UNITY OF TYPE IN THE GREAT CLASSES; AND MORPHOLOGICAL STRUCTURES
_Unity of Type_{454}.
{454} _Origin_, Ed. i. p. 434, vi. p. 595. Ch. VIII corresponds to
a section of Ch. XIII in the _Origin_, Ed. i.
Scarcely anything is more wonderful or has been oftener insisted on than
that the organic beings in each great class, though living in the most
distant climes and at periods immensely remote, though fitted to widely
different ends in the economy of nature, yet all in their internal
structure evince an obvious uniformity. What, for instance, is more
wonderful than that the hand to clasp, the foot or hoof to walk, the
bat's wing to fly, the porpoise's fin{455} to swim, should all be built
on the same plan? and that the bones in their position and number should
be so similar that they can all be classed and called by the same names.
Occasionally some of the bones are merely represented by an apparently
useless, smooth style, or are soldered closely to other bones, but the
unity of type is not by this destroyed, and hardly rendered less clear.
We see in this fact some deep bond of union between the organic beings
of the same great classes--to illustrate which is the object and
foundation of the natural system. The perception of this bond, I may
add, is the evident cause that naturalists make an ill-defined
distinction between true and adaptive affinities.
{455} _Origin_, Ed. i. p. 434, vi. p. 596. In the _Origin_, Ed. i.
these examples occur under the heading _Morphology_; the author
does not there draw much distinction between this heading and that
of _Unity of Type_.
_Morphology._
There is another allied or rather almost identical class of facts
admitted by the least visionary naturalists and included under the name
of Morphology. These facts show that in an individual organic being,
several of its organs consist of some other organ metamorphosed{456}:
thus the sepals, petals, stamens, pistils, &c. of every plant can be
shown to be metamorphosed leaves; and thus not only can the number,
position and transitional states of these several organs, but likewise
their monstrous changes, be most lucidly explained. It is believed that
the same laws hold good with the gemmiferous vesicles of Zoophytes. In
the same manner the number and position of the extraordinarily
complicated jaws and palpi of Crustacea and of insects, and likewise
their differences in the different groups, all become simple, on the
view of these parts, or rather legs and all metamorphosed appendages,
being metamorphosed legs. The skulls, again, of the Vertebrata are
composed of three metamorphosed vertebrae, and thus we can see a meaning
in the number and strange complication of the bony case of the brain. In
this latter instance, and in that of the jaws of the Crustacea, it is
only necessary to see a series taken from the different groups of each
class to admit the truth of these views. It is evident that when in each
species of a group its organs consist of some other part metamorphosed,
that there must also be a "unity of type" in such a group. And in the
cases as that above given in which the foot, hand, wing and paddle are
said to be constructed on a uniform type, if we could perceive in such
parts or organs traces of an apparent change from some other use or
function, we should strictly include such parts or organs in the
department of morphology: thus if we could trace in the limbs of the
Vertebrata, as we can in their ribs, traces of an apparent change from
being processes of the vertebrae, it would be said that in each species
of the Vertebrata the limbs were "metamorphosed spinal processes," and
that in all the species throughout the class the limbs displayed a
"unity of type{457}."
{456} See _Origin_, Ed. i. p. 436, vi. p. 599, where the parts of
the flower, the jaws and palpi of Crustaceans and the vertebrate
skull are given as examples.
{457} The author here brings _Unity of Type_ and _Morphology_
together.
These wonderful parts of the hoof, foot, hand, wing, paddle, both in
living and extinct animals, being all constructed on the same framework,
and again of the petals, stamina, germens, &c. being metamorphosed
leaves, can by the creationist be viewed only as ultimate facts and
incapable of explanation; whilst on our theory of descent these facts
all necessary follow: for by this theory all the beings of any one
class, say of the mammalia, are supposed to be descended from one
parent-stock, and to have been altered by such slight steps as man
effects by the selection of chance domestic variations. Now we can see
according to this view that a foot might be selected with longer and
longer bones, and wider connecting membranes, till it became a swimming
organ, and so on till it became an organ by which to flap along the
surface or to glide over it, and lastly to fly through the air: but in
such changes there would be no tendency to alter the framework of the
internal inherited structure. Parts might become lost (as the tail in
dogs, or horns in cattle, or the pistils in plants), others might become
united together (as in the feet of the Lincolnshire breed of pigs{458},
and in the stamens of many garden flowers); parts of a similar nature
might become increased in number (as the vertebrae in the tails of pigs,
&c., &c. and the fingers and toes in six-fingered races of men and in
the Dorking fowls), but analogous differences are observed in nature and
are not considered by naturalists to destroy the uniformity of the
types. We can, however, conceive such changes to be carried to such
length that the unity of type might be obscured and finally be
undistinguishable, and the paddle of the Plesiosaurus has been advanced
as an instance in which the uniformity of type can hardly be
recognised{459}. If after long and gradual changes in the structure of
the co-descendants from any parent stock, evidence (either from
monstrosities or from a graduated series) could be still detected of the
function, which certain parts or organs played in the parent stock,
these parts or organs might be strictly determined by their former
function with the term "metamorphosed" appended. Naturalists have used
this term in the same metaphorical manner as they have been obliged to
use the terms of affinity and relation; and when they affirm, for
instance, that the jaws of a crab are metamorphosed legs, so that one
crab has more legs and fewer jaws than another, they are far from
meaning that the jaws, either during the life of the individual crab or
of its progenitors, were really legs. By our theory this term assumes
its literal meaning{460}; and this wonderful fact of the complex jaws of
an animal retaining numerous characters, which they would probably have
retained if they had really been metamorphosed during many successive
generations from true legs, is simply explained.
{458} The solid-hoofed pigs mentioned in _Var. under Dom._, Ed. ii.
vol. II. p. 424 are not _Lincolnshire pigs_. For other cases see
Bateson, _Materials for the Study of Variation_, 1894, pp. 387-90.
{459} In the margin C. Bell is given as authority, apparently for
the statement about Plesiosaurus. See _Origin_, Ed. i. p. 436, vi.
p. 598, where the author speaks of the "general pattern" being
obscured in "extinct gigantic sea lizards." In the same place the
suctorial Entomostraca are added as examples of the difficulty of
recognising the type.
{460} _Origin_, Ed. i. p. 438, vi. p. 602.
_Embryology_.
The unity of type in the great classes is shown in another and very
striking manner, namely, in the stages through which the embryo passes
in coming to maturity{461}. Thus, for instance, at one period of the
embryo, the wings of the bat, the hand, hoof or foot of the quadruped,
and the fin of the porpoise do not differ, but consist of a simple
undivided bone. At a still earlier period the embryo of the fish, bird,
reptile and mammal all strikingly resemble each other. Let it not be
supposed this resemblance is only external; for on dissection, the
arteries are found to branch out and run in a peculiar course, wholly
unlike that in the full-grown mammal and bird, but much less unlike that
in the full-grown fish, for they run as if to aerate blood by
branchiae{462} on the neck, of which even the slit-like orifices can be
discerned. How wonderful it is that this structure should be present in
the embryos of animals about to be developed into such different forms,
and of which two great classes respire only in the air. Moreover, as the
embryo of the mammal is matured in the parent's body, and that of the
bird in an egg in the air, and that of the fish in an egg in the water,
we cannot believe that this course of the arteries is related to any
external conditions. In all shell-fish (Gasteropods) the embryo passes
through a state analogous to that of the Pteropodous Mollusca: amongst
insects again, even the most different ones, as the moth, fly and
beetle, the crawling larvae are all closely analogous: amongst the
Radiata, the jelly-fish in its embryonic state resembles a polype, and
in a still earlier state an infusorial animalcule--as does likewise the
embryo of the polype. From the part of the embryo of a mammal, at one
period, resembling a fish more than its parent form; from the larvae of
all orders of insects more resembling the simpler articulate animals
than their parent insects{463}; and from such other cases as the embryo
of the jelly-fish resembling a polype much nearer than the perfect
jelly-fish; it has often been asserted that the higher animal in each
class passes through the state of a lower animal; for instance, that the
mammal amongst the vertebrata passes through the state of a fish{464}:
but Mueller denies this, and affirms that the young mammal is at no time
a fish, as does Owen assert that the embryonic jelly-fish is at no time
a polype, but that mammal and fish, jelly-fish and polype pass through
the same state; the mammal and jelly-fish being only further developed
or changed.
{461} _Origin_, Ed. i. p. 439, vi. p. 604.
{462} The uselessness of the branchial arches in mammalia is
insisted on in the _Origin_, Ed. i. p. 440, vi. p. 606. Also the
uselessness of the spots on the young blackbird and the stripes of
the lion-whelp, cases which do not occur in the present Essay.
{463} In the _Origin_, Ed. i. pp. 442, 448, vi. pp. 608, 614 it is
pointed out that in some cases the young form resembles the adult,
_e.g._ in spiders; again, that in the Aphis there is no "worm-like
stage" of development.
{464} In the _Origin_, Ed. i. p. 449, vi. p. 618, the author speaks
doubtfully about the recapitulation theory.
As the embryo, in most cases, possesses a less complicated structure
than that into which it is to be developed, it might have been thought
that the resemblance of the embryo to less complicated forms in the same
great class, was in some manner a necessary preparation for its higher
development; but in fact the embryo, during its growth, may become less,
as well as more, complicated{465}. Thus certain female Epizoic
Crustaceans in their mature state have neither eyes nor any organs of
locomotion; they consist of a mere sack, with a simple apparatus for
digestion and procreation; and when once attached to the body of the
fish, on which they prey, they never move again during their whole
lives: in their embryonic condition, on the other hand, they are
furnished with eyes, and with well articulated limbs, actively swim
about and seek their proper object to become attached to. The larvae,
also, of some moths are as complicated and are more active than the
wingless and limbless females, which never leave their pupa-case, never
feed and never see the daylight.
{465} This corresponds to the _Origin_, Ed. i. p. 441, vi. p. 607,
where, however, the example is taken from the Cirripedes.
_Attempt to explain the facts of embryology._
I think considerable light can be thrown by the theory of descent on
these wonderful embryological facts which are common in a greater or
less degree to the whole animal kingdom, and in some manner to the
vegetable kingdom: on the fact, for instance, of the arteries in the
embryonic mammal, bird, reptile and fish, running and branching in the
same courses and nearly in the same manner with the arteries in the
full-grown fish; on the fact I may add of the high importance to
systematic naturalists{466} of the characters and resemblances in the
embryonic state, in ascertaining the true position in the natural system
of mature organic beings. The following are the considerations which
throw light on these curious points.
{466} _Origin_, Ed. i. p. 449, vi. p. 617.
In the economy, we will say of a feline animal{467}, the feline
structure of the embryo or of the sucking kitten is of quite secondary
importance to it; hence, if a feline animal varied (assuming for the
time the possibility of this) and if some place in the economy of
nature favoured the selection of a longer-limbed variety, it would be
quite unimportant to the production by natural selection of a
long-limbed breed, whether the limbs of the embryo and kitten were
elongated if they _became_ so _as soon_ as the animal had to provide
food for itself. And if it were found after continued selection and the
production of several new breeds from one parent-stock, that the
successive variations had supervened, not very early in the youth or
embryonic life of each breed (and we have just seen that it is quite
unimportant whether it does so or not), then it obviously follows that
the young or embryos of the several breeds will continue resembling each
other more closely than their adult parents{468}. And again, if two of
these breeds became each the parent-stock of several other breeds,
forming two genera, the young and embryos of these would still retain a
greater resemblance to the one original stock than when in an adult
state. Therefore if it could be shown that the period of the slight
successive variations does not always supervene at a very early period
of life, the greater resemblance or closer unity in type of animals in
the young than in the full-grown state would be explained. Before
practically{469} endeavouring to discover in our domestic races whether
the structure or form of the young has or has not changed in an exactly
corresponding degree with the changes of full-grown animals, it will be
well to show that it is at least quite _possible_ for the primary
germinal vesicle to be impressed with a tendency to produce some change
on the growing tissues which will not be fully effected till the animal
is advanced in life.
{467} This corresponds to the _Origin_, Ed. i. pp. 443-4, vi. p.
610: the "feline animal" is not used to illustrate the
generalisation, but is so used in the Essay of 1842, p. 42.
{468} _Origin_, Ed. i. p. 447, vi. p. 613.
{469} In the margin is written "Get young pigeons"; this was
afterwards done, and the results are given in the _Origin_, Ed. i.
p. 445, vi. p. 612.
From the following peculiarities of structure being inheritable and
appearing only when the animal is full-grown--namely, general size,
tallness (not consequent on the tallness of the infant), fatness either
over the whole body, or local; change of colour in hair and its loss;
deposition of bony matter on the legs of horses; blindness and deafness,
that is changes of structure in the eye and ear; gout and consequent
deposition of chalk-stones; and many other diseases{470}, as of the
heart and brain, &c., &c.; from all such tendencies being I repeat
inheritable, we clearly see that the germinal vesicle is impressed with
some power which is wonderfully preserved during the production of
infinitely numerous cells in the ever changing tissues, till the part
ultimately to be affected is formed and the time of life arrived at. We
see this clearly when we select cattle with any peculiarity of their
horns, or poultry with any peculiarity of their second plumage, for such
peculiarities cannot of course reappear till the animal is mature.
Hence, it is certainly _possible_ that the germinal vesicle may be
impressed with a tendency to produce a long-limbed animal, the full
proportional length of whose limbs shall appear only when the animal is
mature{471}.
{470} In the _Origin_, Ed. i. the corresponding passages are at pp.
8, 13, 443, vi. pp. 8, 15, 610. In the _Origin_, Ed. i. I have not
found a passage so striking as that which occurs a few lines lower
"that the germinal vesicle is impressed with some power which is
wonderfully preserved, &c." In the _Origin_ this _preservation_ is
rather taken for granted.
{471} <In the margin is written> Aborted organs show, perhaps,
something about period <at> which changes supervene in embryo.
In several of the cases just enumerated we know that the first cause of
the peculiarity, when _not_ inherited, lies in the conditions to which
the animal is exposed during mature life, thus to a certain extent
general size and fatness, lameness in horses and in a lesser degree
blindness, gout and some other diseases are certainly in some degree
caused and accelerated by the habits of life, and these peculiarities
when transmitted to the offspring of the affected person reappear at a
nearly corresponding time of life. In medical works it is asserted
generally that at whatever period an hereditary disease appears in the
parent, it tends to reappear in the offspring at the same period. Again,
we find that early maturity, the season of reproduction and longevity
are transmitted to corresponding periods of life. Dr Holland has
insisted much on children of the same family exhibiting certain diseases
in similar and peculiar manners; my father has known three brothers{472}
die in very old age in a _singular_ comatose state; now to make these
latter cases strictly bear, the children of such families ought
similarly to suffer at corresponding times of life; this is probably not
the case, but such facts show that a tendency in a disease to appear at
particular stages of life can be transmitted through the germinal
vesicle to different individuals of the same family. It is then
certainly possible that diseases affecting widely different periods of
life can be transmitted. So little attention is paid to very young
domestic animals that I do not know whether any case is on record of
selected peculiarities in young animals, for instance, in the first
plumage of birds, being transmitted to their young. If, however, we turn
to silk-worms{473}, we find that the caterpillars and coccoons (which
must correspond to a _very early_ period of the embryonic life of
mammalia) vary, and that these varieties reappear in the offspring
caterpillars and coccoons.
{472} See p. 42, note 5.{Note 160}
{473} The evidence is given in _Var. under Dom._, I. p. 316.
I think these facts are sufficient to render it probable that at
whatever period of life any peculiarity (capable of being inherited)
appears, whether caused by the action of external influences during
mature life, or from an affection of the primary germinal vesicle, it
_tends_ to reappear in the offspring at the corresponding period of
life{474}. Hence (I may add) whatever effect training, that is the full
employment or action of every newly selected slight variation, has in
fully developing and increasing such variation, would only show itself
in mature age, corresponding to the period of training; in the second
chapter I showed that there was in this respect a marked difference in
natural and artificial selection, man not regularly exercising or
adapting his varieties to new ends, whereas selection by nature
presupposes such exercise and adaptation in each selected and changed
part. The foregoing facts show and presuppose that slight variations
occur at various periods of life _after birth_; the facts of
monstrosity, on the other hand, show that many changes take place before
birth, for instance, all such cases as extra fingers, hare-lip and all
sudden and great alterations in structure; and these when inherited
reappear during the embryonic period in the offspring. I will only add
that at a period even anterior to embryonic life, namely, during the
_egg_ state, varieties appear in size and colour (as with the
Hertfordshire duck with blackish eggs{475}) which reappear in the egg;
in plants also the capsule and membranes of the seed are very variable
and inheritable.
{474} _Origin_, Ed. i. p. 444, vi. p. 610.
{475} In _Var. under Dom._, Ed. ii. vol. I. p. 295, such eggs are
said to be laid early in each season by the black Labrador duck. In
the next sentence in the text the author does not distinguish the
characters of the vegetable capsule from those of the ovum.
If then the two following propositions are admitted (and I think the
first can hardly be doubted), viz. that variation of structure takes
place at all times of life, though no doubt far less in amount and
seldomer in quite mature life{476} (and then generally taking the form
of disease); and secondly, that these variations tend to reappear at a
corresponding period of life, which seems at least probable, then we
might _a priori_ have expected that in any selected breed the _young_
animal would not partake in a corresponding degree the peculiarities
characterising the _full-grown_ parent; though it would in a lesser
degree. For during the thousand or ten thousand selections of slight
increments in the length of the limbs of individuals necessary to
produce a long-limbed breed, we might expect that such increments would
take place in different individuals (as we do not certainly know at what
period they do take place), some earlier and some later in the embryonic
state, and some during early youth; and these increments would reappear
in their offspring only at corresponding periods. Hence, the entire
length of limb in the new long-limbed breed would only be acquired at
the latest period of life, when that one which was latest of the
thousand primary increments of length supervened. Consequently, the
foetus of the new breed during the earlier part of its existence would
remain much less changed in the proportions of its limbs; and the
earlier the period the less would the change be.
{476} This seems to me to be more strongly stated here than in the
_Origin_, Ed. i.
Whatever may be thought of the facts on which this reasoning is
grounded, it shows how the embryos and young of different species might
come to remain less changed than their mature parents; and practically
we find that the young of our domestic animals, though differing, differ
less than their full-grown parents. Thus if we look at the young
puppies{477} of the greyhound and bulldog--(the two most obviously
modified of the breeds of dog)--we find their puppies at the age of six
days with legs and noses (the latter measured from the eyes to the tip)
of the same length; though in the proportional thicknesses and general
appearance of these parts there is a great difference. So it is with
cattle, though the young calves of different breeds are easily
recognisable, yet they do not differ so much in their proportions as the
full-grown animals. We see this clearly in the fact that it shows the
highest skill to select the best forms early in life, either in horses,
cattle or poultry; no one would attempt it only a few hours after birth;
and it requires great discrimination to judge with accuracy even during
their full youth, and the best judges are sometimes deceived. This shows
that the ultimate proportions of the body are not acquired till near
mature age. If I had collected sufficient facts to firmly establish the
proposition that in artificially selected breeds the embryonic and young
animals are not changed in a corresponding degree with their mature
parents, I might have omitted all the foregoing reasoning and the
attempts to explain how this happens; for we might safely have
transferred the proposition to the breeds or species naturally selected;
and the ultimate effect would necessarily have been that in a number of
races or species descended from a common stock and forming several
genera and families the embryos would have resembled each other more
closely than full-grown animals. Whatever may have been the form or
habits of the parent-stock of the Vertebrata, in whatever course the
arteries ran and branched, the selection of variations, supervening
after the first formation of the arteries in the embryo, would not tend
from variations supervening at corresponding periods to alter their
course at that period: hence, the similar course of the arteries in the
mammal, bird, reptile and fish, must be looked at as a most ancient
record of the embryonic structure of the common parent-stock of these
four great classes.
{477} _Origin_, Ed. i. p. 444, vi. p. 611.
A long course of selection might cause a form to become more simple, as
well as more complicated; thus the adaptation of a crustaceous{478}
animal to live attached during its whole life to the body of a fish,
might permit with advantage great simplification of structure, and on
this view the singular fact of an embryo being more complex than its
parent is at once explained.
{478} _Origin_, Ed. i. p. 441, vi. p. 607.
_On the graduated complexity in each great class._
I may take this opportunity of remarking that naturalists have observed
that in most of the great classes a series exists from very complicated
to very simple beings; thus in Fish, what a range there is between the
sand-eel and shark,--in the Articulata, between the common crab and the
Daphnia{479},--between the Aphis and butterfly, and between a mite and a
spider{480}. Now the observation just made, namely, that selection might
tend to simplify, as well as to complicate, explains this; for we can
see that during the endless geologico-geographical changes, and
consequent isolation of species, a station occupied in other districts
by less complicated animals might be left unfilled, and be occupied by a
degraded form of a higher or more complicated class; and it would by no
means follow that, when the two regions became united, the degraded
organism would give way to the aboriginally lower organism. According to
our theory, there is obviously no power tending constantly to exalt
species, except the mutual struggle between the different individuals
and classes; but from the strong and general hereditary tendency we
might expect to find some tendency to progressive complication in the
successive production of new organic forms.
{479} Compare _Origin_, Ed. i. p. 419, vi. p. 575.
{480} <Note in original.> Scarcely possible to distinguish between
non-development and retrograde development.
_Modification by selection of the forms of immature animals._
I have above remarked that the feline{481} form is quite of secondary
importance to the embryo and to the kitten. Of course, during any great
and prolonged change of structure in the mature animal, it might, and
often would be, indispensable that the form of the embryo should be
changed; and this could be effected, owing to the hereditary tendency at
corresponding ages, by selection, equally well as in mature age: thus if
the embryo tended to become, or to remain, either over its whole body or
in certain parts, too bulky, the female parent would die or suffer more
during parturition; and as in the case of the calves with large hinder
quarters{482}, the peculiarity must be either eliminated or the species
become extinct. Where an embryonic form has to seek its own food, its
structure and adaptation is just as important to the species as that of
the full-grown animal; and as we have seen that a peculiarity appearing
in a caterpillar (or in a child, as shown by the hereditariness of
peculiarities in the milk-teeth) reappears in its offspring, so we can
at once see that our common principle of the selection of slight
accidental variations would modify and adapt a caterpillar to a new or
changing condition, precisely as in the full-grown butterfly. Hence
probably it is that caterpillars of different species of the Lepidoptera
differ more than those embryos, at a corresponding early period of life,
do which remain inactive in the womb of their parents. The parent during
successive ages continuing to be adapted by selection for some one
object, and the larva for quite another one, we need not wonder at the
difference becoming wonderfully great between them; even as great as
that between the fixed rock-barnacle and its free, crab-like offspring,
which is furnished with eyes and well-articulated, locomotive
limbs{483}.
{481} See p. 42, where the same illustration is used.
{482} _Var. under Dom._, Ed. ii. vol. I. p. 452.
{483} _Origin_, Ed. i. p. 441, vi. p. 607.
_Importance of embryology in classification._
We are now prepared to perceive why the study of embryonic forms is of
such acknowledged importance in classification{484}. For we have seen
that a variation, supervening at any time, may aid in the modification
and adaptation of the full-grown being; but for the modification of the
embryo, only the variations which supervene at a very early period can
be seized on and perpetuated by selection: hence there will be less
power and less tendency (for the structure of the embryo is mostly
unimportant) to modify the young: and hence we might expect to find at
this period similarities preserved between different groups of species
which had been obscured and quite lost in the full-grown animals. I
conceive on the view of separate creations it would be impossible to
offer any explanation of the affinities of organic beings thus being
plainest and of the greatest importance at that period of life when
their structure is not adapted to the final part they have to play in
the economy of nature.
{484} _Origin_, Ed. i. p. 449, vi. p. 617.
_Order in time in which the great classes have first appeared._
It follows strictly from the above reasoning only that the embryos of
(for instance) existing vertebrata resemble more closely the embryo of
the parent-stock of this great class than do full-grown existing
vertebrata resemble their full-grown parent-stock. But it may be argued
with much probability that in the earliest and simplest condition of
things the parent and embryo must have resembled each other, and that
the passage of any animal through embryonic states in its growth is
entirely due to subsequent variations affecting _only_ the more mature
periods of life. If so, the embryos of the existing vertebrata will
shadow forth the full-grown structure of some of those forms of this
great class which existed at the earlier periods of the earth's
history{485}: and accordingly, animals with a fish-like structure ought
to have preceded birds and mammals; and of fish, that higher organized
division with the vertebrae extending into one division of the tail ought
to have preceded the equal-tailed, because the embryos of the latter
have an unequal tail; and of Crustacea, entomostraca ought to have
preceded the ordinary crabs and barnacles--polypes ought to have
preceded jelly-fish, and infusorial animalcules to have existed before
both. This order of precedence in time in some of these cases is
believed to hold good; but I think our evidence is so exceedingly
incomplete regarding the number and kinds of organisms which have
existed during all, especially the earlier, periods of the earth's
history, that I should put no stress on this accordance, even if it held
truer than it probably does in our present state of knowledge.
{485} _Origin_, Ed. i. p. 449, vi. p. 618.