ON THE PROGRESS OF EVOLUTION.
From this scarcely unavoidable but unsatisfactory side-glance upon the
old school, which looks down with so great an air of superiority upon
Darwin's “intellectual dream” and the “giddy enthusiasm” of its
friends, I turn to the more congenial task of considering the
developmental history of the Crustacea from the point of view of the
Darwinian theory.
Darwin himself, in the thirteenth chapter of his book, has already
discussed the conclusions derived from his hypotheses in the domain of
developmental history. For a more detailed application of them,
however, it is necessary in the first place to trace these general
conclusions a little further than he has there done.
The changes by which young animals depart from their parents, and the
gradual accumulation of which causes the production of new species,
genera, and families, may occur at an earlier or later period of
life,—in the young state, or at the period of sexual maturity. For the
latter is by no means always, as in the Insecta, a period of repose;
most other animals even then continue to grow and to undergo changes.
(See above, the remarks on the males of the Amphipoda.) Some
variations, indeed, from their very nature, can only occur when the
young animal has attained the adult stage of development. Thus the Sea
Caterpillars (_Polynoë_) at first possess only a few body-segments,
which, during development, gradually increase to a number which is
different in different species, but constant in the same species; now
before a young animal could exceed the number of segments of its
parents, it must of course have attained that number. We may assume a
similar supplementary progress wherever the deviation of the
descendants consists in an addition of new segments and limbs.
_Descendants therefore reach a new goal, either by deviating sooner or
later whilst still on the way towards the form of their parents, or by
passing along this course without deviation, but then, instead of
standing still, advance still farther._
The former mode will have had a predominant action where the posterity
of common ancestors constitutes a group of forms standing upon the same
level in essential features, as the whole of the Amphipoda, Crabs, or
Birds. On the other hand we are led to the assumption of the second
mode of progress, when we seek to deduce from a common original form,
animals some of which agree with young states of others.
In the former case the developmental history of the descendants can
only agree with that of their ancestors up to a certain point at which
their courses separate,—as to their structure in the adult state it
will teach us nothing. _In the second case the entire development of
the progenitors is also passed through by the descendants, and,
therefore, so far as the production of a species depends upon this
second mode of progress, the historical development of the species will
be mirrored in its developmental history._ In the short period of a few
weeks or months, the changing forms of the embryo and larvæ will pass
before us, a more or less complete and more or less true picture of the
transformations through which the species, in the course of untold
thousands of years, has struggled up to its present state.
(Figs. 65 to 67. Young Tubicolar worms, magnified with the simple lens:
65. Without operculum, Protula-stage. 66. With a barbate opercular
peduncle, Filograna-stage; With a naked opercular peduncle,
Serpula-stage. Figs. 65–67. Young Tubicolar worms, magnified with the
simple lens: 65.[1] Without operculum, _Protula_-stage; 66. With a
barbate opercular peduncle, _Filograna_-stage; 67. With a naked
opercular peduncle, _Serpula_-stage.
One of the simplest examples is furnished by the development of the
Tubicolar Annelids; but from its very simplicity it appears well
adapted to open the eyes of many who, perhaps, would rather not see,
and it may therefore find a place here. Three years ago I found on the
walls of one of my glasses some small worm-tubes (Fig. 65), the
inhabitants of which bore three pairs of barbate branchial filaments,
and had no operculum. According to this we should have been obliged to
refer them to the genus _Protula._ A few days afterwards one of the
branchial filaments had become thickened at the extremity into a
clavate operculum (Fig. 66), when the animals reminded me, by the
barbate opercular peduncle, of the genus _Filograna,_ only that the
latter possesses two opercula. In three days more, during which a new
pair of branchial filaments had sprouted forth, the opercular peduncle
had lost its lateral filaments (Fig. 67), and the worms had become
_Serpulæ._ Here the supposition at once presents itself that the
primitive tubicolar worm was a _Protula,_—that some of its descendants,
which had already become developed into perfect _Protulæ,_ subsequently
improved themselves by the formation of an operculum which might
protect their tubes from inimical intruders,—and that subsequent
descendants of these latter finally lost the lateral filaments of the
opercular peduncle, which they, like their ancestors, had developed.
What say the schools to this case? Whence and for what purpose, if the
_Serpulæ_ were produced or created as ready-formed species, these
lateral filaments of the opercular peduncle? To allow them to sprout
forth merely for the sake of an invariable plan of structure, even when
they must be immediately retracted again as superfluous, would
certainly be an evidence rather of childish trifling or dictatorial
pedantry, than of infinite wisdom. But no, I am mistaken; from the
beginning of all things the Creator knew, that one day the inquisitive
children of men would grope about after analogies and homologies, and
that Christian naturalists would busy themselves with thinking out his
Creative ideas; at any rate, in order to facilitate the discernment by
the former that the opercular peduncle of the _Serpulæ_ is homologous
with a branchial filament, He allowed it to make a detour in its
development, and pass through the form of a barbate branchial filament.
_The historical record preserved in developmental history is gradually_
EFFACED _as the development strikes into a constantly straighter course
from the egg to the perfect animal, and it is frequently_ SOPHISTICATED
_by the struggle for existence which the free-living larvæ have to
undergo._
Thus as the law of inheritance is by no means strict, as it gives room
for individual variations with regard to the form of the parents, this
is also the case with the succession in time of the developmental
processes. Every father of a family who has taken notice of such
matters, is well aware that even in children of the same parents, the
teeth, for example, are not cut or changed, either at the same age, or
in the same order. Now in general it will be useful to an animal to
obtain as early as possible those advantages by which it sustains
itself in the struggle for existence. A precocious appearance of
peculiarities originally acquired at a later period will generally be
advantageous, and their retarded appearance disadvantageous; the
former, when it appears accidentally, will be preserved by natural
selection. It is the same with every change which gives to the larval
stages, rendered multifarious by crossed and oblique characters, a more
straightforward direction, simplifies and abridges the process of
development, and forces it back to an earlier period of life, and
finally into the life of the egg.
As this conversion of a development passing through different young
states into a more direct one, is not the consequence of a mysterious
inherent impulse, but dependent upon advances accidentally presenting
themselves, it may take place in the most nearly allied animals in the
most various ways, and require very different periods of time for its
completion. There is one thing, however, that must not be overlooked
here. The historical development of a species can hardly ever have
taken place in a continuously uniform flow; periods of rest will have
alternated with periods of rapid progress. But forms, which in periods
of rapid progress were severed from others after a short duration, must
have impressed themselves less deeply upon the developmental history of
their descendants, than those which repeated themselves unchanged,
through a long series of successive generations in periods of rest.
These more fixed forms, less inclined to variation, will present a more
tenacious resistance in the transition to direct development, and will
maintain themselves in a more uniform manner and to the last, however
different may be the course of this process in other respects.
In general, as already stated, it will be advantageous to the young to
commence the struggle for existence in the form of their parents and
furnished with all their advantages—in general, but not without
exceptions. It is perfectly clear that a brood capable of locomotion is
almost indispensable to attached animals, and that the larvæ of
sluggish Mollusca, or of worms burrowing in the ground, etc., by
swarming briskly through the sea perform essential services by
dispersing the species over wider spaces. In other cases a
metamorphosis is rendered indispensable by the circumstance that a
division of labour has been set up between the various periods of life;
for example, that the larvæ have exclusively taken upon themselves the
business of nourishment. A further circumstance to be taken into
consideration is the size of the eggs,—a simpler structure may be
produced with less material than a more compound one,—the more
imperfect the larva, the smaller may the egg be, and the larger is the
number of these that the mother can furnish with the same expenditure
of material. As a rule, I believe indeed, this advantage of a more
numerous brood will not by any means outweigh that of a more perfect
brood, but it will do so in those cases in which the chief difficulty
of the young animals consists in finding a suitable place for their
development, and in which, therefore, it is of importance to disperse
the greatest possible number of germs, as in many parasites.
As the conversion of the original development with metamorphosis into
direct development is here under discussion, this may be the proper
place to say a word as to the already indicated absence of
metamorphosis in fresh-water and terrestrial animals the marine allies
of which still undergo a transformation. This circumstance seems to be
explicable in two ways. Either species without a metamorphosis migrated
especially into the fresh waters, or the metamorphosis was more rapidly
got rid of in the emigrants than in their fellows remaining in the sea.
Animals without a metamorphosis would naturally transfer themselves
more easily to a new residence, as they had only themselves and not at
the same time multifarious young forms to adapt to the new conditions.
But in the case of animals with a metamorphosis, the mortality among
the larvæ, always considerable, must have become still greater under
new than under accustomed conditions, every step towards the
simplification of the process of development must therefore have given
them a still greater preponderance over their fellows, and the effacing
of the metamorphosis must have gone on more rapidly. What has taken
place in each individual case, whether the species has immigrated after
it had lost the metamorphosis, or lost the metamorphosis after its
immigration, will not always be easy to decide. When there are marine
allies without, or with only a slight metamorphosis, like the Lobster
as the cousin of the Cray-fish, we may take up the former supposition;
when allies with a metamorphosis still live upon the land or in fresh
water, as in the case of _Gecarcinus,_ we may adopt the latter.
That besides this gradual extinction of the primitive history, a
_falsification_ of the record preserved in the developmental history
takes place by means of the struggle for existence which the
free-living young states have to undergo, requires no further
exposition. For it is perfectly evident that the struggle for existence
and natural selection combined with this, must act in the same way, in
change and development, upon larvæ which have to provide for
themselves, as upon adult animals. The changes of the larvæ,
independent of the progress of the adult animal, will become the more
considerable, the longer the duration of the life of the larva in
comparison to that of the adult animal, the greater the difference in
their mode of life, and the more sharply marked the division of labour
between the different stages of development. These processes have to a
certain extent an action opposed to the gradual extinction of the
primitive history; they increase the differences between the individual
stages of development, and it will be easily seen how even a
straightforward course of development may be again converted by them
into a development with metamorphosis. By this means many, and it seems
to me valid reasons may be brought up in favour of the opinion that the
most ancient Insects approached more nearly to the existing Orthoptera,
and perhaps to the wingless Blattidæ, than to any other order, and that
the “complete metamorphosis” of the Beetles, Lepidoptera, etc., is of
later origin. There were, I believe, perfect Insects before larvæ and
pupæ; but, on the contrary, Nauplii and Zoëæ far earlier than perfect
Prawns. In contradistinction to the _inherited_ metamorphosis of the
Prawns, we may call that of the Coleoptera, Lepidoptera, etc. an
_acquired_ metamorphosis.[2]
Which of the different modes of development at present occurring in a
class of animals may claim to be that approaching most nearly to the
original one, is easy to judge from the above statements.
_The primitive history of a species will be preserved in its
developmental history the more perfectly, the longer the series of
young states through which it passes by uniform steps; and the more
truly, the less the mode of life of the young departs from that of the
adults, and the less the peculiarities of the individual young states
can be conceived as transferred back from later ones in previous
periods of life, or as independently acquired._
Let us apply this to the Crustacea.
[1] Fig. 65 is drawn from memory, as the little animals, which I at
first took for young _Protulae,_ only attracted my attention when I
remarked the appearance of the operculum, which induced me to draw
them.
[2] I will here briefly give my reasons for the opinion that the
so-called “complete metamorphosis” of Insects, in which these animals
quit the egg as grubs or caterpillars, and afterwards become quiescent
pupæ incapable of feeding, was not inherited from the primitive
ancestor of all Insects, but acquired at a later period.
The order Orthoptera, including the Pseudoneuroptera (Ephemera,
_Libellula,_ etc.) appears to approach nearest to the primitive
form of Insects. In favour of this view we have:—
1. The structure of their buccal organs, especially the formation
of the labium, “which retains, either perfectly or approximately,
the original form of a second pair of maxillæ” (Gerstäcker).
2. The segmentation of the abdomen; “like the labium, the abdomen
also very generally retains its original segmentation, which is
shown in the development of eleven segments” (Gerstäcker). The
Orthoptera with eleven segments in the abdomen, agree perfectly in
the number of their body-segments with the Prawn-larva represented
in Fig. 33, or indeed, with the higher Crustacea (Podophthalma and
Edriophthalma) in general, in which the historically youngest last
thoracic segment (see Chapter 12), which is sometimes
late-developed, or destitute of appendages, or even deficient, is
still wanting.
3. That, as in the Crustacea, the sexual orifice and anus are
placed upon different segments; “whilst the former is situated in
the ninth segment, the latter occurs in the eleventh” (Gerstäcker).
4. Their palæontological occurrence; “in a fossil state the
Orthoptera make their appearance the earliest of all Insects,
namely as early as the Carboniferous formation, in which they
exceed all others in number” (Gerstäcker).
5. The absence of uniformity of habit at the present day in an
order so small when compared with the Coleoptera, Hymenoptera, etc.
For this also is usually a phenomenon characteristic of very
ancient groups of forms which have already overstepped the climax
of their development, and is explicable by extinction in mass. A
Beetle or a Butterfly is to be recognised as such at the first
glance, but only a thorough investigation can demonstrate the
mutual relationships of _Termes, Blatta, Mantis, Forficula,
Ephemera, Libellula,_ etc. I may refer to a corresponding
remarkable example from the vegetable world: amongst Ferns the
genera _Aneimia, Schizæa_ and _Lygodium,_ belonging to the group
_Schizæaceæ_ which is very poor in species, differ much more from
each other than any two forms of the group _Polypodiaceæ_ which
numbers its thousands of species.
If, from all this, it seems right to regard the Orthoptera as the
order of Insects approaching most nearly to the common primitive
form, we must also expect that their mode of development will agree
better with that of the primitive form, than, for example, that of
the Lepidoptera, in the same way that some of the Prawns (_Penéus_)
approaching most closely the primitive form of the Decapoda, have
most truly preserved their original mode of development. Now, the
majority of the Orthoptera quit the egg in a form which is
distinguished from that of the adult Insect almost solely by the
want of wings; these larvæ then soon acquire rudiments of wings,
which appear more strongly developed after every moult. Even this
perfectly gradual transition from the youngest larva to the
sexually mature Insect, preserves in a far higher degree the
picture of an original mode of development, than does the so-called
complete metamorphosis of the Coleoptera, Lepidoptera, or Diptera,
with its abruptly separated larva-, pupa- and imago-states.
The most ancient Insects would probably have most resembled these
wingless larvæ of the existing Orthoptera. The circumstance that
there are still numerous wingless species among the Orthoptera, and
that some of these (_Blattidæ_) are so like certain Crustacea
(Isopods) in habit that both are indicated by the same name
(“_Baratta_”) by the people in this country, can scarcely be
regarded as of any importance.
The contrary supposition that the oldest Insects possessed a
“complete metamorphosis,” and that the “incomplete metamorphosis”
of the Orthoptera and Hemiptera is only of later origin, is met by
serious difficulties. If all the classes of Arthropoda (Crustacea,
Insecta, Myriopoda and Arachnida) are indeed all branches of a
common stem (and of this there can scarcely be a doubt), it is
evident that the water-inhabiting and water-breathing Crustacea
must be regarded as the original stem from which the other
terrestrial classes, with their tracheal respiration, have branched
off. But nowhere among the Crustacea is there a mode of development
comparable to the “complete metamorphosis” of the Insecta, nowhere
among the young or adult Crustacea are there forms which might
resemble the maggots of the Diptera or Hymenoptera, the larvæ of
the Coleoptera, or the caterpillars of the Lepidoptera, still less
any bearing even a distant resemblance to the quiescent pupæ of
these animals. The pupæ, indeed, cannot at all be regarded as
members of an original developmental series, the individual stages
of which represent permanent ancestral states, for an animal like
the mouthless and footless pupa of the Silkworm, enclosed by a
thick cocoon, can never have formed the final, sexually mature
state of an Arthropod.
In the development of the Insecta we never see new segments added
to those already present in the youngest larvæ, but we do see
segments which were distinct in the larva afterwards become fused
together or disappear. Considering the parallelism which prevails
throughout organic nature between palæontological and embryonic
development, it is therefore improbable that the oldest Insects
should have possessed fewer segments than some of their
descendants. But the larva of the Coleoptera, Lepidoptera, etc.,
never have more than nine abdominal segments, it is therefore not
probable that they represent the original young form of the oldest
Insects, and that the Orthoptera, with an abdomen of eleven
segments, should have been subsequently developed from them.
Taking into consideration on the one hand these difficulties, and
on the other the arguments which indicate the Orthoptera as the
order most nearly approaching the primitive form, it is my opinion
that the “incomplete metamorphosis” of the Orthoptera is the
primitive one, _inherited_ from the original parents of all
Insects, and the “complete metamorphosis” of the Coleoptera,
Diptera, etc., a subsequently _acquired_ one.