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 a.s.sumes 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, a.s.sumes 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 a.s.sumes 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 a.n.a.logy farther. Under certain circ.u.mstances, 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 cla.s.sed 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 pa.s.ses 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 circ.u.mstances, 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 pa.s.s into the active state.
Thus, so far as outward form and the general character of the cycle of modifications, through which the organism pa.s.ses 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 mult.i.tudinous 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 ma.s.s 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 ma.s.s 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 ca.n.a.ls; 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 acc.u.mulate at the lower end of the gullet. From this they gradually project, surrounded by a ball of water, which at length pa.s.ses 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 a.s.similated. 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 pa.s.s 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 circ.u.mstance 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
[1861]
Natural history is the name familiarly applied to the study of the properties of such natural bodies as minerals, plants, and animals; the sciences which embody the knowledge man has acquired upon these subjects are commonly termed Natural Sciences, in contradistinction to other so- called "physical" sciences; and those who devote themselves especially to the pursuit of such sciences have been and are commonly termed "Naturalists."
Linnaeus was a naturalist in this wide sense, and his "Systema Naturae" was a work upon natural history, in the broadest acceptation of the term; in it, that great methodising spirit embodied all that was known in his time of the distinctive characters of minerals, animals, and plants. But the enormous stimulus which Linnaeus gave to the investigation of nature soon rendered it impossible that any one man should write another "Systema Naturae," and extremely difficult for any one to become even a naturalist such as Linnaeus was.
Great as have been the advances made by all the three branches of science, of old included under the t.i.tle of natural history, there can be no doubt that zoology and botany have grown in an enormously greater ratio than mineralogy; and hence, as I suppose, the name of "natural history" has gradually become more and more definitely attached to these prominent divisions of the subject, and by "naturalist" people have meant more and more distinctly to imply a student of the structure and function of living beings.
However this may be, it is certain that the advance of knowledge has gradually widened the distance between mineralogy and its old a.s.sociates, while it has drawn zoology and botany closer together; so that of late years it has been found convenient (and indeed necessary) to a.s.sociate the sciences which deal with vitality and all its phenomena under the common head of "biology"; and the biologists have come to repudiate any blood-relationship with their foster-brothers, the mineralogists.
Certain broad laws have a general application throughout both the animal and the vegetable worlds, but the ground common to these kingdoms of nature is not of very wide extent, and the multiplicity of details is so great, that the student of living beings finds himself obliged to devote his attention exclusively either to the one or the other. If he elects to study plants, under any aspect, we know at once what to call him. He is a botanist, and his science is botany. But if the investigation of animal life be his choice, the name generally applied to him will vary according to the kind of animals he studies, or the particular phenomena of animal life to which he confines his attention. If the study of man is his object, he is called an anatomist, or a physiologist, or an ethnologist; but if he dissects animals, or examines into the mode in which their functions are performed, he is a comparative anatomist or comparative physiologist. If he turns his attention to fossil animals, he is a palaeontologist. If his mind is more particularly directed to the specific description, discrimination, cla.s.sification, and distribution of animals, he is termed a zoologist.
For the purpose of the present discourse, however, I shall recognise none of these t.i.tles save the last, which I shall employ as the equivalent of botanist, and I shall use the term zoology is denoting the whole doctrine of animal life, in contradistinction to botany, which signifies the whole doctrine of vegetable life.
Employed in this sense, zoology, like botany, is divisible into three great but subordinate sciences, morphology, physiology, and distribution, each of which may, to a very great extent, be studied independently of the other.
Zoological morphology is the doctrine of animal form or structure.
Anatomy is one of its branches; development is another; while cla.s.sification is the expression of the relations which different animals bear to one another, in respect of their anatomy and their development.
Zoological distribution is the study of animals in relation to the terrestrial conditions which obtain now, or have obtained at any previous epoch of the earth"s history.
Zoological physiology, lastly, is the doctrine of the functions or actions of animals. It regards animal bodies as machines impelled by certain forces, and performing an amount of work which can be expressed in terms of the ordinary forces of nature. The final object of physiology is to deduce the facts of morphology, on the one hand, and those of distribution on the other, from the laws of the molecular forces of matter.
Such is the scope of zoology. But if I were to content myself with the enunciation of these dry definitions, I should ill exemplify that method of teaching this branch of physical science, which it is my chief business to-night to recommend. Let us turn away then from abstract definitions. Let us take some concrete living thing, some animal, the commoner the better, and let us see how the application of common sense and common logic to the obvious facts it presents, inevitably leads us into all these branches of zoological science.
I have before me a lobster. When I examine it, what appears to be the most striking character it presents? Why, I observe that this part which we call the tail of the lobster, is made up of six distinct hard rings and a seventh terminal piece. If I separate one of the middle rings, say the third, I find it carries upon its under surface a pair of limbs or appendages, each of which consists of a stalk and two terminal pieces. So that I can represent a transverse section of the ring and its appendages upon the diagram board in this way.
If I now take the fourth ring, I find it has the same structure, and so have the fifth and the second; so that, in each of these divisions of the tail, I find parts which correspond with one another, a ring and two appendages; and in each appendage a stalk and two end pieces. These corresponding parts are called, in the technical language of anatomy, "h.o.m.ologous parts." The ring of the third division is the "h.o.m.ologue" of the ring of the fifth, the appendage of the former is the h.o.m.ologue of the appendage of the latter. And, as each division exhibits corresponding parts in corresponding places, we say that all the divisions are constructed upon the same plan. But now let us consider the sixth division. It is similar to, and yet different from, the others. The ring is essentially the same as in the other divisions; but the appendages look at first as if they were very different; and yet when we regard them closely, what do we find? A stalk and two terminal divisions, exactly as in the others, but the stalk is very short and very thick, the terminal divisions are very broad and flat, and one of them is divided into two pieces.
I may say, therefore, that the sixth segment is like the others in plan, but that it is modified in its details.
The first segment is like the others, so far as its ring is concerned, and though its appendages differ from any of those yet examined in the simplicity of their structure, parts corresponding with the stem and one of the divisions of the appendages of the other segments can be readily discerned in them.
Thus it appears that the lobster"s tail is composed of a series of segments which are fundamentally similar, though each presents peculiar modifications of the plan common to all. But when I turn to the forepart of the body I see, at first, nothing but a great shield-like sh.e.l.l, called technically the "carapace," ending in front in a sharp spine, on either side of which are the curious compound eyes, set upon the ends of stout movable stalks. Behind these, on the under side of the body, are two pairs of long feelers, or antennae, followed by six pairs of jaws folded against one another over the mouth, and five pairs of legs, the foremost of these being the great pinchers, or claws, of the lobster.
It looks, at first, a little hopeless to attempt to find in this complex ma.s.s a series of rings, each with its pair of appendages, such as I have shown you in the abdomen, and yet it is not difficult to demonstrate their existence. Strip off the legs, and you will find that each pair is attached to a very definite segment of the under wall of the body; but these segments, instead of being the lower parts of free rings, as in the tail, are such parts of rings which are all solidly united and bound together; and the like is true of the jaws, the feelers, and the eye- stalks, every pair of which is borne upon its own special segment. Thus the conclusion is gradually forced upon us, that the body of the lobster is composed of as many rings as there are pairs of appendages, namely, twenty in all, but that the six hindmost rings remain free and movable, while the fourteen front rings become firmly soldered together, their backs forming one continuous shield--the carapace.
Unity of plan, diversity in execution, is the lesson taught by the study of the rings of the body, and the same instruction is given still more emphatically by the appendages. If I examine the outermost jaw I find it consists of three distinct portions, an inner, a middle, and an outer, mounted upon a common stem; and if I compare this jaw with the legs behind it, or the jaws in front of it, I find it quite easy to see, that, in the legs, it is the part of the appendage which corresponds with the inner division, which becomes modified into what we know familiarly as the "leg," while the middle division disappears, and the outer division is hidden under the carapace. Nor is it more difficult to discern that, in the appendages of the tail, the middle division appears again and the outer vanishes; while, on the other hand, in the foremost jaw, the so- called mandible, the inner division only is left; and, in the same way, the parts of the feelers and of the eye-stalks can be identified with those of the legs and jaws.
But whither does all this tend? To the very remarkable conclusion that a unity of plan, of the same kind as that discoverable in the tail or abdomen of the lobster, pervades the whole organisation of its skeleton, so that I can return to the diagram representing any one of the rings of the tail, which I drew upon the board, and by adding a third division to each appendage, I can use it as a sort of scheme or plan of any ring of the body. I can give names to all the parts of that figure, and then if I take any segment of the body of the lobster, I can point out to you exactly, what modification the general plan has undergone in that particular segment; what part has remained movable, and what has become fixed to another; what has been excessively developed and metamorphosed and what has been suppressed.
But I imagine I hear the question, How is all this to be tested? No doubt it is a pretty and ingenious way of looking at the structure of any animal; but is it anything more? Does Nature acknowledge, in any deeper way, this unity of plan we seem to trace?
The objection suggested by these questions is a very valid and important one, and morphology was in an unsound state so long as it rested upon the mere perception of the a.n.a.logies which obtain between fully formed parts.