groups. But with organic beings the case is different, and the
view above given accords with their natural arrangement in group under
group; and no other explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what
is meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions—that is, by
one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those
common to the dog-genus, and then, by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that something
more is meant by the Natural System; they believe that it reveals the
plan of the Creator; but unless it be specified whether order in time
or space, or both, or what else is meant by the plan of the Creator, it
seems to me that nothing is thus added to our knowledge. Expressions
such as that famous one by Linnæus, which we often meet with in a more
or less concealed form, namely, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that some
deeper bond is included in our classifications than mere resemblance. I
believe that this is the case, and that community of descent—the one
known cause of close similarity in organic beings—is the bond, which,
though observed by various degrees of modification, is partially
revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely “adaptive or analogical characters;” but to
the consideration of these resemblances we shall recur. It may even be
given as a general rule, that the less any part of the organisation is
concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says,
“The generative organs, being those which are most remotely related to
the habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an
essential character.” With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again, in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service in
classification. This depends on their constancy throughout many allied
groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which
acts only on serviceable characters.
That the mere physiological importance of an organ does not determine
its classificatory value, is almost proved by the fact, that in allied
groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is
widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who, in speaking of certain organs in the
Proteaceæ, says their generic importance, “like that of all their
parts, not only in this, but, as I apprehend in every natural family,
is very unequal, and in some cases seems to be entirely lost.” Again,
in another work he says, the genera of the Connaraceæ “differ in having
one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular æstivation. Any one of these characters singly
is frequently of more than generic importance, though here even, when
all taken together, they appear insufficient to separate Cnestis from
Connarus.” To give an example among insects: in one great division of
the Hymenoptera, the antennæ, as Westwood has remarked, are most
constant in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no
one will say that the antennæ in these two divisions of the same order
are of unequal physiological importance. Any number of instances could
be given of the varying importance for classification of the same
important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
position of the rudimentary florets is of the highest importance in the
classification of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition
of whole groups. For instance, whether or not there is an open passage
from the nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles—the inflection of
the angle of the lower jaw in Marsupials—the manner in which the wings
of insects are folded—mere colour in certain Algæ—mere pubescence on
parts of the flower in grasses—the nature of the dermal covering, as
hair or feathers, in the Vertebrata. If the Ornithorhynchus had been
covered with feathers instead of hair, this external and trifling
character would have been considered by naturalists as an important aid
in determining the degree of affinity of this strange creature to
birds.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of more or
less importance. The value indeed of an aggregate of characters is very
evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance, and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has
been found that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is invariably constant. The importance of an aggregate of
characters, even when none are important, alone explains the aphorism
enunciated by Linnæus, namely, that the characters do not give the
genus, but the genus gives the character; for this seems founded on the
appreciation of many trifling points of resemblance, too slight to be
defined. Certain plants, belonging to the Malpighiaceæ, bear perfect
and degraded flowers; in the latter, as A. de Jussieu has remarked,
“The greater number of the characters proper to the species, to the
genus, to the family, to the class, disappear, and thus laugh at our
classification.” When Aspicarpa produced in France, during several
years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type
of the order, yet M. Richard sagaciously saw, as Jussieu observes, that
this genus should still be retained among the Malpighiaceæ. This case
well illustrates the spirit of our classifications.
Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value. This
principle has been broadly confessed by some naturalists to be the true
one; and by none more clearly than by that excellent botanist, Aug. St.
Hilaire. If several trifling characters are always found in
combination, though no apparent bond of connexion can be discovered
between them, especial value is set on them. As in most groups of
animals, important organs, such as those for propelling the blood, or
for aerating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classification;
but in some groups all these, the most important vital organs, are
found to offer characters of quite subordinate value. Thus, as Fritz
Müller has lately remarked, in the same group of crustaceans, Cypridina
is furnished with a heart, while in two closely allied genera, namely
Cypris and Cytherea, there is no such organ; one species of Cypridina
has well-developed branchiæ, while another species is destitute of
them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryological characters are the most important of all; and this
doctrine has very generally been admitted as true. Nevertheless, their
importance has sometimes been exaggerated, owing to the adaptive
characters of larvæ not having been excluded; in order to show this,
Fritz Müller arranged, by the aid of such characters alone, the great
class of crustaceans, and the arrangement did not prove a natural one.
But there can be no doubt that embryonic, excluding larval characters,
are of the highest value for classification, not only with animals but
with plants. Thus the main divisions of flowering plants are founded on
differences in the embryo—on the number and position of the cotyledons,
and on the mode of development of the plumule and radicle. We shall
immediately see why these characters possess so high a value in
classification, namely, from the natural system being genealogical in
its arrangement.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but with crustaceans, any such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed
by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted
on their arbitrary value. Instances could be given among plants and
insects, of a group first ranked by practised naturalists as only a
genus, and then raised to the rank of a subfamily or family; and this
has been done, not because further research has detected important
structural differences, at first overlooked, but because numerous
allied species, with slightly different grades of difference, have been
subsequently discovered.
All the foregoing rules and aids and difficulties in classification may
be explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification—that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, all true classification being genealogical—that
community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
_arrangement_ of the groups within each class, in due subordination and
relation to each other, must be strictly genealogical in order to be
natural; but that the _amount_ of difference in the several branches or
groups, though allied in the same degree in blood to their common
progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by the
forms being ranked under different genera, families, sections or
orders. The reader will best understand what is meant, if he will take
the trouble to refer to the diagram in the fourth chapter. We will
suppose the letters A to L to represent allied genera existing during
the Silurian epoch, and descended from some still earlier form. In
three of these genera (A, F, and I) a species has transmitted modified
descendants to the present day, represented by the fifteen genera
(_a_14 to _z_14) on the uppermost horizontal line. Now, all these
modified descendants from a single species are related in blood or
descent in the same degree. They may metaphorically be called cousins
to the same millionth degree, yet they differ widely and in different
degrees from each other. The forms descended from A, now broken up into
two or three families, constitute a distinct order from those descended
from I, also broken up into two families. Nor can the existing species
descended from A be ranked in the same genus with the parent A, or
those from I with parent I. But the existing genus F14 may be supposed
to have been but slightly modified, and it will then rank with the
parent genus F; just as some few still living organisms belong to
Silurian genera. So that the comparative value of the differences
between these organic beings, which are all related to each other in
the same degree in blood, has come to be widely different.
Nevertheless, their genealogical _arrangement_ remains strictly true,
not only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants at each
successive stage. If, however, we suppose any descendant of A or of I
to have become so much modified as to have lost all traces of its
parentage in this case, its place in the natural system will be lost,
as seems to have occurred with some few existing organisms. All the
descendants of the genus F, along its whole line of descent, are
supposed to have been but little modified, and they form a single
genus. But this genus, though much isolated, will still occupy its
proper intermediate position. The representation of the groups as here
given in the diagram on a flat surface, is much too simple. The
branches ought to have diverged in all directions. If the names of the
groups had been simply written down in a linear series the
representation would have been still less natural; and it is
notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature among the beings of the same
group. Thus, the natural system is genealogical in its arrangement,
like a pedigree. But the amount of modification which the different
groups have undergone has to be expressed by ranking them under
different so-called genera, subfamilies, families, sections, orders,
and classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, were to be included, such an arrangement
would be the only possible one. Yet it might be that some ancient
languages had altered very little and had given rise to few new
languages, whilst others had altered much owing to the spreading,
isolation and state of civilisation of the several co-descended races,
and had thus given rise to many new dialects and languages. The various
degrees of difference between the languages of the same stock would
have to be expressed by groups subordinate to groups; but the proper or
even the only possible arrangement would still be genealogical; and
this would be strictly natural, as it would connect together all
languages, extinct and recent, by the closest affinities, and would
give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties
under the varieties; and in some cases, as with the domestic pigeon,
with several other grades of difference. Nearly the same rules are
followed as in classifying species. Authors have insisted on the
necessity of arranging varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the Swedish
and common turnip together, though the esculent and thickened stems are
so similar. Whatever part is found to be most constant, is used in
classing varieties: thus the great agriculturist Marshall says the
horns are very useful for this purpose with cattle, because they are
less variable than the shape or colour of the body, &c.; whereas with
sheep the horns are much less serviceable, because less constant. In
classing varieties, I apprehend that if we had a real pedigree, a
genealogical classification would be universally preferred; and it has
been attempted in some cases. For we might feel sure, whether there had
been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of
points. In tumbler pigeons, though some of the subvarieties differ in
the important character of the length of the beak, yet all are kept
together from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any
thought on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
that of species, the two sexes; and how enormously these sometimes
differ in the most important characters is known to every naturalist:
scarcely a single fact can be predicated in common of the adult males
and hermaphrodites of certain cirripedes, and yet no one dreams of
separating them. As soon as the three Orchidean forms, Monachanthus,
Myanthus, and Catasetum, which had previously been ranked as three
distinct genera, were known to be sometimes produced on the same plant,
they were immediately considered as varieties; and now I have been able
to show that they are the male, female, and hermaphrodite forms of the
same species. The naturalist includes as one species the various larval
stages of the same individual, however much they may differ from each
other and from the adult; as well as the so-called alternate
generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties,
not from their partial resemblance to the parent-form, but because they
are descended from it.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvæ
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, all under the so-called natural system? I believe it has
been unconsciously used; and thus only can I understand the several
rules and guides which have been followed by our best systematists. As
we have no written pedigrees, we are forced to trace community of
descent by resemblances of any kind. Therefore, we choose those
characters which are the least likely to have been modified, in
relation to the conditions of life to which each species has been
recently exposed. Rudimentary structures on this view are as good as,
or even sometimes better than other parts of the organisation. We care
not how trifling a character may be—let it be the mere inflection of
the angle of the jaw, the manner in which an insect’s wing is folded,
whether the skin be covered by hair or feathers—if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms with such different habits, only by
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them be
ever so trifling, concur throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent,
that these characters have been inherited from a common ancestor; and
we know that such aggregated characters have especial value in
classification.
We can understand why a species or a group of species may depart from
its allies, in several of its most important characteristics, and yet
be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet, if these extreme
forms are connected together by a chain of intermediate groups, we may
at once infer their community of descent, and we put them all into the
same class. As we find organs of high physiological importance—those
which serve to preserve life under the most diverse conditions of
existence—are generally the most constant, we attach especial value to
them; but if these same organs, in another group or section of a group,
are found to differ much, we at once value them less in our
classification. We shall presently see why embryological characters are
of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera,
because all the species of the same genus, inhabiting any distinct and
isolated region, are in all probability descended from the same
parents.
_Analogical Resemblances._—We can understand, on the above views, the
very important distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to this subject,
and he has been ably followed by Macleay and others. The resemblance in
the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals and fishes,
are analogical. So is the resemblance between a mouse and a shrew-mouse
(Sorex), which belong to different orders; and the still closer
resemblance, insisted on by Mr. Mivart, between the mouse and a small
marsupial animal (Antechinus) of Australia. These latter resemblances
may be accounted for, as it seems to me, by adaptation for similarly
active movements through thickets and herbage, together with
concealment from enemies.
Among insects there are innumerable instances; thus Linnæus, misled by
external appearances, actually classed an homopterous insect as a moth.
We see something of the same kind even with our domestic varieties, as
in the strikingly similar shape of the body in the improved breeds of
the Chinese and common pig, which are descended from distinct species;
and in the similarly thickened stems of the common and specifically
distinct Swedish turnip. The resemblance between the greyhound and
race-horse is hardly more fanciful than the analogies which have been
drawn by some authors between widely different animals.
On the view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive characters, although of the utmost importance to
the welfare of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent, may have
become adapted to similar conditions, and thus have assumed a close
external resemblance; but such resemblances will not reveal—will rather
tend to conceal their blood-relationship. We can thus also understand
the apparent paradox, that the very same characters are analogical when
one group is compared with another, but give true affinities when the
members of the same group are compared together: thus the shape of the
body and fin-like limbs are only analogical when whales are compared
with fishes, being adaptations in both classes for swimming through the
water; but between the the several members of the whale family, the
shape of the body and the fin-like limbs offer characters exhibiting
true affinity; for as these parts are so nearly similar throughout the
whole family, we cannot doubt that they have been inherited from a
common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite
distinct beings between single parts or organs, which have been adapted
for the same functions. A good instance is afforded by the close
resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus—animals which are widely sundered in the natural system. But
this resemblance is confined to general appearance, as in the
prominence of the canines, and in the cutting shape of the molar teeth.
For the teeth really differ much: thus the dog has on each side of the
upper jaw four pre-molars and only two molars; while the Thylacinus has
three pre-molars and four molars. The molars also differ much in the
two animals in relative size and structure. The adult dentition is
preceded by a widely different milk dentition. Any one may, of course,
deny that the teeth in either case have been adapted for tearing flesh,
through the natural selection of successive variations; but if this be
admitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs—of widely different insects
possessing luminous organs—and of orchids and asclepiads having
pollen-masses with viscid discs, come under this same head of
analogical resemblances. But these cases are so wonderful that they
were introduced as difficulties or objections to our theory. In all
such cases some fundamental difference in the growth or development of
the parts, and generally in their matured structure, can be detected.
The end gained is the same, but the means, though appearing
superficially to be the same, are essentially different. The principle
formerly alluded to under the term of _analogical variation_ has
probably in these cases often come into play; that is, the members of
the same class, although only distantly allied, have inherited so much
in common in their constitution, that they are apt to vary under
similar exciting causes in a similar manner; and this would obviously
aid in the acquirement through natural selection of parts or organs,
strikingly like each other, independently of their direct inheritance
from a common progenitor.
As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar
circumstances—to inhabit, for instance, the three elements of land, air
and water—we can perhaps understand how it is that a numerical
parallelism has sometimes been observed between the subgroups of
distinct classes. A naturalist, struck with a parallelism of this
nature, by arbitrarily raising or sinking the value of the groups in
several classes (and all our experience shows that their valuation is
as yet arbitrary), could easily extend the parallelism over a wide
range; and thus the septenary, quinary, quaternary and ternary
classifications have probably arisen.
There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life,
but has been gained for the sake of protection. I allude to the
wonderful manner in which certain butterflies imitate, as first
described by Mr. Bates, other and quite distinct species. This
excellent observer has shown that in some districts of South America,
where, for instance, an Ithomia abounds in gaudy swarms, another
butterfly, namely, a Leptalis, is often found mingled in the same
flock; and the latter so closely resembles the Ithomia in every shade
and stripe of colour, and even in the shape of its wings, that Mr.
Bates, with his eyes sharpened by collecting during eleven years, was,
though always on his guard, continually deceived. When the mockers and
the mocked are caught and compared, they are found to be very different
in essential structure, and to belong not only to distinct genera, but
often to distinct families. Had this mimicry occurred in only one or
two instances, it might have been passed over as a strange coincidence.
But, if we proceed from a district where one Leptalis imitates an
Ithomia, another mocking and mocked species, belonging to the same two
genera, equally close in their resemblance, may be found. Altogether no
less than ten genera are enumerated, which include species that imitate
other butterflies. The mockers and mocked always inhabit the same
region; we never find an imitator living remote from the form which it
imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes
other Lepidoptera mimicking the same Ithomia: so that in the same
place, species of three genera of butterflies and even a moth are found
all closely resembling a butterfly belonging to a fourth genus. It
deserves especial notice that many of the mimicking forms of the
Leptalis, as well as of the mimicked forms, can be shown by a graduated
series to be merely varieties of the same species; while others are
undoubtedly distinct species. But why, it may be asked, are certain
forms treated as the mimicked and others as the mimickers? Mr. Bates
satisfactorily answers this question by showing that the form which is
imitated keeps the usual dress of the group to which it belongs, while
the counterfeiters have changed their dress and do not resemble their
nearest allies.
We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit
on the true explanation. The mocked forms, which always abound in
numbers, must habitually escape destruction to a large extent,
otherwise they could not exist in such swarms; and a large amount of
evidence has now been collected, showing that they are distasteful to
birds and other insect-devouring animals. The mocking forms, on the
other hand, that inhabit the same district, are comparatively rare, and
belong to rare groups; hence, they must suffer habitually from some
danger, for otherwise, from the number of eggs laid by all butterflies,
they would in three or four generations swarm over the whole country.
Now if a member of one of these persecuted and rare groups were to
assume a dress so like that of a well-protected species that it
continually deceived the practised eyes of an entomologist, it would
often deceive predaceous birds and insects, and thus often escape
destruction. Mr. Bates may almost be said to have actually witnessed
the process by which the mimickers have come so closely to resemble the
mimicked; for he found that some of the forms of Leptalis which mimic
so many other butterflies, varied in an extreme degree. In one district
several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much
commoner than the others, and this closely mocked another form of
Ithomia. From facts of this nature, Mr. Bates concludes that the
Leptalis first varies; and when a variety happens to resemble in some
degree any common butterfly inhabiting the same district, this variety,
from its resemblance to a flourishing and little persecuted kind, has a
better chance of escaping destruction from predaceous birds and
insects, and is consequently oftener preserved; “the less perfect
degrees of resemblance being generation after generation eliminated,
and only the others left to propagate their kind.” So that here we have
an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay Archipelago
and Africa, and with some other insects. Mr. Wallace has also detected
one such case with birds, but we have none with the larger quadrupeds.
The much greater frequency of imitation with insects than with other
animals, is probably the consequence of their small size; insects
cannot defend themselves, excepting indeed the kinds furnished with a
sting, and I have never heard of an instance of such kinds mocking
other insects, though they are mocked; insects cannot easily escape by
flight from the larger animals which prey on them; therefore, speaking
metaphorically, they are reduced, like most weak creatures, to trickery
and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the
imitated form was subsequently and gradually modified through any
agency, the imitating form would be led along the same track, and thus
be altered to almost any extent, so that it might ultimately assume an
appearance or colouring wholly unlike that of the other members of the
family to which it belonged. There is, however, some difficulty on this
head, for it is necessary to suppose in some cases that ancient members
belonging to several distinct groups, before they had diverged to their
present extent, accidentally resembled a member of another and
protected group in a sufficient degree to afford some slight
protection, this having given the basis for the subsequent acquisition
of the most perfect resemblance.
_On the Nature of the Affinities connecting Organic Beings._—As the
modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which
they belong large and their parents dominant, they are almost sure to
spread widely, and to seize on more and more places in the economy of
nature. The larger and more dominant groups within each class thus tend
to go on increasing in size, and they consequently supplant many
smaller and feebler groups. Thus, we can account for the fact that all
organisms, recent and extinct, are included under a few great orders
and under still fewer classes. As showing how few the higher groups are
in number, and how widely they are spread throughout the world, the
fact is striking that the discovery of Australia has not added an
insect belonging to a new class, and that in the vegetable kingdom, as
I learn from Dr. Hooker, it has added only two or three families of
small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some degree
intermediate between existing groups. As some few of the old and
intermediate forms having transmitted to the present day descendants
but little modified, these constitute our so-called osculant or
aberrant groups. The more aberrant any form is, the greater must be the
number of connecting forms which have been exterminated and utterly
lost. And we have evidence of aberrant groups having suffered severely
from extinction, for they are almost always represented by extremely
few species; and such species as do occur are generally very distinct
from each other, which again implies extinction. The genera
Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species, instead of as at
present by a single one, or by two or three. We can, I think, account
for this fact only by looking at aberrant groups as forms which have
been conquered by more successful competitors, with a few members still
preserved under unusually favourable conditions.
Mr. Waterhouse has remarked that when a member belonging to one group
of animals exhibits an affinity to a quite distinct group, this
affinity in most cases is general and not special: thus, according to
Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, that is, not to any one Marsupial species more
than to another. As these points of affinity are believed to be real
and not merely adaptive, they must be due in accordance with our view
to inheritance from a common progenitor. Therefore, we must suppose
either that all Rodents, including the bizcacha, branched off from some
ancient Marsupial, which will naturally have been more or less
intermediate in character with respect to all existing Marsupials; or
that both Rodents and Marsupials branched off from a common progenitor,
and that both groups have since undergone much modification in
divergent directions. On either view we must suppose that the bizcacha
has retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character
of their common progenitor, or of some early member of the group. On
the other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected
that the resemblance is only analogical, owing to the Phascolomys
having become adapted to habits like those of a Rodent. The elder De
Candolle has made nearly similar observations on the general nature of
the affinities of distinct families of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor, together
with their retention by inheritance of some characters in common, we
can understand the excessively complex and radiating affinities by
which all the members of the same family or higher group are connected
together. For the common progenitor of a whole family, now broken up by
extinction into distinct groups and subgroups, will have transmitted
some of its characters, modified in various ways and degrees, to all
the species; and they will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the
diagram so often referred to), mounting up through many predecessors.
As it is difficult to show the blood-relationship between the numerous
kindred of any ancient and noble family, even by the aid of a
genealogical tree, and almost impossible to do so without this aid, we
can understand the extraordinary difficulty which naturalists have
experienced in describing, without the aid of a diagram, the various
affinities which they perceive between the many living and extinct
members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinctness
of whole classes from each other—for instance, of birds from all other
vertebrate animals—by the belief that many ancient forms of life have
been utterly lost, through which the early progenitors of birds were
formerly connected with the early progenitors of the other and at that
time less differentiated vertebrate classes. There has been much less
extinction of the forms of life which once connected fishes with
Batrachians. There has been still less within some whole classes, for
instance the Crustacea, for here the most wonderfully diverse forms are
still linked together by a long and only partially broken chain of
affinities. Extinction has only defined the groups: it has by no means
made them; for if every form which has ever lived on this earth were
suddenly to reappear, though it would be quite impossible to give
definitions by which each group could be distinguished, still a natural
classification, or at least a natural arrangement, would be possible.
We shall see this by turning to the diagram: the letters, A to L, may
represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet
the arrangement in the diagram would still hold good and would be
natural; for, on the principle of inheritance, all the forms descended,
for instance from A, would have something in common. In a tree we can
distinguish this or that branch, though at the actual fork the two
unite and blend together. We could not, as I have said, define the
several groups; but we could pick out types, or forms, representing
most of the characters of each group, whether large or small, and thus
give a general idea of the value of the differences between them. This
is what we should be driven to, if we were ever to succeed in
collecting all the forms in any one class which have lived throughout
all time and space. Assuredly we shall never succeed in making so
perfect a collection: nevertheless, in certain classes, we are tending
toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we
can separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction
and divergence of character in the descendants from any one
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages under one species, although they may have
but few characters in common; we use descent in classing acknowledged
varieties, however different they may be from their parents; and I
believe that this element of descent is the hidden bond of connexion
which naturalists have sought under the term of the Natural System. On
this idea of the natural system being, in so far as it has been
perfected, genealogical in its arrangement, with the grades of
difference expressed by the terms genera, families, orders, &c., we can
understand the rules which we are compelled to follow in our
classification. We can understand why we value certain resemblances far
more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations
between one group and another, we summarily reject analogical or
adaptive characters, and yet use these same characters within the
limits of the same group. We can clearly see how it is that all living
and extinct forms can be grouped together within a few great classes;
and how the several members of each class are connected together by the
most complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of the affinities between
the members of any one class; but when we have a distinct object in
view, and do not look to some unknown plan of creation, we may hope to
make sure but slow progress.
Professor Haeckel in his “Generelle Morphologie” and in another works,
has recently brought his great knowledge and abilities to bear on what
he calls phylogeny, or the lines of descent of all organic beings. In
drawing up the several series he trusts chiefly to embryological
characters, but receives aid from homologous and rudimentary organs, as
well as from the successive periods at which the various forms of life
are believed to have first appeared in our geological formations. He
has thus boldly made a great beginning, and shows us how classification
will in the future be treated.
_Morphology._
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term “unity of
type;” or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included
under the general term of Morphology. This is one of the most
interesting departments of natural history, and may almost be said to
be its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include similar bones, in
the same relative positions? How curious it is, to give a subordinate
though striking instance, that the hind feet of the kangaroo, which are
so well fitted for bounding over the open plains—those of the climbing,
leaf-eating koala, equally well fitted for grasping the branches of
trees—those of the ground-dwelling, insect or root-eating,
bandicoots—and those of some other Australian marsupials—should all be
constructed on the same extraordinary type, namely with the bones of
the second and third digits extremely slender and enveloped within the
same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that
the hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the
more striking by the American opossums, which follow nearly the same
habits of life as some of their Australian relatives, having feet
constructed on the ordinary plan. Professor Flower, from whom these
statements are taken, remarks in conclusion: “We may call this
conformity to type, without getting much nearer to an explanation of
the phenomenon;” and he then adds “but is it not powerfully suggestive
of true relationship, of inheritance from a common ancestor?”
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together
in the same invariable order. We never find, for instance, the bones of
the arm and forearm, or of the thigh and leg, transposed. Hence the
same names can be given to the homologous bones in widely different
animals. We see the same great law in the construction of the mouths of
insects: what can be more different than the immensely long spiral
proboscis of a sphinx-moth, the curious folded one of a bee or bug, and
the great jaws of a beetle? Yet all these organs, serving for such
widely different purposes, are formed by infinitely numerous
modifications of an upper lip, mandibles, and two pairs of maxillæ. The
same law governs the construction of the mouths and limbs of
crustaceans. So it is with the flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine
of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the “Nature of Limbs.”
On the ordinary view of the independent creation of each being, we can
only say that so it is; that it has pleased the Creator to construct
all the animals and plants in each great class on a uniform plan; but
this is not a scientific explanation.
The explanation is to a large extent simple, on the theory of the
selection of successive slight modifications, each being profitable in
some way to the modified form, but often affecting by correlation other
parts of the organisation. In changes of this nature, there will be
little or no tendency to alter the original pattern, or to transpose
the parts. The bones of a limb might be shortened and flattened to any
extent, becoming at the same time enveloped in thick membrane, so as to
serve as a fin; or a webbed hand might have all its bones, or certain
bones, lengthened to any extent, with the membrane connecting them
increased, so as to serve as a wing; yet all these modifications would
not tend to alter the framework of the bones or the relative connexion
of the parts. If we suppose that an early progenitor—the archetype, as
it may be called—of all mammals, birds and reptiles, had its limbs
constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the
homologous construction of the limbs throughout the class. So with the
mouths of insects, we have only to suppose that their common progenitor
had an upper lip, mandibles, and two pairs of maxillæ, these parts
being perhaps very simple in form; and then natural selection will
account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally
lost, by the reduction and ultimately by the complete abortion of
certain parts, by the fusion of other parts, and by the doubling or
multiplication of others, variations which we know to be within the
limits of possibility. In the paddles of the gigantic extinct
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems thus to have become partially obscured.
There is another and equally curious branch of our subject; namely,
serial homologies, or the comparison of the different parts or organs
in the same individual, and not of the same parts or organs in
different members of the same class. Most physiologists believe that
the bones of the skull are homologous—that is, correspond in number and
in relative connexion—with the elemental parts of a certain number of
vertebræ. The anterior and posterior limbs in all the higher vertebrate
classes are plainly homologous. So it is with the wonderfully complex
jaws and legs of crustaceans. It is familiar to almost every one, that
in a flower the relative position of the sepals, petals, stamens, and
pistils, as well as their intimate structure, are intelligible on the
view that they consist of metamorphosed leaves, arranged in a spire. In
monstrous plants, we often get direct evidence of the possibility of
one organ being transformed into another; and we can actually see,
during the early or embryonic stages of development in flowers, as well
as in crustaceans and many other animals, that organs, which when
mature become extremely different are at first exactly alike.
How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be enclosed in a box composed of
such numerous and such extraordinarily shaped pieces of bone apparently
representing vertebræ? As Owen has remarked, the benefit derived from
the yielding of the separate pieces in the act of parturition by
mammals, will by no means explain the same construction in the skulls
of birds and reptiles. Why should similar bones have been created to
form the wing and the leg of a bat, used as they are for such totally
different purposes, namely flying and walking? Why should one
crustacean, which has an extremely complex mouth formed of many parts,
consequently always have fewer legs; or conversely, those with many
legs have simpler mouths? Why should the sepals, petals, stamens, and
pistils, in each flower, though fitted for such distinct purposes, be
all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some
animals first became divided into a series of segments, or how they
became divided into right and left sides, with corresponding organs,
for such questions are almost beyond investigation. It is, however,
probable that some serial structures are the result of cells
multiplying by division, entailing the multiplication of the parts
developed from such cells. It must suffice for our purpose to bear in
mind that an indefinite repetition of the same part or organ is the
common characteristic, as Owen has remarked, of all low or little
specialised forms; therefore the unknown progenitor of the Vertebrata
probably possessed many vertebræ; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also
formerly seen that parts many times repeated are eminently liable to
vary, not only in number, but in form. Consequently such parts, being
already present in considerable numbers, and being highly variable,
would naturally afford the materials for adaptation to the most
different purposes; yet they would generally retain, through the force
of inheritance, plain traces of their original or fundamental
resemblance. They would retain this resemblance all the more, as the
variations, which afforded the basis for their subsequent modification
through natural selection, would tend from the first to be similar; the
parts being at an early stage of growth alike, and being subjected to
nearly the same conditions. Such parts, whether more or less modified,
unless their common origin became wholly obscured, would be serially
homologous.
In the great class of molluscs, though the parts in distinct species
can be shown to be homologous, only a few serial homologies; such as
the valves of Chitons, can be indicated; that is, we are seldom enabled
to say that one part is homologous with another part in the same
individual. And we can understand this fact; for in molluscs, even in
the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part as we find in the other great
classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first appears,
as has lately been well shown in a remarkable paper by Mr. E. Ray
Lankester, who has drawn an important distinction between certain
classes of cases which have all been equally ranked by naturalists as
homologous. He proposes to call the structures which resemble each
other in distinct animals, owing to their descent from a common
progenitor with subsequent modification, _homogenous;_ and the
resemblances which cannot thus be accounted for, he proposes to call
_homoplastic_. For instance, he believes that the hearts of birds and
mammals are as a whole homogenous—that is, have been derived from a
common progenitor; but that the four cavities of the heart in the two
classes are homoplastic—that is, have been independently developed. Mr.
Lankester also adduces the close resemblance of the parts on the right
and left sides of the body, and in the successive segments of the same
individual animal; and here we have parts commonly called homologous
which bear no relation to the descent of distinct species from a common
progenitor. Homoplastic structures are the same with those which I have
classed, though in a very imperfect manner, as analogous modifications
or resemblances. Their formation may be attributed in part to distinct
organisms, or to distinct parts of the same organism, having varied in
an analogous manner; and in part to similar modifications, having been
preserved for the same general purpose or function, of which many
instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ; the jaws of crabs as metamorphosed legs; the stamens and
pistils in flowers as metamorphosed leaves; but it would in most cases
be more correct, as Professor Huxley has remarked, to speak of both
skull and vertebræ, jaws and legs, &c., as having been metamorphosed,
not one from the other, as they now exist, but from some common and
simpler element. Most naturalists, however, use such language only in a
metaphorical sense: they are far from meaning that during a long course
of descent, primordial organs of any kind—vertebræ in the one case and
legs in the other—have actually been converted into skulls or jaws. Yet
so strong is the appearance of this having occurred that naturalists
can hardly avoid employing language having this plain signification.
According to the views here maintained, such language may be used
literally; and the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters, which they probably would have retained
through inheritance, if they had really been metamorphosed from true
though extremely simple legs, is in part explained.
_Development and Embryology._
This is one of the most important subjects in the whole round of
natural history. The metamorphoses of insects, with which every one is
familiar, are generally effected abruptly by a few stages; but the
transformations are in reality numerous and gradual, though concealed.
A certain ephemerous insect (Chlöeon) during its development, moults,
as shown by Sir J. Lubbock, above twenty times, and each time undergoes
a certain amount of change; and in this case we see the act of
metamorphosis performed in a primary and gradual manner. Many insects,
and especially certain crustaceans, show us what wonderful changes of
structure can be effected during development. Such changes, however,
reach their acme in the so-called alternate generations of some of the
lower animals. It is, for instance, an astonishing fact that a delicate
branching coralline, studded with polypi, and attached to a submarine
rock, should produce, first by budding and then by transverse division,
a host of huge floating jelly-fishes; and that these should produce
eggs, from which are hatched swimming animalcules, which attach
themselves to rocks and become developed into branching corallines; and
so on in an endless cycle. The belief in the essential identity of the
process of alternate generation and of ordinary metamorphosis has been
greatly strengthened by Wagner’s discovery of the larva or maggot of a
fly, namely the Cecidomyia, producing asexually other larvæ, and these
others, which finally are developed into mature males and females,
propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner’s remarkable discovery was
first announced, I was asked how was it possible to account for the
larvæ of this fly having acquired the power of a sexual reproduction.
As long as the case remained unique no answer could be given. But
already Grimm has shown that another fly, a Chironomus, reproduces
itself in nearly the same manner, and he believes that this occurs
frequently in the order. It is the pupa, and not the larva, of the
Chironomus which has this power; and Grimm further shows that this
case, to a certain extent, “unites that of the Cecidomyia with the
parthenogenesis of the Coccidæ;” the term parthenogenesis implying that
the mature females of the Coccidæ are capable of producing fertile eggs
without the concourse of the male. Certain animals belonging to several
classes are now known to have the power of ordinary reproduction at an
unusually early age; and we have only to accelerate parthenogenetic
reproduction by gradual steps to an earlier and earlier age—Chironomus
showing us an almost exactly intermediate stage, viz., that of the
pupa—and we can perhaps account for the marvellous case of the
Cecidomyia.
It has already been stated that various parts in the same individual,
which are exactly alike during an early embryonic period, become widely
different and serve for widely different purposes in the adult state.
So again it has been shown that generally the embryos of the most
distinct species belonging to the same class are closely similar, but
become, when fully developed, widely dissimilar. A better proof of this
latter fact cannot be given than the statement by Von Baer that “the
embryos of mammalia, of birds, lizards and snakes, probably also of
chelonia, are in the earliest states exceedingly like one another, both
as a whole and in the mode of development of their parts; so much so,
in fact, that we can often distinguish the embryos only by their size.
In my possession are two little embryos in spirit, whose names I have
omitted to attach, and at present I am quite unable to say to what
class they belong. They may be lizards or small birds, or very young
mammalia, so complete is the similarity in the mode of formation of the
head and trunk in these animals. The extremities, however, are still
absent in these embryos. But even if they had existed in the earliest
stage of their development we should learn nothing, for the feet of
lizards and mammals, the wings and feet of birds, no less than the
hands and feet of man, all arise from the same fundamental form.” The
larvæ of most crustaceans, at corresponding stages of development,
closely resemble each other, however different the adults may become;
and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each
other in their immature plumage; as we see in the spotted feathers in
the young of the thrush group. In the cat tribe, most of the species
when adult are striped or spotted in lines; and stripes or spots can be
plainly distinguished in the whelp of the lion and the puma. We
occasionally, though rarely, see something of the same kind in plants;
thus the first leaves of the ulex or furze, and the first leaves of the
phyllodineous acacias, are pinnate or divided like the ordinary leaves
of the leguminosæ.
The points of structure, in which the embryos of widely different
animals within the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
courses of the arteries near the branchial slits are related to similar
conditions—in the young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in such
a relation than we have to believe that the similar bones in the hand
of a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one supposes that the stripes on the whelp of a
lion, or the spots on the young blackbird, are of any use to these
animals.
The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period
of activity may come on earlier or later in life; but whenever it comes
on, the adaptation of the larva to its conditions of life is just as
perfect and as beautiful as in the adult animal. In how important a
manner this has acted, has recently been well shown by Sir J. Lubbock
in his remarks on the close similarity of the larvæ of some insects
belonging to very different orders, and on the dissimilarity of the
larvæ of other insects within the same order, according to their habits
of life. Owing to such adaptations the similarity of the larvæ of
allied animals is sometimes greatly obscured; especially when there is
a division of labour during the different stages of development, as
when the same larva has during one stage to search for food, and during
another stage has to search for a place of attachment. Cases can even
be given of the larvæ of allied species, or groups of species,
differing more from each other than do the adults. In most cases,
however, the larvæ, though active, still obey, more or less closely,
the law of common embryonic resemblance. Cirripedes afford a good
instance of this: even the illustrious Cuvier did not perceive that a
barnacle was a crustacean: but a glance at the larva shows this in an
unmistakable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, though differing widely in external
appearance, have larvæ in all their stages barely distinguishable.
The embryo in the course of development generally rises in
organisation. I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by organisation being
higher or lower. But no one probably will dispute that the butterfly is
higher than the caterpillar. In some cases, however, the mature animal
must be considered as lower in the scale than the larva, as with
certain parasitic crustaceans. To refer once again to cirripedes: the
larvæ in the first stage have three pairs of locomotive organs, a
simple single eye, and a probosciformed mouth, with which they feed
largely, for they increase much in size. In the second stage, answering
to the chrysalis stage of butterflies, they have six pairs of
beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennæ; but they have a closed and
imperfect mouth, and cannot feed: their function at this stage is, to
search out by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become
attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennæ, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes
may be considered as either more highly or more lowly organised than
they were in the larval condition. But in some genera the larvæ become
developed into hermaphrodites having the ordinary structure, or into
what I have called complemental males; and in the latter the
development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and
every other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as
in some necessary manner contingent on growth. But there is no reason
why, for instance, the wing of a bat, or the fin of a porpoise, should
not have been sketched out with all their parts in proper proportion,
as soon as any part became visible. In some whole groups of animals and
in certain members of other groups this is the case, and the embryo
does not at any period differ widely from the adult: thus Owen has
remarked in regard to cuttle-fish, “there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the
embryo are completed.” Land-shells and fresh-water crustaceans are born
having their proper forms, while the marine members of the same two
great classes pass through considerable and often great changes during
their development. Spiders, again, barely undergo any metamorphosis.
The larvæ of most insects pass through a worm-like stage, whether they
are active and adapted to diversified habits, or are inactive from
being placed in the midst of proper nutriment, or from being fed by
their parents; but in some few cases, as in that of Aphis, if we look
to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Müller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple
nauplius-form, and after passing through two or more zoëa-stages, and
then through the mysis-stage, finally acquire their mature structure:
now in the whole great malacostracan order, to which these crustaceans
belong, no other member is as yet known to be first developed under the
nauplius-form, though many appear as zoëas; nevertheless Müller assigns
reasons for his belief, that if there had been no suppression of
development, all these crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology—namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes,
being at an early period of growth alike; the common, but not
invariable, resemblance between the embryos or larvæ of the most
distinct species in the same class; the embryo often retaining, while
within the egg or womb, structures which are of no service to it,
either at that or at a later period of life; on the other hand, larvæ
which have to provide for their own wants, being perfectly adapted to
the surrounding conditions; and lastly, the fact of certain larvæ
standing higher in the scale of organisation than the mature animal
into which they are developed? I believe that all these facts can be
explained as follows.
It is commonly assumed, perhaps from monstrosities affecting the embryo
at a very early period, that slight variations or individual
differences necessarily appear at an equally early period. We have
little evidence on this head, but what we have certainly points the
other way; for it is notorious that breeders of cattle, horses and
various fancy animals, cannot positively tell, until some time after
birth, what will be the merits and demerits of their young animals. We
see this plainly in our own children; we cannot tell whether a child
will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation may have been
caused, but at what period the effects are displayed. The cause may
have acted, and I believe often has acted, on one or both parents
before the act of generation. It deserves notice that it is of no
importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to
a bird which obtained its food by having a much-curved beak whether or
not while young it possessed a beak of this shape, as long as it was
fed by its parents.
I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding
age in the offspring. Certain variations can only appear at
corresponding ages; for instance, peculiarities in the caterpillar,
cocoon, or imago states of the silk-moth; or, again, in the full-grown
horns of cattle. But variations which, for all that we can see might
have appeared either earlier or later in life, likewise tend to
reappear at a corresponding age in the offspring and parent. I am far
from meaning that this is invariably the case, and I could give several
exceptional cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the
parent.
These two principles, namely, that slight variations generally appear
at a not very early period of life, and are inherited at a
corresponding not early period, explain, as I believe, all the above
specified leading facts in embryology. But first let us look to a few
analogous cases in our domestic varieties. Some authors who have
written on Dogs maintain that the greyhound and bull-dog, though so
different, are really closely allied varieties, descended from the same
wild stock, hence I was curious to see how far their puppies differed
from each other. I was told by breeders that they differed just as much
as their parents, and this, judging by the eye, seemed almost to be the
case; but on actually measuring the old dogs and their six-days-old
puppies, I found that the puppies had not acquired nearly their full
amount of proportional difference. So, again, I was told that the foals
of cart and race-horses—breeds which have been almost wholly formed by
selection under domestication—differed as much as the full-grown
animals; but having had careful measurements made of the dams and of
three-days-old colts of race and heavy cart-horses, I find that this is
by no means the case.
As we have conclusive evidence that the breeds of the Pigeon are
descended from a single wild species, I compared the young pigeons
within twelve hours after being hatched. I carefully measured the
proportions (but will not here give the details) of the beak, width of
mouth, length of nostril and of eyelid, size of feet and length of leg,
in the wild parent species, in pouters, fantails, runts, barbs,
dragons, carriers, and tumblers. Now, some of these birds, when mature,
differ in so extraordinary a manner in the length and form of beak, and
in other characters, that they would certainly have been ranked as
distinct genera if found in a state of nature. But when the nestling
birds of these several breeds were placed in a row, though most of them
could just be distinguished, the proportional differences in the above
specified points were incomparably less than in the full-grown birds.
Some characteristic points of difference—for instance, that of the
width of mouth—could hardly be detected in the young. But there was one
remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon, and of the
other breeds, in almost exactly the same proportions as in the adult
stage.
These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, &c., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier
or later in life, if the full-grown animal possesses them. And the
cases just given, more especially that of the pigeons, show that the
characteristic differences which have been accumulated by man’s
selection, and which give value to his breeds, do not generally appear
at a very early period of life, and are inherited at a corresponding
not early period. But the case of the short-faced tumbler, which when
twelve hours old possessed its proper characters, proves that this is
not the universal rule; for here the characteristic differences must
either have appeared at an earlier period than usual, or, if not so,
the differences must have been inherited, not at a corresponding, but
at an earlier age.
Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and
modified through natural selection for different habits. Then, from the
many slight successive variations having supervened in the several
species at a not early age, and having been inherited at a
corresponding age, the young will have been but little modified, and
they will still resemble each other much more closely than do the
adults, just as we have seen with the breeds of the pigeon. We may
extend this view to widely distinct structures and to whole classes.
The fore-limbs, for instance, which once served as legs to a remote
progenitor, may have become, through a long course of modification,
adapted in one descendant to act as hands, in another as paddles, in
another as wings; but on the above two principles the fore-limbs will
not have been much modified in the embryos of these several forms;
although in each form the fore-limb will differ greatly in the adult
state. Whatever influence long continued use or disuse may have had in
modifying the limbs or other parts of any species, this will chiefly or
solely have affected it when nearly mature, when it was compelled to
use its full powers to gain its own living; and the effects thus
produced will have been transmitted to the offspring at a corresponding
nearly mature age. Thus the young will not be modified, or will be
modified only in a slight degree, through the effects of the increased
use or disuse of parts.
With some animals the successive variations may have supervened at a
very early period of life, or the steps may have been inherited at an
earlier age than that at which they first occurred. In either of these
cases the young or embryo will closely resemble the mature parent-form,
as we have seen with the short-faced tumbler. And this is the rule of
development in certain whole groups, or in certain sub-groups alone, as
with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and
some members of the great class of insects. With respect to the final
cause of the young in such groups not passing through any
metamorphosis, we can see that this would follow from the following
contingencies: namely, from the young having to provide at a very early
age for their own wants, and from their following the same habits of
life with their parents; for in this case it would be indispensable for
their existence that they should be modified in the same manner as
their parents. Again, with respect to the singular fact that many
terrestrial and fresh-water animals do not undergo any metamorphosis,
while marine members of the same groups pass through various
transformations, Fritz Müller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing
through any larval stage; for it is not probable that places well
adapted for both the larval and mature stages, under such new and
greatly changed habits of life, would commonly be found unoccupied or
ill-occupied by other organisms. In this case the gradual acquirement
at an earlier and earlier age of the adult structure would be favoured
by natural selection; and all traces of former metamorphoses would
finally be lost.
If, on the other hand, it profited the young of an animal to follow
habits of life slightly different from those of the parent-form, and
consequently to be constructed on a slightly different plan, or if it
profited a larva already different from its parent to change still
further, then, on the principle of inheritance at corresponding ages,
the young or the larvæ might be rendered by natural selection more and
more different from their parents to any conceivable extent.
Differences in the larva might, also, become correlated with successive
stages of its development; so that the larva, in the first stage, might
come to differ greatly from the larva in the second stage, as is the
case with many animals. The adult might also become fitted for sites or
habits, in which organs of locomotion or of the senses, &c., would be
useless; and in this case the metamorphosis would be retrograde.
From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass through
stages of development, perfectly distinct from the primordial condition
of their adult progenitors. Most of our best authorities are now
convinced that the various larval and pupal stages of insects have thus
been acquired through adaptation, and not through inheritance from some
ancient form. The curious case of Sitaris—a beetle which passes through
certain unusual stages of development—will illustrate how this might
occur. The first larval form is described by M. Fabre, as an active,
minute insect, furnished with six legs, two long antennæ, and four
eyes. These larvæ are hatched in the nests of bees; and when the male
bees emerge from their burrows, in the spring, which they do before the
females, the larvæ spring on them, and afterwards crawl on to the
females while paired with the males. As soon as the female bee deposits
her eggs on the surface of the honey stored in the cells, the larvæ of
the Sitaris leap on the eggs and devour them. Afterwards they undergo a
complete change; their eyes disappear; their legs and antennæ become
rudimentary, and they feed on honey; so that they now more closely
resemble the ordinary larvæ of insects; ultimately they undergo a
further transformation, and finally emerge as the perfect beetle. Now,
if an insect, undergoing transformations like those of the Sitaris,
were to become the progenitor of a whole new class of insects, the
course of development of the new class would be widely different from
that of our existing insects; and the first larval stage certainly
would not represent the former condition of any adult and ancient form.
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the
condition of the progenitor of the whole group in its adult state. In
the great class of the Crustacea, forms wonderfully distinct from each
other, namely, suctorial parasites, cirripedes, entomostraca, and even
the malacostraca, appear at first as larvæ under the nauplius-form; and
as these larvæ live and feed in the open sea, and are not adapted for
any peculiar habits of life, and from other reasons assigned by Fritz
Müller, it is probable that at some very remote period an independent
adult animal, resembling the Nauplius, existed, and subsequently
produced, along several divergent lines of descent, the above-named
great Crustacean groups. So again, it is probable, from what we know of
the embryos of mammals, birds, fishes and reptiles, that these animals
are the modified descendants of some ancient progenitor, which was
furnished in its adult state with branchiæ, a swim-bladder, four
fin-like limbs, and a long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have ever lived,
can be arranged within a few great classes; and as all within each
class have, according to our theory, been connected together by fine
gradations, the best, and, if our collections were nearly perfect, the
only possible arrangement, would be genealogical; descent being the
hidden bond of connexion which naturalists have been seeking under the
term of the Natural System. On this view we can understand how it is
that, in the eyes of most naturalists, the structure of the embryo is
even more important for classification than that of the adult. In two
or more groups of animals, however much they may differ from each other
in structure and habits in their adult condition, if they pass through
closely similar embryonic stages, we may feel assured that they are all
descended from one parent-form, and are therefore closely related.
Thus, community in embryonic structure reveals community of descent;
but dissimilarity in embryonic development does not prove discommunity
of descent, for in one of two groups the developmental stages may have
been suppressed, or may have been so greatly modified through
adaptation to new habits of life as to be no longer recognisable. Even
in groups, in which the adults have been modified to an extreme degree,
community of origin is often revealed by the structure of the larvæ; we
have seen, for instance, that cirripedes, though externally so like
shell-fish, are at once known by their larvæ to belong to the great
class of crustaceans. As the embryo often shows us more or less plainly
the structure of the less modified and ancient progenitor of the group,
we can see why ancient and extinct forms so often resemble in their
adult state the embryos of existing species of the same class. Agassiz
believes this to be a universal law of nature; and we may hope
hereafter to see the law proved true. It can, however, be proved true
only in those cases in which the ancient state of the progenitor of the
group has not been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or by such
variations having been inherited at an earlier age than that at which
they first appeared. It should also be borne in mind, that the law may
be true, but yet, owing to the geological record not extending far
enough back in time, may remain for a long period, or for ever,
incapable of demonstration. The law will not strictly hold good in
those cases in which an ancient form became adapted in its larval state
to some special line of life, and transmitted the same larval state to
a whole group of descendants; for such larval state will not resemble
any still more ancient form in its adult state.
Thus, as it seems to me, the leading facts in embryology, which are
second to none in importance, are explained on the principle of
variations in the many descendants from some one ancient progenitor,
having appeared at a not very early period of life, and having been
inherited at a corresponding period. Embryology rises greatly in
interest, when we look at the embryo as a picture, more or less
obscured, of the progenitor, either in its adult or larval state, of
all the members of the same great class.
_Rudimentary, Atrophied, and Aborted Organs._
Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some
part or other is not in a rudimentary condition. In the mammalia, for
instance, the males possess rudimentary mammæ; in snakes one lobe of
the lungs is rudimentary; in birds the “bastard-wing” may safely be
considered as a rudimentary digit, and in some species the whole wing
is so far rudimentary that it cannot be used for flight. What can be
more curious than the presence of teeth in foetal whales, which when
grown up have not a tooth in their heads; or the teeth, which never cut
through the gums, in the upper jaws of unborn calves?
Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to
the same identical species, which have either full-sized and perfect
wings, or mere rudiments of membrane, which not rarely lie under
wing-covers firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally occurs
with the mammæ of male mammals, which have been known to become well
developed and to secrete milk. So again in the udders of the genus Bos,
there are normally four developed and two rudimentary teats; but the
latter in our domestic cows sometimes become well developed and yield
milk. In regard to plants, the petals are sometimes rudimentary, and
sometimes well developed in the individuals of the same species. In
certain plants having separated sexes Kölreuter found that by crossing
a species, in which the male flowers included a rudiment of a pistil,
with an hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much increased in
size; and this clearly shows that the rudimentary and perfect pistils
are essentially alike in nature. An animal may possess various parts in
a perfect state, and yet they may in one sense be rudimentary, for they
are useless: thus the tadpole of the common salamander or water-newt,
as Mr. G.H. Lewes remarks, “has gills, and passes its existence in the
water; but the Salamandra atra, which lives high up among the
mountains, brings forth its young full-formed. This animal never lives
in the water. Yet if we open a gravid female, we find tadpoles inside
her with exquisitely feathered gills; and when placed in water they
swim about like the tadpoles of the water-newt. Obviously this aquatic
organisation has no reference to the future life of the animal, nor has
it any adaptation to its embryonic condition; it has solely reference
to ancestral adaptations, it repeats a phase in the development of its
progenitors.”
An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is
to allow the pollen-tubes to reach the ovules within the ovarium. The
pistil consists of a stigma supported on the style; but in some
Compositæ, the male florets, which of course cannot be fecundated, have
a rudimentary pistil, for it is not crowned with a stigma; but the
style remains well developed and is clothed in the usual manner with
hairs, which serve to brush the pollen out of the surrounding and
conjoined anthers. Again, an organ may become rudimentary for its
proper purpose, and be used for a distinct one: in certain fishes the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or
lung. Many similar instances could be given.
Useful organs, however little they may be developed, unless we have
reason to suppose that they were formerly more highly developed, ought
not to be considered as rudimentary. They may be in a nascent
condition, and in progress towards further development. Rudimentary
organs, on the other hand, are either quite useless, such as teeth
which never cut through the gums, or almost useless, such as the wings
of an ostrich, which serve merely as sails. As organs in this condition
would formerly, when still less developed, have been of even less use
than at present, they cannot formerly have been produced through
variation and natural selection, which acts solely by the preservation
of useful modifications. They have been partially retained by the power
of inheritance, and relate to a former state of things. It is, however,
often difficult to distinguish between rudimentary and nascent organs;
for we can judge only by analogy whether a part is capable of further
development, in which case alone it deserves to be called nascent.
Organs in this condition will always be somewhat rare; for beings thus
provided will commonly have been supplanted by their successors with
the same organ in a more perfect state, and consequently will have
become long ago extinct. The wing of the penguin is of high service,
acting as a fin; it may, therefore, represent the nascent state of the
wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimentary. Owen
considers the simple filamentary limbs of the Lepidosiren as the
“beginnings of organs which attain full functional development in
higher vertebrates;” but, according to the view lately advocated by Dr.
Günther, they are probably remnants, consisting of the persistent axis
of a fin, with the lateral rays or branches aborted. The mammary glands
of the Ornithorhynchus may be considered, in comparison with the udders
of a cow, as in a nascent condition. The ovigerous frena of certain
cirripedes, which have ceased to give attachment to the ova and are
feebly developed, are nascent branchiæ.
Rudimentary organs in the individuals of the same species are very
liable to vary in the degree of their development and in other
respects. In closely allied species, also, the extent to which the same
organ has been reduced occasionally differs much. This latter fact is
well exemplified in the state of the wings of female moths belonging to
the same family. Rudimentary organs may be utterly aborted; and this
implies, that in certain animals or plants, parts are entirely absent
which analogy would lead us to expect to find in them, and which are
occasionally found in monstrous individuals. Thus in most of the
Scrophulariaceæ the fifth stamen is utterly aborted; yet we may
conclude that a fifth stamen once existed, for a rudiment of it is
found in many species of the family, and this rudiment occasionally
becomes perfectly developed, as may sometimes be seen in the common
snap-dragon. In tracing the homologies of any part in different members
of the same class, nothing is more common, or, in order fully to
understand the relations of the parts, more useful than the discovery
of rudiments. This is well shown in the drawings given by Owen of the
leg bones of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal rule, that a rudimentary part is of greater size in the
embryo relatively to the adjoining parts, than in the adult; so that
the organ at this early age is less rudimentary, or even cannot be said
to be in any degree rudimentary. Hence rudimentary organs in the adult
are often said to have retained their embryonic condition.
I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment; for
the same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and useless.
In works on natural history, rudimentary organs are generally said to
have been created “for the sake of symmetry,” or in order “to complete
the scheme of nature.” But this is not an explanation, merely a
restatement of the fact. Nor is it consistent with itself: thus the
boa-constrictor has rudiments of hind limbs and of a pelvis, and if it
be said that these bones have been retained “to complete the scheme of
nature,” why, as Professor Weismann asks, have they not been retained
by other snakes, which do not possess even a vestige of these same
bones? What would be thought of an astronomer who maintained that the
satellites revolve in elliptic courses round their planets “for the
sake of symmetry,” because the planets thus revolve round the sun? An
eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or matter
injurious to the system; but can we suppose that the minute papilla,
which often represents the pistil in male flowers, and which is formed
of mere cellular tissue, can thus act? Can we suppose that rudimentary
teeth, which are subsequently absorbed, are beneficial to the rapidly
growing embryonic calf by removing matter so precious as phosphate of
lime? When a man’s fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon believe that
these vestiges of nails are developed in order to excrete horny matter,
as that the rudimentary nails on the fin of the manatee have been
developed for this same purpose.
On the view of descent with modification, the origin of rudimentary
organs is comparatively simple; and we can understand to a large extent
the laws governing their imperfect development. We have plenty of cases
of rudimentary organs in our domestic productions, as the stump of a
tail in tailless breeds, the vestige of an ear in earless breeds of
sheep—the reappearance of minute dangling horns in hornless breeds of
cattle, more especially, according to Youatt, in young animals—and the
state of the whole flower in the cauliflower. We often see rudiments of
various parts in monsters; but I doubt whether any of these cases throw
light on the origin of rudimentary organs in a state of nature, further
than by showing that rudiments can be produced; for the balance of
evidence clearly indicates that species under nature do not undergo
great and abrupt changes. But we learn from the study of our domestic
productions that the disuse of parts leads to their reduced size; and
that the result is inherited.
It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more
and more complete reduction of a part, until at last it became
rudimentary—as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have
ultimately lost the power of flying. Again, an organ, useful under
certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this case
natural selection will have aided in reducing the organ, until it was
rendered harmless and rudimentary.
Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ
might, also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered
useless may well be variable, for their variations can no longer be
checked by natural selection. All this agrees well with what we see
under nature. Moreover, at whatever period of life either disuse or
selection reduces an organ, and this will generally be when the being
has come to maturity and to exert its full powers of action, the
principle of inheritance at corresponding ages will tend to reproduce
the organ in its reduced state at the same mature age, but will seldom
affect it in the embryo. Thus we can understand the greater size of
rudimentary organs in the embryo relatively to the adjoining parts, and
their lesser relative size in the adult. If, for instance, the digit of
an adult animal was used less and less during many generations, owing
to some change of habits, or if an organ or gland was less and less
functionally exercised, we may infer that it would become reduced in
size in the adult descendants of this animal, but would retain nearly
its original standard of development in the embryo.
There remains, however, this difficulty. After an organ has ceased
being used, and has become in consequence much reduced, how can it be
still further reduced in size until the merest vestige is left; and how
can it be finally quite obliterated? It is scarcely possible that
disuse can go on producing any further effect after the organ has once
been rendered functionless. Some additional explanation is here
requisite which I cannot give. If, for instance, it could be proved
that every part of the organisation tends to vary in a greater degree
towards diminution than toward augmentation of size, then we should be
able to understand how an organ which has become useless would be
rendered, independently of the effects of disuse, rudimentary and would
at last be wholly suppressed; for the variations towards diminished
size would no longer be checked by natural selection. The principle of
the economy of growth, explained in a former chapter, by which the
materials forming any part, if not useful to the possessor, are saved
as far as is possible, will perhaps come into play in rendering a
useless part rudimentary. But this principle will almost necessarily be
confined to the earlier stages of the process of reduction; for we
cannot suppose that a minute papilla, for instance, representing in a
male flower the pistil of the female flower, and formed merely of
cellular tissue, could be further reduced or absorbed for the sake of
economising nutriment.
Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a
former state of things, and have been retained solely through the power
of inheritance—we can understand, on the genealogical view of
classification, how it is that systematists, in placing organisms in
their proper places in the natural system, have often found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue for its derivation. On
the view of descent with modification, we may conclude that the
existence of organs in a rudimentary, imperfect, and useless condition,
or quite aborted, far from presenting a strange difficulty, as they
assuredly do on the old doctrine of creation, might even have been
anticipated in accordance with the views here explained.
_Summary._
In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups—that the
nature of the relationships by which all living and extinct organisms
are united by complex, radiating, and circuitous lines of affinities
into a few grand classes—the rules followed and the difficulties
encountered by naturalists in their classifications—the value set upon
characters, if constant and prevalent, whether of high or of the most
trifling importance, or, as with rudimentary organs of no
importance—the wide opposition in value between analogical or adaptive
characters, and characters of true affinity; and other such rules—all
naturally follow if we admit the common parentage of allied forms,
together with their modification through variation and natural
selection, with the contingencies of extinction and divergence of
character. In considering this view of classification, it should be
borne in mind that the element of descent has been universally used in
ranking together the sexes, ages, dimorphic forms, and acknowledged
varieties of the same species, however much they may differ from each
other in structure. If we extend the use of this element of descent—the
one certainly known cause of similarity in organic beings—we shall
understand what is meant by the Natural System: it is genealogical in
its attempted arrangement, with the grades of acquired difference
marked by the terms, varieties, species, genera, families, orders, and
classes.
On this same view of descent with modification, most of the great facts
in Morphology become intelligible—whether we look to the same pattern
displayed by the different species of the same class in their
homologous organs, to whatever purpose applied, or to the serial and
lateral homologies in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the leading
facts in embryology; namely, the close resemblance in the individual
embryo of the parts which are homologous, and which when matured become
widely different in structure and function; and the resemblance of the
homologous parts or organs in allied though distinct species, though
fitted in the adult state for habits as different as is possible. Larvæ
are active embryos, which have become specially modified in a greater
or less degree in relation to their habits of life, with their
modifications inherited at a corresponding early age. On these same
principles, and bearing in mind that when organs are reduced in size,
either from disuse or through natural selection, it will generally be
at that period of life when the being has to provide for its own wants,
and bearing in mind how strong is the force of inheritance—the
occurrence of rudimentary organs might even have been anticipated. The
importance of embryological characters and of rudimentary organs in
classification is intelligible, on the view that a natural arrangement
must be genealogical.
Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera and families, with which this world is peopled, are all
descended, each within its own class or group, from common parents, and
have all been modified in the course of descent, that I should without
hesitation adopt this view, even if it were unsupported by other facts
or arguments.