Discourses: Biological and Geological — On the Border Territory Between the Animal and the Vegetable Kingdoms Pt 2

Some months ago, Professor Tyndall asked me to examine a drop of infusion of hay, placed under an excellent and powerful microscope, and to tell him what I thought some organisms visible in it were.

I looked and observed, in the first place, multitudes of _Bacteria_ moving about with their ordinary intermittent spasmodic wriggles.

As to the vegetable nature of these there is now no doubt.

Not only does the close resemblance of the _Bacteria_ to unquestionable plants, such as the _Oscillatorioe_ and the lower forms of _Fungi_, justify this conclusion, but the manufacturing test settles the question at once.

It is only needful to add a minute drop of fluid containing _Bacteria_, to water in which tartrate, phosphate, and sulphate of ammonia are dissolved; and, in a very short space of time, the clear fluid becomes milky by reason of their prodigious multiplication, which, of course, implies the manufacture of living Bacterium-stuff out of these merely saline matters.

But other active organisms, very much larger than the _Bacteria_, attaining in fact the comparatively gigantic dimensions of 1/3000 of an inch or more, incessantly crossed the field of view.

Each of these had a body shaped like a pear, the small end being slightly incurved and produced into a long curved filament, or _cilium_, of extreme tenuity.

Behind this, from the concave side of the incurvation, proceeded another long cilium, so delicate as to be discernible only by the use of the highest powers and careful management of the light.

In the centre of the pear-shaped body a clear round space could occasionally be discerned, but not always; and careful watching showed that this clear vacuity appeared gradually, and then shut up and disappeared suddenly, at regular intervals.

Such a structure is of common occurrence among the lowest plants and animals, and is known as a _contractile vacuole_.

The little creature thus described sometimes propelled itself with great activity, with a curious rolling motion, by the lashing of the front cilium, while the second cilium trailed behind; sometimes it anchored itself by the hinder cilium and was spun round by the working of the other, its motions resembling those of an anchor buoy in a heavy sea.

Sometimes, when two were in full career towards one another, each would appear dexterously to get out of the other's way; sometimes a crowd would assemble and jostle one another, with as much semblance of individual effort as a spectator on the Grands Mulets might observe with a telescope among the specks representing men in the valley of Chamounix.

The spectacle, though always surprising, was not new to me.

So my reply to the question put to me was, that these organisms were what biologists call _Monads_, and though they might be animals, it was also possible that they might, like the _Bacteria_, be plants.

My friend received my verdict with an expression which showed a sad want of respect for authority.

He would as soon believe that a sheep was a plant.

Naturally piqued by this want of faith, I have thought a good deal over the matter;

and, as I still rest in the lame conclusion I originally expressed, and must even now confess that I cannot certainly say whether this creature is an animal or a plant, I think it may be well to state the grounds of my hesitation at length.

But, in the first place, in order that I may conveniently distinguish this "Monad" from the multitude of other things which go by the same designation, I must give it a name of its own.

I

think (though, for reasons which need not be stated at present, I am not quite sure) that it is identical with the species _Monas lens_ as defined by the eminent French microscopist Dujardin, though his magnifying power was probably insufficient to enable him to see that it is curiously like a much larger form of monad which he has named _Heteromita_.

I shall, therefore, call it not _Monas_, but _Heteromita lens_.

I have been unable to devote to my _Heteromita_ the prolonged study needful to work out its whole history, which would involve weeks, or it may be months, of unremitting attention.

But I the less regret this circumstance, as some remarkable observations recently published by Messrs. Dallinger and Drysdale[5] on certain Monads, relate, in part, to a form so similar to my _Heteromita lens_, that the history of the one may be used to illustrate that of the other.

These most patient and painstaking observers, who employed the highest attainable powers of the microscope and, relieving one another, kept watch day and night over the same individual monads, have been enabled to trace out the whole history of their _Heteromita_; which they found in infusions of the heads of fishes of the Cod tribe.

[Footnote 5: "Researches in the Life-history of a Cercomonad: a Lesson in Biogenesis"; and "Further Researches in the Life-history of the Monads," --_Monthly Microscopical Journal_, 1873. ] Of the four monads described and figured by these investigators, one, as I have said, very closely resembles _Heteromita lens_ in every particular, except that it has a separately distinguishable central particle or "nucleus," which is not certainly to be made out in _Heteromita lens_; and that nothing is said by Messrs. Dallinger and Drysdale of the existence of a contractile vacuole in this monad, though they describe it in another.

Their _Heteromita_, however, multiplied rapidly by fission.

Sometimes a transverse constriction appeared; the hinder half developed a new cilium, and the hinder cilium gradually split from its base to its free end, until it was divided into two; a process which, considering the fact that this fine filament cannot be much more than 1/100000 of an inch in diameter, is wonderful enough.

The constriction of the body extended inwards until the two portions were united by a narrow isthmus; finally, they separated and each swam away by itself, a complete _Heteromita_, provided with its two cilia.

Sometimes the constriction took a longitudinal direction, with the same ultimate result.

In each case the process occupied not more than six or seven minutes.

At this rate, a single _Heteromita_ would give rise to a thousand like itself in the course of an hour, to about a million in two hours, and to a number greater than the generally assumed number of human beings now living in the world in three hours; or, if we give each _Heteromita_ an hour's enjoyment of individual existence, the same result will be obtained in about a day.

The apparent suddenness of the appearance of multitudes of such organisms as these in any nutritive fluid to which one obtains access is thus easily explained.

During these processes of multiplication by fission, the _Heteromita_ remains active; but sometimes another mode of fission occurs.

The body becomes rounded and quiescent, or nearly so; and, while in this resting state, divides into two portions, each of which is rapidly converted into an active _Heteromita_.

A still more remarkable phenomenon is that kind of multiplication which is preceded by the union of two monads, by a process which is termed _conjugation_.

Two active _Heteromitoe_ become applied to one another, and then slowly and gradually coalesce into one body.

The two nuclei run into one; and the mass resulting from the conjugation of the two _Heteromitoe_, thus fused together, has a triangular form.

The two pairs of cilia are to be seen, for some time, at two of the angles, which answer to the small ends of the conjoined monads; but they ultimately vanish, and the twin organism, in which all visible traces of organisation have disappeared, falls into a state of rest.

Sudden wave- like movements of its substance next occur; and, in a short time, the apices of the triangular mass burst, and give exit to a dense yellowish, glairy fluid, filled with minute granules.

This process, which, it will be observed, involves the actual confluence and mixture of the substance of two distinct organisms, is effected in the space of about two hours.

The authors whom I quote say that they "cannot express" the excessive minuteness of the granules in question, and they estimate their diameter at less than 1/200000 of an inch.

Under the highest powers of the microscope, at present applicable, such specks are hardly discernible.

Nevertheless, particles of this size are massive when compared to physical molecules; whence there is no reason to doubt that each, small as it is, may have a molecular structure sufficiently complex to give rise to the phenomena of life.

And, as a matter of fact, by patient watching of the place at which these infinitesimal living particles were discharged, our observers assured themselves of their growth and development into new monads.

In about four hours from their being set free, they had attained a sixth of the length of the parent, with the characteristic cilia, though at first they were quite motionless; and, in four hours more, they had attained the dimensions and exhibited all the activity of the adult.

These inconceivably minute particles are therefore the germs of the _Heteromita_; and from the dimensions of these germs it is easily shown that the body formed by conjugation may, at a low estimate, have given exit to thirty thousand of them; a result of a matrimonial process whereby the contracting parties, without a metaphor, "become one flesh," enough to make a Malthusian despair of the future of the Universe.

I am not aware that the investigators from whom I have borrowed this history have endeavoured to ascertain whether their monads take solid nutriment or not; so that though they help us very much to fill up the blanks in the history of my _Heteromita_, their observations throw no light on the problem we are trying to solve--Is it an animal or is it a plant?

Undoubtedly it is possible to bring forward very strong arguments in favour of regarding _Heteromita_ as a plant.

For example, there is a Fungus, an obscure and almost microscopic mould, termed _Peronospora infestans_.

Like many other Fungi, the _Peronosporoe_ are parasitic upon other plants; and this particular _Peronospora_ happens to have attained much notoriety and political importance, in a way not without a parallel in the career of notorious politicians, namely, by reason of the frightful mischief it has done to mankind.

For it is this _Fungus_ which is the cause of the potato disease; and, therefore, _Peronospora infestans_ (doubtless of exclusively Saxon origin, though not accurately known to be so) brought about the Irish famine.

The plants afflicted with the malady are found to be infested by a mould, consisting of fine tubular filaments, termed _hyphoe_, which burrow through the substance of the potato plant, and appropriate to themselves the substance of their host; while, at the same time, directly or indirectly, they set up chemical changes by which even its woody framework becomes blackened, sodden, and withered.

In structure, however, the _Peronospora_ is as much a mould as the common _Penicillium_; and just as the _Penicillium_ multiplies by the breaking up of its hyphoe into separate rounded bodies, the spores; so, in the _Peronospora_, certain of the hyphoe grow out into the air through the interstices of the superficial cells of the potato plant, and develop spores.

Each of these hyphoe usually gives off several branches.

The ends of the branches dilate and become closed sacs, which eventually drop off as spores.

The spores falling on some part of the same potato plant, or carried by the wind to another, may at once germinate, throwing out tubular prolongations which become hyphoe, and burrow into the substance of the plant attacked.

But, more commonly, the contents of the spore divide into six or eight separate portions.

The coat of the spore gives way, and each portion then emerges as an independent organism, which has the shape of a bean, rather narrower at one end than the other, convex on one side, and depressed or concave on the opposite.

From the depression, two long and delicate cilia proceed, one shorter than the other, and directed forwards.

Close to the origin of these cilia, in the substance of the body, is a regularly pulsating, contractile vacuole.

The shorter cilium vibrates actively, and effects the locomotion of the organism, while the other trails behind; the whole body rolling on its axis with its pointed end forwards.

The eminent botanist, De Bary, who was not thinking of our problem, tells us, in describing the movements of these "Zoospores," that, as they swim about, "Foreign bodies are carefully avoided, and the whole movement has a deceptive likeness to the voluntary changes of place which are observed in microscopic animals."

After swarming about in this way in the moisture on the surface of a leaf or stem (which, film though it may be, is an ocean to such a fish) for half an hour, more or less, the movement of the zoospore becomes slower, and is limited to a slow turning upon its axis, without change of place.

It then becomes quite quiet, the cilia disappear, it assumes a spherical form, and surrounds itself with a distinct, though delicate, membranous coat.

A protuberance then grows out from one side of the sphere, and rapidly increasing in length, assumes the character of a hypha.

The latter penetrates into the substance of the potato plant, either by entering a stomate, or by boring through the wall of an epidermic cell, and ramifies, as a mycelium, in the substance of the plant, destroying the tissues with which it comes in contact.

As these processes of multiplication take place very rapidly, millions of spores are soon set free from a single infested plant; and, from their minuteness, they are readily transported by the gentlest breeze.

Since, again, the zoospores set free from each spore, in virtue of their powers of locomotion, swiftly disperse themselves over the surface, it is no wonder that the infection, once started, soon spreads from field to field, and extends its ravages over a whole country.

However, it does not enter into my present plan to treat of the potato disease, instructively as its history bears upon that of other epidemics;

and I have selected the case of the _Peroganspora_ simply because it affords an example of an organism, which, in one stage of its existence, is truly a "Monad," indistinguishable by any important character from our _Heteromita_, and extraordinarily like it in some respects.

And yet this "Monad" can be traced, step by step, through the series of metamorphoses which I have described, until it assumes the features of an organism, which is as much a plant as is an oak or an elm.

Moreover, it would be possible to pursue the analogy farther.

Under certain circumstances, a process of conjugation takes place in the _Peronospora_.

Two separate portions of its protoplasm become fused together, surround themselves with a thick coat and give rise to a sort of vegetable egg called an _oospore_.

After a period of rest, the contents of the oospore break up into a number of zoospores like those already described, each of which, after a period of activity, germinates in the ordinary way.

This process obviously corresponds with the conjugation and subsequent setting free of germs in the _Heteromita_.

But it may be said that the _Peronospora_ is, after all, a questionable sort of plant; that it seems to be wanting in the manufacturing power, selected as the main distinctive character of vegetable life; or, at any rate, that there is no proof that it does not get its protein matter ready made from the potato plant.

Let us, therefore, take a case which is not open to these objections.

There are some small plants known to botanists as members of the genus _Colcochaete_, which, without being truly parasitic, grow upon certain water-weeds, as lichens grow upon trees.

The little plant has the form of an elegant green star, the branching arms of which are divided into cells.

Its greenness is due to its chlorophyll, and it undoubtedly has the manufacturing power in full degree, decomposing carbonic acid and setting oxygen free, under the influence of sunlight.

But the protoplasmic contents of some of the cells of which the plant is made up occasionally divide, by a method similar to that which effects the division of the contents of the _Peronospora_ spore; and the severed portions are then set free as active monad-like zoospores.

Each is oval and is provided at one extremity with two long active cilia.

Propelled by these, it swims about for a longer or shorter time, but at length comes to a state of rest and gradually grows into a _Coleochaete_.

Moreover, as in the _Peronospora_, conjugation may take place and result in an oospore; the contents of which divide and are set free as monadiform germs.

If the whole history of the zoospores of _Peronospora_ and of _Coleochaete_ were unknown, they would undoubtedly be classed among "Monads" with the same right as _Heteromita_; why then may not _Heteromita_ be a plant, even though the cycle of forms through which it passes shows no terms quite so complex as those which occur in _Peronospora_ and _Coleochaete_?

And, in fact, there are some green organisms, in every respect characteristically plants, such as _Chlamydomonas_, and the common _Volvox_, or so-called "Globe animalcule," which run through a cycle of forms of just the same simple character as those of _Heteromita_.

The name of _Chlamydomonas_ is applied to certain microscopic green bodies, each of which consists of a protoplasmic central substance invested by a structureless sac.

The latter contains cellulose, as in ordinary plants; and the chlorophyll which gives the green colour enables the _Chlamydomonas_ to decompose carbonic acid and fix carbon as they do.

Two long cilia protrude through the cell-wall, and effect the rapid locomotion of this "monad," which, in all respects except its mobility, is characteristically a plant.

Under ordinary circumstances, the _Chlamydomonas_ multiplies by simple fission, each splitting into two or into four parts, which separate and become independent organisms.

Sometimes, however, the _Chlamydomonas_ divides into eight parts, each of which is provided with four instead of two cilia.

These "zoospores" conjugate in pairs, and give rise to quiescent bodies, which multiply by division, find eventually pass into the active state.

Thus, so far as outward form and the general character of the cycle of modifications, through which the organism passes in the course of its life, are concerned, the resemblance between _Chlamydomonas_ and _Heteromita_ is of the closest description.

And on the face of the matter there is no ground for refusing to admit that _Heteromita_ may be related to _Chlamydomonas_, as the colourless fungus is to the green alga.

_Volvox_ may be compared to a hollow sphere, the wall of which is made up of coherent Chlamydomonads; and which progresses with a rotating motion effected by the paddling of the multitudinous pairs of cilia which project from its surface.

Each _Volvox_-monad, moreover, possesses a red pigment spot, like the simplest form of eye known among animals.

The methods of fissive multiplication and of conjugation observed in the monads of this locomotive globe are essentially similar to those observed in _Chlamydomonas_; and, though a hard battle has been fought over it, _Volvox_ is now finally surrendered to the Botanists.

Thus there is really no reason why _Heteromita_ may not be a plant; and this conclusion would be very satisfactory, if it were not equally easy to show that there is really no reason why it should not be an animal.

For there are numerous organisms presenting the closest resemblance to _Heteromita_, and, like it, grouped under the general name of "Monads," which, nevertheless, can be observed to take in solid nutriment, and which, therefore, have a virtual, if not an actual, mouth and digestive cavity, and thus come under Cuvier's definition of an animal.

Numerous forms of such animals have been described by Ehrenberg, Dujardin, H. James Clark, and other writers on the _Infusoria_.

Indeed, in another infusion of hay in which my _Heteromita lens_ occurred, there were innumerable such infusorial animalcules belonging to the well-known species _Colpoda cucullus_.

[6] [Footnote 6: Excellently described by Stein, almost all of whose statements I have verified.

] Full-sized specimens of this animalcule attain a length of between 1/300 or 1/400 of an inch, so that it may have ten times the length and a thousand times the mass of a _Heteromita_.

In shape, it is not altogether unlike _Heteromita_.

The small end, however, is not produced into one long cilium, but the general surface of the body is covered with small actively vibrating ciliary organs, which are only longest at the small end.

At the point which answers to that from which the two cilia arise in _Heteromita_, there is a conical depression, the mouth; and, in young specimens, a tapering filament, which reminds one of the posterior cilium of _Heteromita_, projects from this region.

The body consists of a soft granular protoplasmic substance, the middle of which is occupied by a large oval mass called the "nucleus"; while, at its hinder end, is a "contractile vacuole," conspicuous by its regular rhythmic appearances and disappearances.

Obviously, although the _Colpoda_ is not a monad, it differs from one only in subordinate details.

Moreover, under certain conditions, it becomes quiescent, incloses itself in a delicate case or _cyst_, and then divides into two, four, or more portions, which are eventually set free and swim about as active _Colpodoe_.

But this creature is an unmistakable animal, and full-sized _Colpodoe_ may be fed as easily as one feeds chickens.

It is only needful to diffuse very finely ground carmine through the water in which they live, and, in a very short time, the bodies of the _Colpodoe_ are stuffed with the deeply-coloured granules of the pigment.

And if this were not sufficient evidence of the animality of _Colpoda_, there comes the fact that it is even more similar to another well-known animalcule, _Paramoecium_, than it is to a monad.

But _Paramoecium_ is so huge a creature compared with those hitherto discussed--it reaches 1/120 of an inch or more in length--that there is no difficulty in making out its organisation in detail; and in proving that it is not only an animal, but that it is an animal which possesses a somewhat complicated organisation.

For example, the surface layer of its body is different in structure from the deeper parts.

There are two contractile vacuoles, from each of which radiates a system of vessel-like canals; and not only is there a conical depression continuous with a tube, which serve as mouth and gullet, but the food ingested takes a definite course, and refuse is rejected from a definite region.

Nothing is easier than to feed these animals, and to watch the particles of indigo or carmine accumulate at the lower end of the gullet.

From this they gradually project, surrounded by a ball of water, which at length passes with a jerk, oddly simulating a gulp, into the pulpy central substance of the body, there to circulate up one side and down the other, until its contents are digested and assimilated.

Nevertheless, this complex animal multiplies by division, as the monad does, and, like the monad, undergoes conjugation.

It stands in the same relation to _Heteromita_ on the animal side, as _Coleochaete_ does on the plant side.

Start from either, and such an insensible series of gradations leads to the monad that it is impossible to say at any stage of the progress where the line between the animal and the plant must be drawn.

There is reason to think that certain organisms which pass through a monad stage of existence, such as the _Myxomycetes_, are, at one time of their lives, dependent upon external sources for their protein matter, or are animals; and, at another period, manufacture it, or are plants.

And seeing that the whole progress of modern investigation is in favour of the doctrine of continuity, it is a fair and probable speculation--though only a speculation--that, as there are some plants which can manufacture protein out of such apparently intractable mineral matters as carbonic acid, water, nitrate of ammonia, metallic and earthy salts; while others need to be supplied with their carbon and nitrogen in the somewhat less raw form of tartrate of ammonia and allied compounds; so there may be yet others, as is possibly the case with the true parasitic plants, which can only manage to put together materials still better prepared--still more nearly approximated to protein--until we arrive at such organisms as the _Psorospermioe_ and the _Panhistophyton_, which are as much animal as vegetable in structure, but are animal in their dependence on other organisms for their food.

The singular circumstance observed by Meyer, that the _Torula_ of yeast, though an indubitable plant, still flourishes most vigorously when supplied with the complex nitrogenous substance, pepsin; the probability that the _Peronospora_ is nourished directly by the protoplasm of the potato-plant; and the wonderful facts which have recently been brought to light respecting insectivorous plants, all favour this view; and tend to the conclusion that the difference between animal and plant is one of degree rather than of kind, and that the problem whether, in a given case, an organism is an animal or a plant, may be essentially insoluble.

VII A LOBSTER; OR, THE STUDY OF ZOOLOGY