THE SPECIES QUESTION
After the discovery of the ameba by Rösel v. Rosenhof and the
introduction of the Linnean system of nomenclature, the number of new
species of amebas that were reported increased rapidly. But in 1856
Carter suggested that what had been described as _A. radiosa_ probably
was a young stage of _A. proteus_. With the general acceptance of the
Darwinian Natural Selection Hypothesis, the ameba came to be looked upon
as standing at the bottom of the scale of organisms, and consequently
was supposed to lack generally such characters as the higher forms
possessed. The ameba became the representative of the “primordial slime”
from which by slow stages the other organisms were evolved. So of the
sixty odd species which had been described up to Leidy’s (’79) time,
Leidy, following the suggestion of Carter, was inclined to think that
the great majority of these represented only changes of shape of about
four species (not including the several species that were then known to
be parasitic). Since Leidy’s time the prevailing tendency has been to
regard most of the “new” species as mere environmental or life cycle
stages of a very few species. A very noted exception to this tendency,
however, has been Penard’s (’02) great work on the amebas and other
rhizopods of the Leman Basin, in which he describes forty-five species
of amebas (including _Gloidium_, _Protamoeba_, _Amoeba_, _Dinamoeba_,
_Pelomyxa_), paying attention mainly to the readily observed ectoplasmic
and endoplasmic characters, and the appearance of the resting nucleus.
The remarkable discoveries of Vahlkampf (’05) of the nuclear changes
during the division process turned the attention of numerous
investigators to this field, and the ectoplasmic and endoplasmic
characters thenceforward received scant attention. Thus Calkins (’04)
came to suggest as Carter had done many years before, that _A. radiosa_
was merely a young form of _A. proteus_. And Doflein (’07) intimated
that the protoplasmic characters of _vespertilio_ cannot be
distinguished from those of _verrucosa_, _radiosa_, _polypodia_, _limax_
and _guttula_. Schepotieff (’10) in a similar vein, writes: “Wir werden
demnach so bekannte und so lange Zeit als selbstständige und typische
Amöbenarten aufgefasste Formen wie _A. limax_, _A. polypodia_, und _A.
radiosa_ nur als Umwandlungsstadien andrer Arten bezeichnen dürfen.”
Gläser (’12) remarks: “The most reliable criterion for the
classification of the amebas is the division of the nucleus.” Calkins
(’12) takes the same view on this point and states that in his opinion
the ectoplasmic and endoplasmic characters of amebas conform to four
“types,” viz., _proteus_, _verrucosa_, _vespertilio_ and _limax_. The
enormous amount of work that has been done on the nuclear division
changes as compared with the small amount of work on the cytoplasmic
structure has thus naturally tended to an over-estimation of the
significance of the nuclear changes.
There are objections to making the nuclear changes the basis of the
classification of the amebas.
1. In the first place, to classify the amebas means not only labeling
the different species accurately, but also to assign to them their
proper place in the system of organisms. All organisms are classified
with this purpose in view. This is what is meant by a _natural system
of_ classification as contrasted with an _artificial system_ based on
only a part, arbitrarily selected, of each of the organisms concerned.
In the past all artificial systems have been discarded. It is perhaps
unnecessary to say that a classification based on nuclear characters
would be a highly artificial system. For in no group of organisms has it
been found possible thus far to use the nuclear changes as a basis of
classification. The great amount of labor that has been expended by
cytologists within recent years on the behavior of chromosomes, and the
immense amount of work done by the students of genetics, has failed to
show any specific relation whatever between the external characters of
organisms and the nuclear behavior.[3] In other words, the
peculiarities of mitotic processes have not been found to be correlated
with characters in the somatoplasm. It is to be remembered however that
all living organisms, with the exception of some of the bacteria, are
classified with respect to their external characters, and that in almost
all organisms the number of visible and demonstrable specific characters
becomes rapidly greater as ontogenetic development proceeds.
2. There is considerable disagreement among the investigators of the
nuclear phenomena of amebas as to the actual events occurring during the
division process. Cf. Dobell (’14) and Hartmann (’14) _in re Amoeba
lacertae_; Nägler (’09), Gläser (’12) and Wilson (’16) on the presence
of a centriole in amebas; etc. The work of Schardinger (’99), Wherry
(’13) and Wilson (’16) on the nuclear stages of amebas was done with
care, yet Wilson (’16) still remains in doubt as to whether or not these
investigators all worked on the same species.
3. Awerinzew (’04, ’06) found that the nuclear changes in _Amoeba
proteus_ are similar to those in the heliozoan _Actinosphaerium_; there
being thus greater correspondence in the nuclear changes between species
belonging to different orders than there is between species in the same
genus. Logically therefore _Actinosphaerium_ would have to be placed in
the same genus with _Amoeba proteus_.
4. There is the great practical objection that in many of the larger
species it is extremely difficult to find suitable division stages even
though thousands of individuals are at hand, and the search is continued
for days and weeks by an experienced investigator (Dobell, ’10).
Experimental work, which is usually done with one of the larger species,
would thus be greatly handicapped because of the great difficulty in
determining the nature of the organism employed.
From these considerations it appears that the attempt to classify the
amebas on the basis of the nuclear changes is highly artificial and
exceptional, and if we may judge from past attempts to classify
organisms on the basis of a single character, is foredoomed to failure.
This conclusion does not apply, however, to very minute amebas in which
no specific cytoplasmic characters have yet been established, chiefly
because of their very minuteness; such amebas could be given specific
names for reference but they could not be classified in a natural system
excepting perhaps as a group.
But the definiteness and the consistency with which the nuclear division
stages occur in any given species of ameba, lends support to the
probability that in these animals the relation existing between the
chromatin and the cytoplasm are similar to those observed in higher
animals; and that the laws governing the transmission of cytoplasmic
characters in amebas are quite as inflexible as those governing
somatoplasmic characters in the higher organisms. Among the
investigators of cytologic and genetic phenomena (among the
multicellulars) the belief is practically unanimous that the elaborate
mechanism involved in nuclear division is primarily a design for
distributing the factors concerned in heredity. Now it would be very
strange indeed if a similar and quite as complicated a mechanism in
ameba had no function to perform. For what would be the purpose of the
complicated nuclear changes in ameba if not concerned with heredity? As
has already been seen, however, there are numerous cytoplasmic
characters, in the larger amebas at least, that are inherited from one
generation to the next with as little variation as is observed in other
organisms (Schaeffer, ’16). The recent work of Jennings (’16) on
_Difflugia_ and Hegner (’18) on _Arcella_ also indicates that the
general processes of inheritance in these organisms which are closely
related to amebas, are similar to those observed in higher forms. The
conclusion seems justified therefore that the nuclear changes in amebas
mean essentially the same thing as in other organisms.
We are now therefore in a position to say that amebas are definitely and
thoroughly organized; that they are not really “shapeless”; that they
are not more subject to variation than a higher organism is; and that
each species differs from all others in probably every visible detail.
The large variety of pseudopods observed in different species are seen
not to be the result of physical or extrinsic chemical forces acting
upon ectoplasms differing in some mere physical character as viscosity.
But all these peculiarities are hereditary, and are due to a fundamental
chemical structure of the protoplasm which is specific for the species.
The highly characteristic nature of the pseudopods formed by the amebas
of any species, it is seen, is to be referred to the fundamental
structure of the protoplasm, probably its stereochemical structure. And
what is of especial importance for this discussion, the character of
streaming concerned with pseudopod formation and with movement in
general, which is specific for each species, is likewise found, to some
extent at least, to be conditioned by the specific structure of the
protoplasm.
That the specific character of the pseudopods, and the streaming which
of course lies back of it, is not wholly or perhaps even largely, due to
the specific structure of the protoplasm, is evident from a
consideration of streaming in some other organisms, without a study of
which, streaming in amebas can be only imperfectly understood.
The formation of pseudopods is not necessary to streaming. Occasionally
one sees internal currents unaccompanied by movement or ectoplasm
formation in amebas approximating spherical shape, such as in _Amoeba
blattae_ (Rhumbler, ’98) and rarely also in _proteus_ or _dubia_. But
especially well is such streaming seen in a contracted _Biomyxa_, a
naked foraminifer, and in numerous plant cells. In paramecium and other
ciliates the continuous circulation of the endoplasm,--a true streaming
process,--is an involuntary act. But in _Frontonia_, another large
ciliate, the circulation of the endoplasm is under the control of the
animal, that is to say, voluntary, and is set in motion only when
feeding, the direction of streaming being away from the mouth so as to
drag in the food (see Figure 32, p. 99). If the food particle is a long
filament of _Oscillatoria_, for example, the endoplasm circulates very
much as it does in paramecium, only more rapidly, until the whole
filament is wound up into a coil. Then streaming stops. In the second
place streaming is not necessarily accompanied by the formation of
ectoplasm as observed in ameba. In plant cells the ectoplasm is
practically stationary, while the endoplasm is in continual flux. The
transformation of endoplasm into ectoplasm and vice versa is therefore
not an essential feature of streaming, though it is of locomotion; that
is, ectoplasm is always found between endoplasm and water, though it
might be possible under certain conditions for endoplasm to come into
contact with water without stiffening. And if so, there appears to be
no reason why locomotion might not occur. It appears however under
normal conditions that a moderate tendency to ectoplasm formation
(_proteus, dubia_) leads to greater efficiency in movement than a very
weak (_limicola_) or a very strong (_verrucosa_) tendency to form
ectoplasm.
In the reticulose rhizopods, as is well known there is no ectoplasm of
the kind observed in amebas. The middle of the pseudopod, moreover, is
not the region of most rapid streaming as in ameba, but frequently
becomes congealed, on the contrary, into a rod-like structure. In
general this axial rod has the character of very stiff ectoplasm. The
character of streaming in reticulose rhizopods, however, has received
very little attention, and detailed comparisons are therefore
impossible.
Another interesting property of reticulose pseudopods, which are formed
by a streaming process, is their great power in some species, of rapid
contraction. If a diatom for example, in its movements breaks loose a
pseudopod it is often (though not necessarily) contracted very rapidly,
much more rapidly than could be the case if it were accomplished by
streaming. It frequently happens that knobs are found on a slender
pseudopod. These knobs may move back and forth with great rapidity
without visibly affecting the pseudopod (Figure 12). The process reminds
one of a block sliding on a rope. These observations indicate a very
high degree of elasticity in the formed pseudopods of such a rhizopod as
_Biomyxa_ as compared with a very low degree of elasticity in the
amebas.
It thus appears that the process of streaming is a much more fundamental
phenomenon than most of the theories accounting for ameboid movement
would lead one to suppose; for these theories concern themselves only
with streaming as observed in amebas, and many content themselves with
only two or three species. Since the general features of streaming are
similar no matter where streaming occurs, no theory is likely to gain
acceptance that explains streaming only in one group of organisms.
Streaming in rhizopods, myxomycetes, ciliates, plant cells, is most
rationally looked upon as caused by the same fundamental process; but
the detailed form it takes, especially in freely formed pseudopods, is
undoubtedly conditioned by the structure of the protoplasm, both
physically and chemically, but more especially the latter.
[Illustration: Figure 12. Illustrating the high degree of elasticity in
the pseudopods of _Biomyxa vagans_. In _a_ and _b_ are shown two stages
of a small section of the pseudopodial network, which remained unchanged
while a small lump of protoplasm (near the arrow) moved rapidly up and
down the slender pseudopod. Movement along the whole length of the
pseudopod occupied about half a second. Just exactly what the movement
was due to could not be determined, but the distance between the forks
in the pseudopod did not change, nor did the thickness of the
protoplasmic strand on which the protoplasmic lump moved change
noticeably.]