Popular Science Monthly/Volume 30/March 1887/How a Naturalist Is Trained
|HOW A NATURALIST IS TRAINED.|
EVERY trade, every profession, has its own peculiar methods of procedure, which, while not kept secret, are still unknown to the general public. This ignorance is due to several causes, among which may be mentioned a lack of interest and a lack of any simple account of the processes involved. If one not educated to the legal profession be told the facts in a certain case, and then be turned loose in a large law library, how long would it take him to work up a brief? How would he know what books to consult, where to find decisions bearing on the cases in question, or, when found, how to interpret them, and ascertain their exact relations to the subject in dispute?
There is probably just as much mystery surrounding the way in which the naturalist investigates the secrets of Nature, yet the true student has not the slightest desire to conceal his methods; but, on the other hand, is perfectly willing, even glad, to tell how he arrived at his results to any one who wishes to hear. The student, on first entering a biological laboratory, thinks he has an easy task before him. All that he has to do in order to become a naturalist is to see and to remember what he sees. In a few days this confidence gives way to a spirit of despair. He begins to realize that observation is not so easy as he thought, and that the structures so distinctly shown in anatomical plates arc not so readily discovered in the object before him. He becomes satisfied that in science, as in the other departments of knowledge, there is no royal road to learning. Gradually he acquires the methods, and knowing them his knowledge increases. What at first seemed an impossible task is seen to be really easy, and things at first invisible are soon as plain as day. At first sight it would seem difficult to take an egg only 100 of an inch in diameter, and cut it into slices in any desired manner, and yet it is an every-day operation to section such an egg and convert it into fifty slices.
It is the purpose of this article to tell in general terms the way in which a naturalist, and especially a zoölogist, arrives at his results. To give exact details would expand this article to a large volume and render it extremely abstruse and technical, while a mere outline will be much shorter and (the writer hopes) more interesting. Within the past few years the methods of study and tendencies of biological thought have undergone an immense development; and although each of the nearly four hundred colleges and universities in the United States pretend to give instruction in botany and zoology, there are really less than a dozen where the student can obtain a good and solid foundation in the biological sciences as they exist to-day.
Until school-life begins, a child is a good observer, but the whole training after that period is one adapted, if not intended to repress the forms of observation, and hence the first thing to be taught a student on entering the biological laboratory in one of the institutions of the better class is how to observe. This, though it appears an easy task, is really one of considerable difficulty. First, there is a tendency to consult books so as to learn what is or should be seen; and, secondly, there is a sudden jumping at conclusions from the most superficial examination of the specimen, and these conclusions are adhered to most tenaciously, utterly preventing the formation of any different view.
Together with the formation of habits of observation, it is desirable that a certain amount of facts be obtained, and so the student is set at the dissection of a selected series of animal types; for instance, the sea-anemone, sea-urchin, earth-worm, lobster, clam, and frog. Economy of material is insisted upon, and the admonition is frequently given that each stroke of the scalpel should mean something. Drawing is extremely essential, for, if the student be made to draw exactly what he sees, he will have to look more closely, and, at the same time, the instructor can readily see exactly how well his pupil works, and exactly where his difficulties lie. At first the student declares he can not draw, that he has not the slightest taste for art, and yet, after a very little experience, he makes thoroughly intelligible, if not artistic, representations of what he sees.
A very important point in making these small dissections is making them under water. If one attempt to dissect a clam in the open air, the various parts will settle down and adhere to each other; while, if the operation be performed under water, this difficulty will be avoided, the parts being buoyed up by the surrounding medium. To trace the course of the blood-vessels, injections are resorted to. Some quickly hardening mass, like plaster-of-Paris or melted wax, or gelatine colored by carmine, vermilion, or Prussian blue, is forced into the arteries or veins, and then the student, by following the streaks of color, can readily follow the course of the circulation. When sufficient skill is obtained by dissection of these larger forms, smaller ones may be taken, and after a short time the student experiences but little more difficulty in dissecting a grasshopper or a snail than in a pigeon or turtle.
Besides obtaining a skill in dissection and a capacity for observation, a student is led in this anatomical course to make comparisons between the various objects dissected. This results in a recognition of similarities and differences, and exercises the reasoning faculties. The value of the mathematical sciences in logical training is often insisted upon, but to the writer it seems as if the biological sciences were even more important from this standpoint. In mathematics, given such and such premises, there can be but one conclusion; there is no alternative, while in zoölogical reasoning there is an element of uncertainty to be eliminated. Each fact observed must be weighed, and its relative importance determined, before conclusions can be drawn, and even then it is frequently necessary to estimate the relative probability of two or more alternatives, thus giving an exercise to the powers of ratiocination which is utterly lacking in the remorseless logic of the mathematical theorem.
When this general foundation of facts and methods is obtained, more special studies are taken up; and since embryological research includes most of the processes involved, we will suppose that the student is next introduced to this fascinating field which is now so assiduously cultivated by scholars all over the world. As a rule, it may be stated that animals living on the land or in fresh water differ considerably in their mode of development from their near relatives in the sea. Many ancestral features which are retained in marine forms have become eliminated in the others, and hence the study of the growth of salt-water forms from the egg to the adult throws far more light on the relationships and ancestry of the different groups than does that of the terrestrial and fluviatile species. Again, the sea affords a wealth of life far beyond that of the land and fresh water, a wealth to be estimated not only in number of individuals but of species as well. Whole groups of animals are solely marine, while others are represented on the earth or in rivers and ponds by a few small and insignificant forms.
For this reason the student of embryology betakes himself every year to the shore, so that, while being recuperated by the sea-breeze, he may continue his studies and add to the total of human knowledge. Marine laboratories for this purpose are scattered the whole length of our coast, from North Carolina to Eastport, some being mere temporary affairs, others permanent stations. In 1885 public or private laboratories existed at Beaufort, Newport, Nantucket, "Wood's Holl, Salem, Annisquam, Mount Desert, and Eastport. The absolutely necessary furnishings of such a laboratory are extremely few, but to them one may add as far as purse and inclination admit. There must be tables, chairs, and glass dishes, while each student must have a microscope and accessoires. Then come a boat (a row-boat is sufficient for all ordinary work) and apparatus for collecting. Finally, a small stock of chemicals and apparatus for microscopical work complete the list of necessities.
Possibly the most common way for obtaining material for embryological study is by use of the skimming-net. This consists of a brass ring about a foot in diameter, to which is attached a net of fine gauze and cords for dragging it behind a boat. The whole operation of skimming is very simple. Two persons are required, one to row the boat, the other to attend the net. The latter allows the net to trail behind, keeping the cords so that part of the mouth is above and part below the surface of the water, so that as much as possible of the surface-water will be strained through the gauze. At intervals the net is hauled in, turned inside out, and rinsed by immersion and agitation in a bucket of water kept in the bottom of the boat. In this way everything entangled in the meshes of the net is transferred to the pail. The net is again put out and the operation repeated.
Skimming may be performed at any hour of the day; but in the daytime the forms collected will differ considerably from those captured at night, and, besides, will not be nearly so numerous. The best place for skimming is a spot where two tidal currents meet, forming a line of scum upon the surface; the best time is in the evening, when the surface of the water is calm and smooth. Then the wealth of forms and individuals is almost incredible, and no one who has never seen the operation can have the slightest conception of the results. Each time the net is hauled from the water it shines like molten gold from the phosphorescence of the myriads of animals by which it is covered. On closer examination it is seen that spots of other colors exist among the prevailing yellow light: bright red, blue, bluish-green, emerald-green, and white occur, and after some experience one learns to recognize the presence of a few species by the color of the light.
Only two or three hauls of the skimming-net are necessary to insure an abundance of material for study, and at no time need the student spend more than half an hour in this work, while frequently ten or fifteen minutes are ample. The laboratory is now sought, and the contents of the bucket in which the net was rinsed are poured into shallow glass dishes placed between the student and a lamp. Then, and not till then, does one begin to realize the enormous amount of life in the sea. In half an hour's skimming not a thousand gallons of water will pass through the net, and yet but a single glance at the dishes convinces one that millions—yes, millions—of individuals have been captured! The water is roily with minute animals and embryos, whirling, dancing, and jerking about in the strangest manner.
On different nights the relative proportion of forms will vary. Tonight not a single specimen may be taken of a species which last night was very abundant; but at all times a large proportion of the captures will be found to be copepod crustaceans—small forms not over a quarter of an inch in length, which swim about in a jerky manner by means of violent strokes of their long antenna?. To enumerate all the forms which might be taken by skimming would prove a difficult task, but some of the more prominent forms are readily recognized by the peculiarities of their motions. The Crustacea move by jerks, the embryo worms and mollusks, on the other hand, whirl away in a mazy waltz; while the jelly-fish swim lazily away by the languid contractions of their umbrellas.
While a general view of the results of surface-skimming is interesting and instructive, our student has other work before him. He is to take one species of embryo and follow it through its transformations. Soon after the dishes are placed before the light, most of the forms will be found congregated at the lightest side of the dish. A lamp seems to exert the same fascination on them as on the moths of a summer's night. The student, armed with a magnifying-glass, now picks out from the dish the forms he desires to study by means of a medicine-dropper, and transfers them to a separate dish, where they may have an abundance of water, and, when sufficient material has been picked out, the real study begins.
The first thing is to ascertain everything of the external and internal structure that can be seen in the living animal. For this purpose it is placed on a glass slide in a drop of sea-water and carefully studied under the microscope. In this, as in all embryological work, drawing is absolutely necessary. Pages of description will not take the place of pictorial representation. After the whole is studied, then comes a study of the different parts, drawings and notes being made of each. An embryo is continually growing, and it becomes necessary to take into account every stage of growth. The embryo of to-morrow will be different from that of to-day, and the changes must be recorded. Some of the embryos are therefore kept, the water being changed, as often as necessary, and these serve for to-morrow's study, the drawings of to-day furnishing a basis of comparison. In many cases it is a comparatively easy task to rear embryos until the adult condition is recognizable, but at other times it is found impossible to keep them in confinement for more than two or three days. In the first case it is an easy task to identify the forms studied, but in the other the difficulty is considerable. Subsequent skimmings must be made in the hopes of securing the later stages of development, while an endeavor to find the animal which produces the eggs frequently meets with success. Comparison with the studies of other investigators is also an important aid to identification.
If, however, the eggs are taken directly from the parents, this trouble is wholly avoided, although other difficulties are introduced. Suppose, for instance, that one wishes to study the development of one of the fishes, the first step is to obtain males and females with the generative products ripe. A gentle stroking will serve to expel both eggs and milt, and then these are mixed together and "artificial impregnation" is affected. In the case of worms, oysters, and clams, the same result can be obtained by mincing the generative organs of ripe males and females, mixing them together, and then straining off the larger portions which, by their decay, would soon pollute the water. In the case of crabs and shrimps the eggs are borne attached to the abdominal legs of the mothers, and by capturing these females an abundant supply of material can be obtained. The parents can readily be preserved alive in lobster-cars or similar contrivances, and furnish eggs as they are needed.
Artificial impregnation is a very valuable process, for, by its aid, every stage in development may be obtained. Eggs and milt may be mixed under the microscope, and all the phenomena of the maturation of the egg and its impregnation can then be followed as well as the processes of segmentation which result in the conversion of the single-celled egg into the many-celled embryo. Interesting as a description of these phenomena would be, we must pass them by, for we have not yet described one of the most important processes of study.
Studying embryos, even the most transparent ones, by simply watching them under the microscope, leaves many features of the method of formation of the internal structure unknown, while in the case of opaque forms it reveals not a single feature except those of the surface. A knowledge of these internal points is, however, just as important as of the external modifications of form. In many, yes, in almost every case, the embryo is too small to be dissected, but by converting it into a series of slices or sections, and then studying these, structures and processes of growth are revealed which otherwise would remain entirely unknown. This method of section-cutting and the processes of preserving the sections thus obtained is almost entirely a growth of the last ten years. It is true that for a long time naturalists have resorted to it, but so crude were the instruments and so faulty the technique that section-cutting could hardly be said to exist in comparison with its importance to-day.
Here, as elsewhere, details would be out of place in an article of this character, but an outline of the processes involved in section-cutting will show the capacities of modern research as well as the methods which our student must master before he can take his place among the advanced workers of to-day. It must be said, in passing, that for every form some process is best adapted, and that what works well for one is often utterly unsuited for a closely related species. No general rule can be laid down by which the student can at once say that such and such methods are best adapted to give good results; the exact course of procedure in any case can only be determined by experiment.
Were it attempted to cut the fresh egg into sections, the result would be an ignominious failure. There are various preparatory processes necessary, and in all of these care must be exercised that the reagents employed do not produce abnormal effects. First, the egg must be hardened, and here there is a choice among a number of chemicals—alcohol, chromic acid, bichromate of potash, osmic or nitric acids, corrosive sublimate, etc.—each of which has its especial advantages and disadvantages. Even in the method of killing the egg previous to hardening, there are a number of methods to choose from. The hardening reagents all serve to kill, but not equally well, for they do not all work with the same rapidity. In the use of all, care has to be exercised to prevent contraction.
"Were we to cut the hardened egg, our sections, without further treatment, would reveal but little, for they would be very transparent, and one portion would closely resemble another. So staining is resorted to, and, where practicable, it is preferable to stain before cutting the sections. Of stains for microscopic purposes there are many, and the value of each depends upon the fact that the different elements of cells and tissues will absorb it in varying quantities. Most used of all is some preparation of carmine which stains certain portions red, leaving others uncolored. Of these carmine solutions the student has no less than twenty to choose from. Next in order comes hæmatoxylon, or extract of logwood, which, when combined with alum, stains certain portions blue or purple. Osmic acid also stains a brown or a black, according to the structure and the length of exposure. Nitrate of silver is also frequently used for certain purposes, while of the anilines only eosin and Bismarck-brown have any great value.
In order to section the egg we must employ some means to hold it firmly, and for this purpose various substances are employed, paraffin or celloidin being the most common. The requisites of an imbedding substance are that it be possible to make it thoroughly impregnate every part of the egg, and also that it be of such a consistency as to be readily cut into the thinnest sections. The egg is imbedded in paraffin by completely replacing all the water in it by alcohol, this in turn by some solvent of paraffin, as turpentine or oil of clove, and then by keeping it for a time in melted paraffin. Then egg and paraffin may be cut as if only paraffin were present. In the case of celloidin (a solid form of gun-cotton) the intermediate reagents are alcohol and a mixture of alcohol and ether. The process involves some time to accomplish thoroughly, and here, as elsewhere, neglect of details is sure to result in failure.
In order to cut the sections, special instruments (microtomes they are called) have been devised, and are now made of a high degree of accuracy and excellence. So delicately are they made, that it is possible to cut an egg into a series of sections so thin that it would require twenty-five or even more of them to equal in thickness the paper on which this magazine is printed. In the early days of section-cutting no such facilities were available, and the apparatus described in the hand-books of microscopy even five years ago were utterly inadequate to produce good results. Of modem microtomes there are now four distinct types in use, two having the knife stationary, the other two having it moved through a fixed and definite plane. It is not necessary to describe these here; those who wish may find accounts and figures of them in recent works, like Whitman's "Methods of Research in Microscopical Anatomy and Embryology."
Very recently a new "kink" has been introduced into section-cutting which has relieved the student from a great deal of drudgery. It has been found that by trimming the block of paraffin square, and by having the edge of the section-knife at right angles to the line of stroke, the successive sections would adhere together by their edges, and form long ribbons, and that thus a large number of sections could be mounted as readily as one by the former method. For some purposes this serial section-cutting" has no especial advantages, but where it is desired to preserve every section it is indispensable. To mount them, however, requires special processes. The one most in use is the following:
A glass slide is coated on one side with a mixture of clove-oil and collodion, which forms a viscid, sticky surface. On this the ribbons of sections are laid and then the whole is exposed for some time to a gentle heat in a water-bath. This melts the paraffin and allows the section to drop into the sticky mixture where every part, even if separate from the rest, is firmly and securely held in its proper position. A continuation of the heat evaporates the clove-oil until the sections are fastened to the slide by a thin film of pure collodion. The paraffin is now washed away by turpentine or chloroform, Canada balsam and a bit of thin glass are applied, and then every section is permanently preserved and ready for study, it may be immediately or in a year or two.
Usually most of the time spent in a summer marine laboratory is devoted to studying the external features of development and in preparing material and slides of sections which can be examined more carefully later. With a slide which contains all of the sections cut from a single egg the student can reconstruct all the details of the embryo and can trace the course and limits of every organ and vessel. By comparing this with the sections of a later and an earlier stage, he sees how the organs arise, and how during growth they are modified in size and shape, as well as in their relations one to another. In studying the living embryo, time is not taken into account. It usually happens that material is obtained late in the day or in the evening. It takes no rest, but develops minute by minute, hour by hour, and the student, if he really wishes to master his problems must be reconciled to turn night into day. His egg is changing constantly, and, with pencil in hand, he sits with one eye to his microscope, watching the growth. Hour after hour thus passes away, each minute revealing some new feature, until at last his eyes tire and he is forced to quit. He seeks his bed with regret, for he has been compelled to leave some of the most interesting objects that the human eye ever saw. Consider for a moment the differences between an adult fish and its minute egg, clear as a drop of dew, and then try to imagine the changes which are necessary to convert the one into the other. Such things as these our student was forced to leave, and leave, knowing that the egg would not wait for him, but on the morrow would be far advanced, and there would be a great gap in his notes. While he was looking, it seemed as if he could almost see actual personal life. Changes were constantly occurring: now he saw a cell divide into two, just as though a knife had passed through it; again, he was seeing the way the cells thus formed were arranging themselves in layers and building up the embryo. Such scenes as these have a fascination beyond description, and the student can not repress the feeling that, could he wait a little longer, or were his microscope a little more powerful, he could actually see the force that accomplished these marvelous results. On the morrow, when he returns to his study, the feeling is the same, and no matter if he be witnessing some phase of development, even for the hundredth time, he is as interested as at first. At such times it seems as if the solution to the great problem, "What is life?" were really close at hand.
So far it has been an easy task to describe the process of conversion of an untrained person into an original investigator, but there is another side—a psychological one—which baffles description. All that is necessary in order to perform the various operations which have been thus briefly outlined are power of observation and skill in manipulation; but the facts thus gained must be interpreted in order to render them of real value. A paper which merely details the facts observed of course has its value; but if it adds a comparison of them with the phenomena which occur in other forms, and tells or even suggests their meaning, it then takes a far higher rank. This, however, takes thought, and who can describe the way in which one thinks?
The student tries to master every fact in the development of his embryos, and then compares these facts with what was already known of the development of other forms. In this way he recognizes similarities and differences, for both of which explanations are to be sought. Even in the development of the specimen studied there are many phenomena which have their own meaning, and which, properly interpreted, throw much light on its ancestry and line of descent. In general terms this interpretation is effected by framing an hypothesis which will embrace some of the facts, and then testing it in every conceivable manner. When an objection arises, the first step is to see if it really be founded on fact, or upon a misconception, and then, if it be valid, the attempt is made to reconcile it with the hypothesis. It frequently happens that several hypotheses are formulated before a satisfactory one is found.
Such in brief outline is the training, or rather a part of the training, which is necessary to make a naturalist to-day. The time is past when one can collect a few bugs or shells and then straightway proceed to describe so many new species. Description of species is a necessary work, but it is not the highest kind of work. Far more important, far more ennobling, far more interesting are the deeper problems of how an animal grows, why it grows in the way it does. The training necessary for work of this character requires as long a time, as much patience, and as much perseverance as does any of the so-called learned professions; but when one becomes an original investigator there is no respite. It means continual work, continual study. If one stop, even for less than a year, be can never catch up. Others have been at work while he was idle, and he is left behind. Pecuniarily a naturalist's work does not pay; the same amount of energy and ability spent in any other direction would bring in ample recompense, but here it does not. It does, however, have its own reward: every new fact discovered gives the student far more pleasure than any money could; each new advance into the terra incognita of Nature lightens the inconveniences of poverty.
And now the question may arise. What is the use of discovering the secrets of Nature if there be no money in it? Yes, that is it. Everything must be reduced to a basis of dollars and cents! The utilitarians who propound such questions do not and can not see any value in learning for learning's sake; it must bring some pecuniary reward. In some instances it is readily seen that such studies have a direct influence in curing or ameliorating some of the ills that human flesh is heir to; in other instances their exact bearings are not immediately apparent. Just one hundred years ago an Italian physician, Galvani by name, discovered some facts which, while interesting, must have seemed at that time wholly without practical bearings. To-day no one can deny that they were fraught with great good to mankind. What would the world be without electricity as a servant? Yet Galvani's experiments contained the germ of all our numerous electrical discoveries. Who can say but that biological studies are to have an equal value in solid gold?