species, with the increase of infertility between them,
would be greatly assisted by two other important factors
in the problem. It has already been shown that, with each
modification of form and habits, and especially with
modifications of color, there arises a disinclination of the
two forms to pair together; and this would produce an amount
of isolation which would greatly assist the specialisation of
the forms in adaptation to their different conditions of life.
Again, evidence has been adduced that change of conditions
or of mode of life is a potent cause of disturbance of the
reproductive system, and, consequently, of infertility. We may
therefore assume that, as the two forms adopted more and more
different modes of life, and perhaps acquired also decided
peculiarities of form and coloration, the infertility between
them would increase or become more general; and as we have
seen that every such increase of infertility would give that
portion of the species in which it arose an advantage over
the remaining portions in which the two varieties were more
fertile together, all this induced infertility would maintain
itself, and still further increase the general infertility
between the two forms of the species."
Here we perceive that Mr. Wallace, after having adopted the theory
of physiological selection in its main elements, next proceeds to
supplement that theory (as Mr. Gulick and myself had previously done),
by showing how greatly the principle of physiological selection
must be assisted by any association with other forms of isolation,
or segregate breeding. The only difference between Mr. Wallace and
ourselves here is, that while he instances but three or four forms of
segregate breeding (or homogamy) with which physiological selection
may be associated, I had previously considered several others in
addition to these, while Mr. Gulick had gone into the matter still
more exhaustively. Therefore, here as elsewhere, I can only account
for the character of Mr. Wallace's criticism by supposing that he read
our papers inattentively in the first instance, and was afterwards
influenced by "unconscious memory" in his subsequent cogitations upon
the problem of cross-sterility.
And now, finally, in order to show this still more completely, I may
quote the whole paragraph which concludes his long discussion of that
problem:
"The preceding argument, it will be seen, depends entirely
upon the assumption that some amount of infertility
characterises the distinct varieties which are in process of
differentiation into species; and it may be objected that of
such infertility there is no proof. This is admitted: but
it is urged that facts have been adduced which render such
infertility probable, at least in some cases, and this is
all that is required. It is by no means necessary that all
varieties should exhibit incipient infertility, but only some
varieties; for we know that, of the innumerable varieties
that occur, but few become developed into distinct species;
and it may be that the absence of infertility, to obviate the
effects of intercrossing, is one of the usual causes of their
failure. All I have attempted to show is, that when incipient
infertility does occur in correlation with other varietal
differences, that infertility can be, and in fact must be,
increased by natural selection; and this, it appears to me, is
a decided step in advance in the solution of the problem."
This serves to convey a very accurate summary of the whole "preceding
argument"; and it is likewise an admirably concise restatement of the
theory of physiological selection. The only points in it to which I
object—considered as an epitome of my own paper—are as follows.
First, Mr. Wallace has not proved quite so good an advocate as he might
have proved, had he looked more closely into the evidence "that some
amount of infertility characterises the distinct varieties which are
in process of differentiation into species." For although he says,
properly enough, that his "preceding argument"—i. e., the theory of
physiological selection—"depends entirely upon the assumption" that
such infertility does "characterise distinct varieties which are in
process of differentiation into species"; still he is wrong in saying
it is "admitted" that in favor of this assumption there is "no proof"
beyond what he has himself "urged" in the way of "facts which render
such infertility probable": there are many other facts which not only
render such infertility probable, but prove it to be actual. Secondly,
although I quite agree with Mr. Wallace in holding that natural
selection must often, as I said in my original paper, "co-operate" with
physiological selection, still I must point out that the particular
form of segregate breeding to which he here alludes is not natural
selection at all; but (as explained in the foot-note to page 15)
physiological selection pure and simple. My objections, however, with
regard to these two points have no reference to the validity of Mr.
Wallace's restatement of my views; and the fact that this restatement
has been given with the most incomprehensible unconsciousness that
it is a restatement, does not appear to me to detract from the
significance of the argumentative suicide in which his entire criticism
is thus found to terminate.[7]
[7] I am the more surprised that Mr. Wallace did not perceive
his almost complete adoption of my views in this latest
publication of his own, because I had previously had occasion
to point out a partial adoption of them in an earlier
publication of his on the same subject. The following is what
I said upon that occasion—viz., in the _Nineteenth Century_,
January, 1888:
"One very obvious and probably frequent instance of what may
be termed collective variation in the reproductive system—or
a variation due to a common cause acting on many individuals
simultaneously—is actually quoted from my paper by Mr.
Wallace himself, namely, changes in the season of flowering
or of pairing, which insure that any section of a species
so affected shall be fertile only within itself. Collective
variation of this kind may be directly due to the incidence
of some common cause, such as changed conditions of life with
respect to food, climate, station, etc.; or, as in the case
of bud-variation, it may be due to a single "sport" affecting
all the blossoms growing upon the same branch. But besides
such direct action of a common cause, it is easy to see
that natural selection, use and disuse, etc., by operating
in the production of organic changes elsewhere, may not
unfrequently react on the sexual system indirectly, and so
induce the sexual change required in a number of individuals
simultaneously."
Now, in his _Darwinism_, Mr. Wallace again reproduces this
instance of "physiological selection," without even yet
appearing to perceive that both in my original paper upon the
subject and in my answer to his criticism as above quoted, I
adduce this particular instance of physiological selection
as a typical one. Therefore, when he now says:—"Another mode
of isolation is brought about by the variety—either owing
to habits, climate, or constitutional change—breeding at a
slightly different time from the parent species: this is known
to produce complete isolation in the case of many varieties of
plants": he is merely restating what I have repeatedly given
as an unquestionable case of physiological selection.
With the self-destruction of this criticism I am left without any other
to answer; and I should not have occupied so much space in dealing
with this one, were it not that the high estimation in which Mr.
Wallace is so deservedly held by all other naturalists is calculated
to render almost incredible the peculiar position to which he has
eventually gravitated with reference to my views—professing hostility
on the one hand, while reproducing them as original on the other. The
misunderstanding of my ideas which this state of matters represents,
might have led me to wonder whether I could possibly have rendered
my meaning more clear in the first instance, were it not that this
misunderstanding extends in an even greater measure to Mr. Gulick's
paper than it does to mine. For seeing that the whole criticism is
founded on the erroneous idea that our theory supposes physiological
selection always to act alone, the misconception becomes positively
ludicrous in its relation to Mr. Gulick's views; seeing that, as
previously stated, Mr. Gulick not only agrees with me in holding that
physiological selection must be greatly fortified by being associated
with any other form of homogamy, but even goes so far as to agree with
Mr. Wallace that, unless it is so fortified, it can never act at all.
So that, as far as physiological selection is concerned, Mr. Gulick's
theory is precisely identical with that of Mr. Wallace, and differs
from his statement of it only in recognising a number of forms of
homogamy, in addition to natural selection, sexual selection, etc.,
with which the principle of physiological selection may be associated.
GEORGE J. ROMANES.
THE IMMORTALITY OF INFUSORIA.
The ingenious hypothesis that Weismann, the eminent Freiburg professor,
promulgated several years ago regarding the vitality of all unicellular
beings, but more especially of the Protozoans, is undoubtedly widely
known. Weismann maintained that the Protozoans were distinguished from
the Metazoans, or organisms composed of a number of cells, by the
curious property they possessed of exemption from decay and death.
The Protozoans exhibited, in the very words of the German savant,
an instance of potential immortality;[8] that is to say, a natural
physiological death did not exist for them; if they perished, it was
by accident or chance, extraneous to the laws of their organisation.
A great many authors have written upon this subject since Weismann,
either in support of his opinion, or in refutation of it, and of them
we may mention principally Goette,[9] Minot,[10] and M. Delboeuf.[11]
[8] _Ueber die Dauer des Lebens._ Jena, 1882.
[9] _Ueber den Ursprung des Todes_, 1883.
[10] _La Mort et l'Individualité._ (_Bulletin Scientifique du
Nord_, 1884-85.)
[11] _La Matière Brute et la Matière Vivante._ Paris 1887.
It is to be observed that this idea of potential immortality is not
the exclusive property of Weismann. We find it clearly indicated by
Ehrenberg. And, moreover, as Bütschli remarks, it is so natural that
it ought to occur of itself to the mind of every tolerably thoughtful
observer that has devoted his time to the study of the biology of these
minute creatures.[12]
[12] _Gedanken über Leben und Tod_ (_Zoologische Anzeige_,
Vol. v, 1882), cited by M. Maupas in _Multiplication des
Infusoires Ciliés—Arch. de Zool. Experimen._, No. 2, 1888.
Weismann founded his theory in part upon metaphysical, or at least
theoretical, considerations, which we deem it useless to discuss at
this point. But it is also supported by observed facts, and these facts
it will be profitable to recapitulate from the very onset. The idea
of the immortality of Infusoria occurs naturally to the mind when one
examines with care what happens when an Infusorian reproduces. We know
that the reproduction consists in a bipartition of the body of the
animal, and that consequently the parent does not die but lives in the
two products of its bipartition. In subsequent multiplications the same
phenomenon is always observed to occur, so that the entire substance
of the parent is found preserved and living in the individuals to
which it gives birth. This process Weismann expressed by the emphatic
statement: In multiplication by division there are no corpses. It is
wholly otherwise with the metazoans, and the reason of this fundamental
difference is easily explained by the comparison of the organisation
of the body of a metazoan with that of a protozoan. Whereas the
protozoan is represented by a single cell that comprehends all the
vital functions, the functions of reproduction as well as those of
nutrition and relation, the metazoan, on the other hand, is composed of
an aggregation, of a colony of distinct cells, among which a division
of labor has been effected varying in complexity with the height that
the animal has attained in the classificatory scale. It results from
this division of labor that in the metazoan certain cells only—those
namely which are called the sexual cells—are entrusted with the office
of the conservation of the species, while the various other cells are
more especially adapted to the conservation of the individual. When
a metazoan reproduces, the sexual cells alone enter into activity,
and after having suffered various modifications, the principal one of
which is fecundation, the sexual cells become the seat of numerous
segmentations that go to constitute a new animal distinct from the
one that gave it birth. The moment the parent individual ceases to
be blended with the individual it produces, it can perish without
imperilling the conservation of the species, and thus it is that death
appears in the animal kingdom as a logical consequence of division of
labor.
We also know that Weismann, in developing these interesting facts,
was led with many other naturalists to establish the doctrine that
every metazoan may be considered as made up of two entirely distinct
groups of cells: 1) of somatic cells, which represent the individual,
and which are invested with the care of its nourishment, its
sense-mechanism, its movements, and all the functions that have to do
with individual life; and 2) of sexual cells, charged with the office
of the maintenance of the species in time. Whereas the somatic cells
are destined to perish, the sexual cells on the contrary, multiplying
by division after the mode of the reproduction of micro-organisms,
represent the protozoan type, which is immortal; and, by the
intermediary agency of the fecundated ovum, the sexual cells pass
from generation to generation, thus forming a material bond between
successive generations. Though we have to succumb to death, there is
at least a portion of us that ought not to die, from the fact that it
is transmittible to our descendants. Naegeli expressed this idea in a
felicitous form, when he compared the species to a creeping branch that
sent out at successive points annual buds. The buds, which die, are the
individuals—that is the somatic group; while the branch that survives
after the death of the buds, and which represents the species, is the
system of sexual cells. Weismann, finally, has described the same
phenomenon by the expression 'continuity of the germinative plasm.'
A great many discussions have arisen with regard to this germinative
plasm; for everything touches upon this domain, and Weismann has
conceived a theory that endeavors to explain not only the phenomenon
of fecundation, but also that of heredity. I cannot mention here the
numerous works upon this subject, and refer the inquisitive reader for
a knowledge of the same to a series of lectures by Professor Balbiani
that I have epitomized in the _Revue Philosophique_ for December 1889.
The theory of the potential immortality of the Infusoria has recently
been attacked by M. Maupas, whose observations tend to show that
natural death, caused by senescence, does obtain among the Infusoria,
and that it is comparable in many points of view to the natural death
of the metazoans. The researches of M. Maupas upon the multiplication
of ciliate Infusoria are of a relatively recent date, having appeared
in 1888 in Vol. VI. of the "Archives de Zoologie Expérimentale."
It is scarcely necessary to say that the ciliate Infusoria can
propagate without previous coition. The agamic mode of reproduction
appears to be almost the same, save in a few details, as that which
follows coition. It consists in a bipartition or division of the body
of the animal along a plane usually perpendicular to the grand axis of
the nucleus, and it is a matter of course that that element takes part
in the division at the same time with the protoplasm. These phenomena
of reproduction it is possible to study upon a grand scale by supplying
Infusoria kept in captivity with abundance of nourishment. The easiest
way is to produce a putrid fermentation by means of vegetable fragments
crushed and macerated in water. The Infusoria contained in this water
find abundant food furnished by the bacteria developed in it, and they
therefore multiply in great numbers. By means of appropriate methods of
treatment and isolation we are able to follow the phenomenon step by
step and to examine what the animal actually becomes after each agamic
bipartition.
Weismann, when he laid the foundation of his theory of the immortality
of Infusoria, supposed that the development of the Infusoria by
bipartition had no limits and could be prolonged indefinitely without
injury to the vitality of the protoplasm. Various authors had already
made observations which were directly in contradiction with this view.
M. Balbiani, in 1860, in a communication entitled, "Observations and
Experiments upon the Phenomena of Fissiparous Reproduction among
Ciliate Infusoria,"[13] concludes thus: "one of the most important
questions ... has been to determine whether this mode of propagation
is really unlimited, or whether, after being continued throughout
a greater or lesser number of generations, it becomes by degrees
enfeebled, finally to disappear completely.... We have established
that this mode of propagation has its limits, and ends invariably
in one of the three following ways: either by the _natural_ and
almost simultaneous _death_ of all the individuals belonging to the
same cycle, or by the recurrence of sexual generation leading to the
termination of one of the cycles and the commencement of a new cycle,
or finally by the phenomenon of encystment, which in fact brings
about only a momentary interruption of the process of reproduction by
fissiparity" M. Balbiani, apropos of this subject, has called attention
to a curious observation made by the celebrated Danish micrographer O.
F. Müller, who lived in the last century. Müller had observed that the
individuals of any one species most ordinarily found in coition were
almost all of small stature. But he took them for the young individuals
of the species. Now these individuals of small size are in reality
the oldest, that is to say, they are the ones that are the result of
a great number of successive bipartitions; and it is to be observed,
that, in a great many species, in proportion as the bipartitions
increase the size of the Infusoria decreases.
[13] _C. R. Acad. des Sciences._ Vol. iv. p. 1191.
In fine, without further concerning ourselves with the history of this
question, we see that according to M. Balbiani the agamic reproduction
of Infusoria has its limits, and that, when coition, that is to say
fecundation, does not intervene, it may terminate by the natural death
of the individuals or in certain species by encystment.
The chief new element contained in the recent researches of M.
Maupas, which were made twenty years after the date of the preceding
investigations, consists in his study of the various phenomena of
senescence that the Infusoria after a long series of bipartitions
present. M. Maupas has established that there exists in the Infusoria
no part, no element, that by itself and by its own faculties, can live
and be maintained indefinitely. The first outward sign of degeneration
is manifested in a reduction of size. The individuals, according as
the number of generations increases, become smaller and smaller.
With _Stylonichia pustulata_, which in the normal state measures one
hundred and sixty μ, the size of the body is seen gradually to fall
to one hundred and thirty-five, one hundred and ten, seventy, and
even to forty μ. When the effects of senescence become marked, the
animal in its external organs undergoes atrophies and new and more
profound degenerations. In _Stylonichia pustulata_ the vibratile buccal
apparatus becomes gradually atrophied and partly disappears, and in
all species the body is reduced and becomes more and more shrunken,
assuming forms and contours very far removed from the specific type.
The degeneration of the nuclear apparatus at once begins. The first
modifications affect the accessory or attendant nucleus, a cut of
which will be found at page 118 of my work on Micro-organisms,[14]
and of which the principal function seems to be the maintenance
and conservation of the species, and which, therefore, ought to be
considered as the real substratum of the immortal plasma. Far from
enjoying the attribute of eternal youth, the accessory nucleus seems on
the contrary to be affected with a weakness greater and more premature
than that of the other parts of the organism. In fact it is this organ
that is first atrophied and that disappears under the influence of
senile degenerescence. Then, in its turn, the principal nucleus is
affected. It takes, according to the species, a different form. Now it
diminishes in volume, now it divides into two minute bodies that assume
irregular contours, and at other times it assumes a ribbon-like shape.
[14] English translation by The Open Court Pub. Co., Chicago.
Longmans & Co., London.
It is interesting to note that even after the disappearance of the
accessory nucleus, whenever the principal nucleus still subsists, the
Infusoria continue to live and divide by fission. This life, says M.
Maupas, has some features of abnormality about it, since it has become
wholly purposeless. The animals still live an individual life, but they
are dead to the life of the species.[15]
[15] Page 262.
In concluding upon this point, I must mention the reservations that
may be entertained with regard to the exactitude of the preceding
observations and the value of the method employed in their attainment.
A competent critic has remarked that it is difficult to assume that
nine hundred and thirty-five specimens of the genus _Stylonichia_
could find the gases necessary for the support of life, seeing that
M. Maupas kept them under the same stage where they only had at their
disposal a mass of water equal to one hundred cubic millimetres; and it
may thus be asked whether the phenomena of senescence produced under
these special conditions were not pathological. This criticism seems to
be especially strengthened by the fact, that according to M. Maupas,
the animalcula placed beneath the shield, all finally congregate at the
edge of the preparation, evidently to seek there the air of which they
are in need.
If we took our stand, however, upon the facts before cited we could
conclude without hesitation that the celebrated thesis of Weismann
regarding the immortality of the ciliate Infusoria had been overthrown.
But the phenomena are not presented with this simplicity. When the
vitality of the Infusoria has become weakened by a considerable
number of agamic reproductions, and the animalcule is upon the point
of dying a natural death, a new biological phenomenon can intervene,
rejuvenating the animal and rendering it capable of reproducing itself
anew for a long series of generations. That phenomenon is fecundation.
* * * * *
In our work upon Micro-Organisms we have spoken at length of the
material process of fecundation in ciliate Infusoria, and of the
phenomena preliminary to it, following as our guide the observations
of Balbiani, Gruber, Bütschli, and Engelmann. It will be necessary to
recur here to that subject and to supplement our preceding exposition
with some important details. Moreover, recent researches, added to
other older ones, afford us interesting information with regard to
the conditions and determining causes of conjugation and also of the
significance of fecundation itself.