ON THE PRINCIPLES OF CLASSIFICATION.
Perhaps some one else, more fortunate than myself, may be able, even
without Darwin, to find the guiding clue through the confusion of
developmental forms, now so totally different in the nearest allies,
now so surprisingly similar in members of the most distant groups,
which we have just cursorily reviewed. Perhaps a sharper eye may be
able, with Agassiz, to make out “the plan established from the
beginning by the Creator,”[1] who may have written here, as a
Portuguese proverb says “straight in crooked lines.”[2] I cannot but
think that we can scarcely speak of a general plan, or typical mode of
development of the Crustacea, differentiated according to the separate
Sections, Orders, and Families, when, for example, among the Macrura,
the River Crayfish leaves the egg in its permanent form; the Lobster
with Schizopodal feet; _Palæmon,_ like the Crabs, as a Zoëa; and
_Penéus,_ like the Cirripedes, as a Nauplius,—and when, still, within
this same sub-order Macrura, _Palinurus, Mysis_ and _Euphausia_ again
present different young forms,—when new limbs sometimes sprout forth as
free rudiments on the ventral surface, and are sometimes formed beneath
the skin which passes smoothly over them, and both modes of development
are found in different limbs of the same animal and in the same pair of
limbs in different animals,—when in the Podophthalma the limbs of the
thorax and abdomen make their appearance sometimes simultaneously, or
sometimes the former and sometimes the latter first, and when further
in each of the two groups the pairs sometimes all appear together, and
sometimes one after the other,—when, among the Hyperina, a simple foot
becomes a chela in _Phronima_ and a chela a simple foot in
_Brachyscelus,_ etc.
And yet, according to the teaching of the school, it is precisely in
youth, precisely in the course of development, that the “Type” is
mostly openly displayed. But let us hear what the Old School has to
tell us as to the significance of developmental history, and its
relation to comparative anatomy and systematic zoology.
Let two of its most approved masters speak.
“Whilst comparative anatomy,” said Johannes Müller, in 1844, in his
lectures upon this science (and the opinions of my memorable teacher
were for many years my own), “whilst comparative anatomy shows us the
infinitely multifarious formation of the same organ in the Animal
Kingdom, it furnishes us at the same time with the means, by the
comparison of these various forms, of recognising the truly essential,
the type of these organs, and separating therefrom everything
unessential. In this, developmental history serves it as a check or
test. Thus, as the idea of development is not that of mere increase of
size, but that of progress from what is not yet distinguished, but
which potentially contains the distinction in itself, to the actually
distinct,—it is clear, that the less an organ is developed, so much the
more does it approach the type, and that, during its development, it
more and more acquires peculiarities. The types discovered by
comparative anatomy and developmental history must therefore agree.”
Then, after Johannes Müller has combated the idea of a graduated scale
of animals, and of the passage through several animal grades during
development, he continues:—“What is true in this idea is, that every
embryo at first bears only the type of its section, from which the type
of the Class, Order, etc., is only afterwards developed.”
In 1856, in an elementary work,[3] in which it is usual to admit only
what are regarded as the assured acquisitions of science, Agassiz
expresses himself as follows:—
“_The ovarian eggs of all animals are perfectly identical,_ small cells
with a vitellus, germinal vesicle and germinal spot” (§ 278). “_The
organs of the body are formed in the sequence of their organic
importance; the most essential always appear first._ Thus the organs of
vegetative life, the intestine, etc., appear later than those of animal
life, the nervous system, skeleton, etc., and these in turn are
preceded by the more general phenomena belonging to the animal as such”
(§ 318). “Thus, in Fishes, the first changes consist in the
segmentation of the vitellus and the formation of a germ, processes
which are common to all classes of animals. Then the dorsal furrow,
characteristic of the Vertebrate, appears—the brain, the organs of the
senses; at a later period are formed the intestine, the limbs, and the
permanent form of the respiratory organs, from which the class is
recognised with certainty. It is only after exclusion that the
peculiarities of the structure of the teeth and fins indicate the genus
and species” (§ 319). “_Hence the embryos of different animals resemble
each other the more, the younger they are_” (§ 320). “Consequently the
high importance of developmental history is indubitable. For, _if the
formation of the organs takes place in the order corresponding to their
importance, this sequence must of itself be a criterion of their
comparative value in classification._ The peculiarities which appear
earlier should be considered of higher value than those which appear
subsequently” (§ 321). “_A system, in order to be true and natural,
must agree with the sequence of the organs in the development of the
embryo_” (§ 322).
I do not know whether any one at the present day will be inclined to
subscribe to this proposition in its whole extent.[4] It is certain,
however, that views essentially similar are still to be met with
everywhere in discussions on classification, and that even within the
last few years, the very sparingly successful attempts to employ
developmental history as the foundation of classification have been
repeated.
But how do these propositions agree with our observations on the
developmental history of the Crustacea? That these observations relate
for the most part to their “free metamorphosis” after their quitting
the egg, cannot prejudice their application to the propositions
enunciated especially with regard to “embryonal development” in the
egg; for Agassiz himself points out (§ 391) that both kinds of change
are of the same nature and of equal importance and that no “radical
distinction” is produced by the circumstance that the former take place
before and the latter after birth.
“_The ovarian eggs of all animals are identical,_ small cells with
vitellus, germinal vesicle and germinal spot.” Yes, somewhat as all
Insects are identical, small animals with head, thorax, and abdomen;
that is to say if, only noticing what is common to them, we leave out
of consideration the difference of their development, the presence or
absence and the multifarious structure of the vitelline membrane, the
varying composition of the vitellus, the different number and formation
of the germinal spots, etc. Numerous examples, which might easily be
augmented, of such profound differences, are furnished by Leydig’s
‘Lehrbuch der Histologie.’ In the Crustacea the ovarian eggs actually
sometimes furnish excellent characters for the discrimination of
species of the same genus; thus, for example, in one _Porcellana_ of
this country they are blackish-green, in a second deep blood-red, and
in a third dark yellow; and within the limits of the same order they
present considerable differences in size, which, as Van Beneden and
Claus have already pointed out, stands in intimate connexion with the
subsequent mode of development.
“_The organs of the body are formed in the sequence of their organic
importance; the most essential always appear first._” This proposition
might be characterised _à priori_ as undemonstrable, since it is
impossible either in general, or for any particular animal, to
establish a sequence of importance amongst equally indispensable parts.
Which is the more important, the lung or the heart—the liver or the
kidney?—the artery or the vein? Instead of giving the preference, with
Agassiz, to the organs of animal life, we might with equal justice give
it to those of vegetative life, as the latter are conceivable without
the former, but not the former without the latter. We might urge that,
according to this proposition, provisional organs as the first produced
must exceed the later-formed permanent organs in importance.
But let us stick to the Crustacea. In _Polyphemus_ Leydig finds the
first traces of the intestinal tube even during segmentation. In
_Mysis_ a provisional tail is first formed, and in _Ligia_ a
maggot-like larva-skin. The simple median eye appears earlier, and
would therefore be more important than the compound paired eyes; the
scale of the antennæ in the Prawns would be more important than the
flagellum; the maxillipedes of the Decapoda would be more important
than the chelæ and ambulatory feet, and the anterior six pairs of feet
in the Isopoda, than the precisely similarly formed seventh pair; in
the Amphipoda the most important of all organs would be the “micropylar
apparatus,” which disappears without leaving a trace soon after
hatching; in _Cyclops_ the setæ of the tail would be more important
than all the natatory feet; in the Cirripedia the posterior antennæ, as
to which we do not know what becomes of them, would be more important
than the cirri, and so forth. The most unimportant of all organs would
be the sexual organs, and the most essential peculiarity would consist
in colour, which is to be referred back to the ovarian egg.
“_The embryos, or young states of different animals, resemble each
other the more, the younger they are,_” or, as Johannes Müller
expresses it, “_they approach the more closely to the common type._”
Different as may be the ideas connected with the word “type,” no one
will dispute that the typical form of the penultimate pair of feet in
the Amphipoda is that of a simple ambulatory foot, and not that of a
chela, for the latter occurs in no single adult Amphipod; we know it
only in the young of the genus _Brachyscelus,_ which therefore in this
respect undoubtedly depart more widely than the adults from the type of
their order. This applies also to the young males of the Shore-hoppers
(_Orchestia_) with regard to the second pair of anterior feet
(_gnathopoda_). In like manner no one will hesitate to accept the
possession of seven pairs of feet as a “typical” peculiarity of the
Edriophthalma, which Agassiz, on this account, names Tetradecapoda; the
young Isopoda, which are Dodecapoda, are also in this respect further
from the “type” than the adults.
It is certainly a rule, and this Darwin’s theory would lead us to
expect, that in the progress of development those forms which are at
first similar gradually depart further from each other; but here, as in
other classes, the exceptions, for which the Old School has no
explanation, are numerous. Not unfrequently we might indeed directly
reverse the proposition and assert that the difference becomes the
greater, the further we go back in the development, and this not only
in those cases in which one of two nearly allied species is directly
developed, and the other passes through several larval stages, such as
the common Crayfish and the Prawns which are produced from
Nauplius-brood. The same may be said, for example, of the Isopoda and
Amphipoda. In the adult animals the number of limbs is the same; at the
first sight of a _Cyrtophium_ or a _Dulichia,_ and even after the
careful examination of a _Tanais,_ we may be in doubt whether we have
an Isopod or an Amphipod before us; in the newly-hatched young the
number of limbs is different, and if we go back to their existence in
the egg, the most passing glance to see whether the curvature is
upwards or downwards suffices to distinguish even the youngest embryos
of the two orders.
In other instances, the courses which lead from a similar
starting-point to a similar goal, separate widely in the middle of the
development, as in the Prawns with Nauplius-brood already described.
Finally, so that even the last possibility may be exhausted, it
sometimes happens that the greatest similarity occurs in the middle of
the development. The most striking example of this is furnished by the
Cirripedia and Rhizocephala, whether we compare the two orders or the
members of each with one another; from a segmentation quite different
in its course (see Figs. 61–64) proceed different forms of Nauplius,
these become converted into exceedingly similar pupæ, and from the pupæ
again proceed sexually mature animals, differing from each other _toto
cœlo._
“_If the formation of the organs occurs in the order corresponding to
their importance, this sequence must of itself be a criterion of their
comparative value in classification._” THAT IS TO SAY, SUPPOSING THE
PHYSIOLOGICAL AND CLASSIFICATIONAL VALUE OF AN ORGAN TO COINCIDE! Just
as in Christian countries there is a catechismal morality, which every
one has upon his lips, but no one considers himself bound to follow, or
expects to see followed by anybody else, so also has Zoology its
dogmas, which are as universally acknowledged, as they are disregarded
in practice. Such a dogma as this is the supposition tacitly made by
Agassiz. Of a hundred who feel themselves compelled to give their
systematic confession of faith as the introduction to a Manual or
Monographic Memoir, ninety-nine will commence by saying that a natural
system cannot be founded upon a single character, but that it has to
take into account all characters, and the general structure of the
animal, but that we must not simply sum up these characters like
equivalent magnitudes, that we must not count but weigh them, and
determine the importance to be ascribed to each of them according to
its physiological significance. This is probably followed by a little
jingle of words in general terms on the comparative importance of
animal and vegetative organs, circulation, respiration, and the like.
But when we come to the work itself, to the discrimination and
arrangement of the species, genera, families, etc., in all probability
not one of the ninety-nine will pay the least attention to these fine
rules, or undertake the hopeless attempt to carry them out in detail.
Agassiz, for example, like Cuvier, and in opposition to the majority of
the German and English zoologists, regards the Radiata as one of the
great primary divisions of the Animal Kingdom, although no one knows
anything about the significance of the radiate arrangement in the life
of these animals, and notwithstanding that the radiate Echinodermata
are produced from bilateral larvæ. The “true Fishes” are divided by him
into Ctenoids and Cycloids, according as the posterior margin of their
scales is denticulated or smooth, a circumstance the importance of
which to the animal must be infinitely small, in comparison to the
peculiarities of the dentition, formation of the fins, number of
vertebræ, etc.
And, to return to our Class of the Crustacea, has any particular
attention been paid in their classification to the distinctions
prevailing in the “most essential organs”? For instance, to the nervous
system? In the Corycæidæ, Claus found all the ventral ganglia fused
together into a single broad mass, and in the Calanidæ a long ventral
chain of ganglia,—the former, therefore, in this respect resembling the
Spider Crabs and the latter the Lobster; but no one would dream on this
account of supposing that there was a relationship between the
Corycæidæ and the Crabs, or the Calanidæ and the Lobsters.—Or to the
organs of circulation? We have among the Copepoda, the Cyclopidæ and
Corycæidæ without a heart, side by side with the Calanidæ and
Pontellidæ with a heart. And in the same way among the Ostracoda, the
_Cypridinæ,_ which I find possess a heart, place themselves side by
side with _Cypris_ and _Cythere_ which have no such organ.—Or to the
respiratory apparatus? Milne-Edwards did this when he separated _Mysis_
and _Leucifer_ from the Decapoda, but he himself afterwards saw that
this was an error. In one _Cypridina_ I find branchiæ of considerable
size, which are entirely wanting in another species, but this does not
appear to me to be a reason for separating these species even
generically.
On the other hand, what do we know of the physiological significance of
the number of segments, and all the other matters which we are
accustomed to regard as typical peculiarities of the different organs,
and to which we usually ascribe the highest systematic value?
“_Those peculiarities which first appear, should be more highly
estimated than those which appear subsequently. A system, in order to
be true and natural, must agree with the sequence of the organs in the
development of the embryo._” If the earlier manifested peculiarities
are to be estimated more highly than those which afterwards make their
appearance, then in those cases in which the structure of the adult
animal requires one position in the system, and that of the larva
another, the latter and not the former must decide the point. As the
_Lernææ_ and Cirripedes, on account of their Nauplius-brood, were
separated from their previous connexions and referred to the Crustacea,
we shall, for the same reason, have to separate _Penëus_ from the
Prawns and unite it with the Copepoda and Cirripedia. But the most
zealous embryomaniac would probably shrink from this course.
A “true and natural system” of the Crustacea to be in accordance with
the sequence of the phenomena would have to take into account in the
first place the various modes of segmentation, then the position of the
embryo, next the number of limbs produced within the egg and so forth,
and might be represented somewhat as follows:—
CLASSIS CRUSTACEA.
Sub-class I. HOLOSCHISTA.—Segmentation complete. No primitive band.
Nauplius-brood.
Ord. 1. _Ceratometopa._)—Nauplius with frontal horns. (Cirripedia,
Rhizocephala.)
Ord. 2. LEIOMETOPA.—Nauplius without frontal horns. (Copepoda, without
_Achtheus,_ etc., Phyllopoda, _Penëus._)
Sub-class II. HEMISCHISTA.—Segmentation not complete.
A. Nototropa.—Embryo bent upwards.
Ord. 3. _Protura._—The tail is first formed. (_Mysis._)
Ord. 3. _Saccomorpha._—A maggot-like larva-skin is first formed.
(_Isopoda._)
B. Gasterotropa.—Embryo bent ventrally.
Ord. 5. _Zoëogona._—Full number of limbs not produced in the egg.
Zoëa-brood. (The majority of the Podophthalmata.)
Ord. 6. _Ametabola._—Full number of limbs produced in the egg.
(_Astacus, Gecarcinus, Amphipoda_ less _Hyperia ?_)
This sample may suffice. The farther we go into details in this
direction, the more brilliantly, as may easily be imagined, does the
naturalness of such an arrangement as this force itself upon us.
All things considered, we may apply the judgment which Agassiz
pronounced upon Darwin’s theory, with far greater justice to the
propositions just examined:—“No theory,” says he, “however plausible it
may be, can be admitted in science, unless it is supported by facts.”
[1] “A plan fully matured in the beginning and undeviatingly pursued;”
or “In the beginning His plan was formed and from it He has never
swerved in any particular” (Agassiz and Gould, ‘Principles of
Zoology’).
[2] “Deos escrive direito em linhas tortas.” To read this remarkable
writing we need the spectacles of Faith, which seldom suit eyes
accustomed to the Microscope.
[3] ‘Principles of Zoology’ Part I. Comparative Physiology. By Louis
Agassiz and A.A. Gould. Revised Edition. Boston, 1856.
[4] Agassiz’ own views have lately become essentially different, so
far as can be made out from Rud. Wagner’s notice of his ‘Essay on
Classification.’ Agassiz himself does not attempt any criticism of the
above cited older views, which, however, are still widely diffused.
With his recent conception I am unfortunately acquainted only from R.
Wagner’s somewhat confused report, and have therefore thought it
better not to attempt any critical remarks upon it.