I.

RECENT EVOLUTIONARY STUDIES IN GERMANY.

Since Darwin’s death, his theory, which in Germany more than elsewhere received its development, has but few decisive steps in advance to point to, even though the circle of its adherents has been enlarged and though in many respects and in special directions it has been rendered more complete and placed upon a firmer foundation. It is a gratifying fact, that most, if not all, of the recent discoveries in zoology, palæontology, and particularly in developmental history, are easily and completely reconcilable with the principles originally established; so that the views which we have reached on this subject have lost more and more the characters of a purely hypothetical fabric.

But the accurate investigations of developmental history have unquestionably furnished the most important material in proof of the theory in question, and the principles established in this department, in the main the results of the labors of German investigators (E. von Baer, Fritz Müller, E. Haeckel), have been verified in a truly surprising manner.

It is true that Darwin himself in no way undervalued the importance of the results of the studies in question, but how little the facts known at the time of the enunciation of his theory of natural selection sufficed, is most clearly proved by the fact that E. von Baer and Louis Agassiz, who at that time were perhaps the greatest authorities in embryology, assumed a hostile attitude towards the new Darwinian theory. Agassiz’s combination of the points of agreement of palæontology and embryology, his explanation of extinct forms as “prophetic types,” proved a veritable hindrance to the perception of the truth, and Carl Vogt, who was his co-worker at that time, appears to have been the last to set up any opposition to the “fundamental biogenetic principle,” that the development of the individual repeats in an abbreviated form the history of his race. Vogt, formerly the champion of advanced views, appears to-day as the leader of the small band of the opposition.

If we compare the recently published fourth edition of Haeckel’s _Anthropogeny_, 1891, and the eighth edition of his _History of Creation_, with the early editions, we cannot help remarking, with considerable astonishment, despite the enormous increase of fresh material, the fact that little in the old plans and principles of the work needs correction. Even the bold generalisations, the inference as to the identity of form of the original beginnings of all of the middle or higher classes of animals, the “Gastræa Theory” of Haeckel, at first so violently opposed, the stress laid upon the equivalence of the blastoderms in the various orders of animals, nay, even many of the animal genealogies, really only asserted as a working hypothesis, have stood the test beyond all expectation; although Du Bois-Reymond insinuated that the pedigrees of the heroes of Homer were more worthy of credit. To appreciate the complete victory of these ideas one need but refer to the discourse _On Recapitulation in Embryology_ with which A. Milnes Marshall opened the meeting of the Biological Section of the British Natural History Society at Leeds, September, 1890.

The very conspicuous irregularities in the formation of organised bodies, which formerly were regarded as monstrosities, or as the freaks and riddles of the formative instinct, the hare-lips, the cleft palates, cases of microcephaly, etc., or the conspicuous want of symmetry in the physical structure of the plaice and sole, formerly made use of by Mivart and Schimper as unassailable counterproofs of Darwin’s doctrines, have shaped themselves into the most decisive verifications of his theory; as in fact, generally, a number of the most splendid evidences of the correctness of the theory have, as the result of exact investigations in organic evolution, proceeded from the most obstinate of its supposed difficulties. Thus, for example, as proof that birds are far removed from the other classes of vertebrates, the circumstance had been cited that certain parts connected with the visual organs are in them situated at the side of the brain, instead of on the dorsal surface, as is the case with the other vertebrates. But a more exact observation has shown that this variation in formation is a secondary result, since in each previous period of development these same organs in the young birds lie, exactly as in the case of the other vertebrates, on the dorsal surface, and only shortly before leaving the egg do they move downward to the sides. In many cases where the development of parts preservable in fossil conditions is under consideration, as for instance portions of the skeleton, the hard integuments, and the teeth, a direct proof may, by comparison, be furnished of the truth of the fact of the correspondence of the embryological formations of living animals with the final and permanent forms of their extinct representatives, a fact which was indeed acknowledged by Agassiz and Vogt, but completely misunderstood. We need only to recall to mind the exact parallelism which Alexander Agassiz and Neumayer have demonstrated to exist in the case of echinoderms, Huxley, Marsh, and others in the formation of the wings of birds, the pelvis of birds, or the hoofs of horses, in order to stamp this view as one that cannot be refuted.

Nevertheless, those opposed to this view, as Carl Vogt, His, Heufen, and others, have not abandoned their position as a hopeless one, and in recent years have relied particularly upon those cases which Haeckel, and before him, Fritz Müller, characterised as a falsification (cenogenesis[16]) or a supplementary alteration and abbreviation of the natural process of development. “Nature is no falsifier,” these opponents proclaim with emphasis, and everything it does is correct and true, and “this false heart alone brings untruth and deceit into the true heaven,” they cry with Wallenstein. People who rely on verbal sophistries merely betray thereby their want of valid counter-arguments. A _mala fides_ on the part of nature can of course never be the subject of discussion among reasonable beings, but a deviation in the process of development of certain varieties from the typical path of the development of the remaining varieties of the species, is _felt_ as a falsification by every investigator who has thoroughly studied the regular processes, for the reason that it has a tendency to _obscure_ the original facts. Thus, for example, in the embryos of certain vertebrates the æsophagus is temporarily completely closed, as Balfour has observed in young sharks, Bles and Marshall in frogs; and this state of affairs may well be considered as a falsification, since an animate being with a closed æsophagus is a natural contradiction, which can never have existed and here happens as a supplementary and temporary process.

As a rule such deviations from the normal course may be classified as consequences of a prolonged residence of the animal germs in the egg or in the womb, the result of which is that owing to the presence of an abundant quantity of nourishing yolk, or through direct connection with the circulatory system of the mother, they in the early stages of their development are relieved of the necessity of acquiring nourishment through their own efforts, and therefore all the contrivances necessary to that purpose may be dispensed with. For this reason we find the primitive processes of development, as Professor Sollas has lately shown, most frequently preserved in marine animals which have never changed nor abandoned their element in the course of the history of their species, in the case of which, therefore, no occasion could ever have arisen for supplementary changes in the process of their development. Much more frequently do we meet with this change in the case of fresh-water animals, for often the rapid currents of their elements, for example a river, will not suffer these to leave the egg in any very helpless larval condition, and in addition fresh water is subject to other unfavorable changes, as the drying up of streams. Also the larvæ of carnivorous animals, which from the very beginning of independent life need more strength to acquire their means of existence, are so completely developed in the richly provisioned eggs in which they take their form, that they emerge therefrom in an almost perfected state of being, as, for example, young sharks and cephalopods. In this kind of animal life, as well as in the case of forms which are brought forth alive from the parent, although they see the light of day much later, comparatively, there takes place not only a great abbreviation of the first stages of existence in the entering upon a more direct path of development, but also changes occur in the form of the original designs because of the limitation of room due to the presence of yolk in the egg, the reason for which is easy to perceive. In many other cases the mechanical cause of the change in development can be directly recognised; for example, in the case of, the tree-toad of the Antilles (_Hylodes marticinensis_), which, owing to the absence of pools lasting through the dry season, is obliged of necessity to remain in the egg during its tadpole stage, that is to say, to skip this stage, as it were; for which reason the formation of external gills in its case is entirely omitted.

The explanation of the origin of new organs seemed at first to afford an insuperable difficulty to the Darwinian theory, since, as Mivart objected, it was not possible to perceive how natural selection could be able to effect the formation of new organs unless they executed corresponding functions from the very beginning. This difficulty, however, has been completely overcome by the theory of altered functions (_Functionswechsel_) which was first proposed by Dorhn, and particularly in recent years by Kleinenberg. According to this theory, in all these cases we have simply to deal with a gradual change in form of already existing organs, which, originally being used to perform one set of functions, are modified so as to perform another. Thus the later developed organs of mastication and the feelers of insects were originally organs of locomotion, legs; and these in the still earlier stages of creeping motion performed appropriate functions as the crooked appendages of the body-rings. The wings of birds were, in their progenitors, forelegs; the tongue of air-breathing vertebrates originated from the fish-bladder, which before that was chiefly an organ of swimming.

The knowledge thus acquired of the natural connection of the processes of evolution also explains, according to Kleinenberg, why organs which are at present completely useless, must yet necessarily appear in the formation of the embryo; for example, the gill-openings in the higher classes of vertebrates, which have no functions to perform at any stage of vertebral development, and which furnished Meckel the first intimation of the fundamental biogenetic law. But as soon as it was explained that the gill-openings furnished the foundation of the development of later-appearing organs with actual functions to perform, it was rendered clear why they should continually recur; namely, because they form the indispensable links of a chain which extends from the dim past of the type in question down to the present time.

There is no doubt that profounder researches in evolutionary history will furnish still more important results: for instance, the more perfect elucidation of the pedigree of mammals; for in this province even our domestic animals are not sufficiently investigated. Every new effort in this direction, for example the recent work of Klever on the evolution of the teeth of the horse, and other investigations concerning the formation of special organs, has invariably shown that much in this field yet remains to be discovered. We have only to recall to mind the recent investigations relating to the development of the pineal gland, which in the last decennium have also led to the discovery of a rudimentary occipital eye, which seems to have actually existed and performed functions in numerous early representatives of the vertebrates, but to-day is simply a fact of history, and has given rise to an organ which Descartes considered as the seat of the soul. We may here also refer to the recent investigations concerning the earlier developmental stages of the duckbills, which have completely confirmed what the theory asserted in advance and required; namely, that they fill the vacancy between the egg-laying reptiles and the mammalia which bring forth their young alive.

Only a few years ago Carl Vogt vehemently opposed the opinion of the duckbills being transitional types, and sought to explain their inferior stage of organisation, which is also evidenced in their low blood temperature, as the results of a stunting process (degeneration, so called). They formed a degenerated branch of marsupials, nothing more. Later, the remarkable yet long anticipated fact was revealed by Haacke and Caldwell, 1884, that the duckbills are egg-laying mammals, a character which certainly could not have been acquired through degeneration, but which simply shows that they are closely related to extinct reptilian forms. In one other respect, namely, with regard to their supply of teeth, the process of degeneration must indeed be admitted. On this point, Poulton and Thomas discovered a few years ago that in their early stages they really do possess true teeth, which, however, just as in the case of certain carnivorous cetacea, later completely disappear, and are replaced by a sort of horny teeth. This, however, is really not a true degeneration, but rather a special adaptation, doubtless beneficial to the animal in some way or other; and with as little reason as we may regard birds as a degenerated race in comparison with their progenitors, because they have lost the numerous teeth which these possessed, with just as little reason can we hold that the duckbills, in their general organisation, have suffered any retrogression worth mentioning. On the contrary, the recent investigations of Marsh and Lemoine concerning the mammals of the Jurassic and Cretaceous periods point more and more distinctly to the conclusion, that there existed among these mammals a very large number which possessed the same degree of organisation as the duckbills of to-day, now represented by only a few species; a supposition which the adherents of the theory of evolution made twenty-five years ago. I do not know that the pedigrees of the heroes of Homer have been so well preserved!

In many other directions, however, speculation of late years in Germany has considerably digressed from the facts of experience and from all probability; especially with reference to the questions of propagation, variation, and heredity. Here, first of all, are to be mentioned the works of Weismann, _Ueber die Continuität des Keimplasmas_ (1885), _Die Bedeutung der sexuellen Fortpflanzung für die Selectionstheorie_ (1886), _Der Rückschritt in der Natur_ (1886), _Die Bedeutung der Richtungskörperchen für die Vererbungstheorie_ (1887), _Die Hypothese der Vererbung von Verletzungen_ (1889), and _Ueber Amphimixis_ (1891).

If we revert to the beginnings of this movement we shall find that it is intimately connected with the more exact study of the processes of fecundation as perfected through the researches of Strassburger, the Hertwigs, and other investigators. In connection with the ideas of Nägeli concerning the so-called idioplasm, the notion was reached that the matter determinative of heredity was contained in the nucleoli, and that by the union of the paternal and maternal nucleoli the sum-total of the parental hereditary tendencies is transmitted to the offspring. This view was to a certain degree verified by the experiments of the brothers Hertwig in removing the nucleoli of the eggs of the sea-urchin; the result being that eggs containing the nucleoli alone, furnished, through artificial impregnation, results resembling the female parent, whereas eggs from which the nucleoli had been removed, furnished germs completely corresponding to the traits of the male parent.

Other processes of fecundation, to which we shall soon recur, had since 1876 produced the impression in the minds of a number of naturalists that the germ-material led an independent life in the bodies of organisms, that it possessed only an internal development, and required from the body nothing but nourishment in order to multiply itself, and to develop its internal powers uninfluenced by the various vicissitudes of the body. In the year 1876 Gustav Jaeger in Germany, and Francis Galton, a cousin of Darwin, almost at the same time in England, called attention to the observation made some time previously, that in certain animals, particularly in insects, the development of the egg into the young offspring begins with the withdrawal of a small portion of the germ from the component substance of the embryo, which remains at first unchanged and only later multiplies. This observation was generalised and accepted. At the commencement of every sexual multiplication the germ-substance, after impregnation, is divided into two parts, according to its future purpose; an ontogenetic or personal part, out of which the body is built up, and a phylogenetic or germinal part, which at first is stored up unused in the individual, but later furnishes new germ-cells. This idea led Weismann to his view of the continuity of the germ-plasm, which forms an unbroken line of descent from the first beginnings of the species and which is simply nourished by the organisms in which it has its temporary abode. From this germ-plasm spring _secondarily_ the cells that go to make up the body (soma); but from these soma-cells no new _germ_-cells can originate, and consequently none of its inherent or adscititious qualities are capable of transmission. The somatic cells make up the mortal and perishable forms of life, while the germ-cells alone insure the further existence and immortality of the race.

It is easy to perceive that these views, if they could be maintained, would completely transform the Darwinian theory. Since, if the somatic cells, that is, the body-parts of animals and plants, with all their adaptations to soil and climate, to definite modes of life, etc., are to be deprived of every power to transmit hereditary characters, then the so-called Lamarckian theory, which should really bear the name of Erasmus Darwin, would be deprived of every foundation which it possesses. Neither the increase in strength of the members of the body, acquired by use and practice, nor their weakness created by their non-use could be inherited; and in just as small a degree could changes caused by external influences, bodily injuries, sickness, entail consequences which were inheritable. This being the case, then also all those views would be untenable which seek to explain the important effects of time as the result of the accumulation and augmentation of the minute impressions of the external environment. If the variations which are generated by means of external influences are not capable of transmission, then the direct adaptation must commence at the beginning in the case of every following generation; an accumulation is impossible.

We can observe, however, in every particular case, the complete harmony in which every living being exists with its surroundings and mode of life; and observe in closely related species the most various adaptations to the elements in which they live: climate, food, nay, even to the particular companions with which they associate; with the result that many plants have shaped the structure of their flowers to conform to the physical anatomy of the insect which ordinarily effects their fertilisation, and that animals assume the figure and form of some associate who is safe from hostile assaults, or even completely adopt different modes of life where it is necessary to enter a life-partnership with a strange animal or plant. But, granting that the most widely extended capability of adaptation is a thing of daily experience, there still arises the question how we shall explain this quality, which can only be brought about by slow degrees, without taking into account the factor of heredity in the transmission of acquired qualities. The theory of Weismann attempts this, in that it takes for granted an infinite variability in the germ-formative materials, and guides the new forms and variations thus begotten into the really true path, that is, into the most successful paths, through the process of natural selection (that is, through the survival of the fittest as regards environment and all other things). According to this doctrine, external circumstances have no direct influence whatever upon the variation of species, as Erasmus Darwin, Lamarck, and the founder of the theory of natural selection and all his followers up to that time supposed, but we have to have recourse to a pure theory of natural selection, and call to our aid an, even now, rather obscure phenomenon, occurring in connection with sexual impregnation, which has been called “the expulsion of the polar bodies,” an extrusion of minute qualities of germ-plasm from the germ-cells while in union. By the processes of crossing, which continually recur, a vast number of the most manifold hereditary tendencies are united in the germ-material. Then certain of these are ejected, so that others acquire supremacy; and in this manner the way is opened for the origination of a vast number of possible combinations. In this way the path is clear to a theory of perfect mechanical variability, in which the germ-material has only to transmit the characters which spontaneously arise in it, and yet affords an investigator endowed with any imagination the possibility of understanding the origin of the great variety and final purpose of the world. It is Frohschammer’s “principle of the imagination as the creator of the world” translated into comprehensible formulæ. The simplicity thus reached by the elimination of all direct influences from the external world, has won the adherency of many investigators following in Darwin’s steps, particularly in England; but whilst Wallace, Galton, Ray Lankester, and others have expressed their full assent to it, other and not less eminent authorities, as Herbert Spencer, Haeckel, Fritz Müller, and Virchow, have emphatically rejected it.

The reasons in favor of this assumption are, as is indeed the whole view itself, mainly of a theoretical nature; the arguments of the opposition are divided into philosophical and experiential propositions. The philosophical opposition is mainly based on the fact that, from the very beginning, there is assigned to the germ-material, as it unceasingly continues its existence, an infinite variety of capacities which the external world cannot affect, and that all progress and advancement takes place as the result of the _loss_ of the originally endowed powers and tendencies. On this theory a family of acrobats or race-horses would not acquire their powers through the gradual augmentation by practice of their feats of skill and endurance, but because these powers were originally resident in them, and every factor incompatible with them was gradually eliminated. On the other hand, these views approach in a dangerous degree to the theories of predestination and preformation, the overthrow of which has been justly regarded as one of the greatest advances of science.

Still more important must be considered the objections of empirical science, which up to this time was completely convinced of the heredity of acquired qualities. Popular experience, as well as that of physicians, universally speaks of inherited disease-germs, and in certain cases, particularly in mental diseases, physicians are so thoroughly convinced of their inheritability that the first question put to the relatives of such sufferers usually is whether the disease has ever appeared in the parents or family of the patient. This fact is so deeply grounded in the general belief, that the modern naturalistic school of novelists, the school of Zola, Ibsen, and their associates, are wont to devote their main efforts to the problem of inherited evils. Now the inheritability of certain evil conditions, even though proved, would not by any means be an absolute disproof of Weismann’s theory; for, inclined as much as we may be to derive diseases from mistakes and sins against a natural mode of life, such as colds, drunkenness, dissipation, mental and bodily over-exertion, we yet cannot deny _a priori_ that blastogenic diseases, or diseases originating in the germ-plasm, may exist, which without any doubt would then be transmissible. It also does not lie beyond the realms of possibility that congenital malformations, such as hare-lips, supernumerary fingers, and the defects which show a remarkable disposition to heredity, fall into this category. These blastogenic germs of disease would then, of course, have to be distinguished from the somatogenic diseases (or the diseases produced in the body by external causes), which never could be inherited.

From this point of view the question as to the hereditary consequences of external injuries has given rise to great efforts to prove experimentally the truth of this belief, which has existed for centuries. In almost every part of the globe we meet with the assertion that hornless cattle, such, for example, as are bred in South America, or the tailless cats of the Isle of Man, or other domestic animals with similar deficiencies, are descended from a progenitor which lost its horns or its tail through disease or other mishap. Since now, recently, similar assertions have again been put forward to the effect that tailless cats are found among the descendants of feline progenitors who have been robbed of their posterior ornaments by an act of violence, and these cases have been discussed in connection with the pangenesis theory of Darwin, according to which each part of the body is believed to supply material contributions to the germ-plasm, Weismann determined to institute experiments on this point. He started the breeding of white mice whose tails were regularly cut off, without finding as a result, from among 840 young ones derived from such mutilated progenitors, a single one having a malformation or missing tail. However, even this experiment cannot be regarded as an absolute proof, as it at first view might seem, and the negative result was foreseen by the writer of these lines. It is a clear conclusion that if in the case of many vertebrates, for example, salamanders and lizards, as well as in the case of most invertebrates, missing limbs and tails are renewed in the course of their lifetime, it would indeed be very remarkable if their renewal should not take place, at least in the case of the complete rejuvenation of new birth.

Darwin himself had concluded, from his own experience and that of others, that injuries and similar inflicted acts of violence are the cause of hereditary consequences only in cases where they bring about some long-continued and wasting disease, and thus produce some permanent effect on the bodily constitution. For this reason, especially injuries to main nerve-tracts in parts near the centres are readily accompanied by hereditary consequences, because they interfere with the nutrition of the members supplied by them. Brown-Séquard has observed in a great number of cases of guinea pigs whose nerve-roots he had severed, that the offspring of the animals operated upon developed diseases of the eyes, ears, and other organs which conformed regularly to the character of the operation, and could therefore be predicted; and also noted malformations and deficiencies, amounting even to the complete disappearance of the eye-balls, such as never arise or have been observed in these animals without violent interference. His positive results regarding the hereditability of the evil consequences of disturbing operations have a decided advantage in numbers and scope over the negative results of Weismann; and it is not clear how the belief in the non-hereditability of somatic conditions will accommodate itself to them.

But if conditions of the body produced by such sudden interferences have under certain circumstances entailed hereditary consequences, how much more should we expect this same result from slowly effected constitutional changes, which external influences, working uninterruptedly for hundreds of years, bring about in an organism which has been transported into a new element, into new surroundings, or into a different climate. Not at all infrequently does the coming together and union of two new organisms beget hereditary changes which can be explained only through the direct influence of the one upon the other. Thus, for example, in the case of plants in hot countries which are protected against the assaults of leaf-devouring ants by body-guards of smaller ants, and also in species of quite different families, as, for example, in _Cecropia_ of the order _Euphorbiaceæ_, and in some _Triplaris_ species among the _Polygonaceæ_, we find little chambers, approachable through small openings in the stems, which serve the ants protecting the plants as dwelling and breeding places. Are we to believe now, in regard to this fact, that these plants, so different in their nature, have produced through voluntary variations the stems which contain these openings, or are we to believe we have to deal here with openings acquired through inheritance which originally were bored in the stems by the ants at the most appropriate points? Surely the first conclusion, which would uphold Weismann’s theory, has but a very slight degree of probability in its favor, whilst the latter, which would overthrow his view, is very highly probable. And such examples could be cited in great numbers.

It is also to be remembered that the power of variation is not exhibited solely in sexually created individuals, as it should be according to Weismann’s theory, but frequently also in non-sexual multiplication, where no amphimixis (mingling) occurs. It is well known that the majority of the sporting varieties of our trees, for example, _Fagus sanguinea_, and the so-called weeping varieties, that is, abnormal varieties with pendent twigs, forms with split, spotted, or white leaves, are wont first to appear on single branches of old trees, in which the continuity of the protoplasm unquestionably existed, but no amphimixis or extrusion of the polar bodies took place. It is also the generally received opinion of naturalists that the lowest classes of animal and plant life are universally multiplied by non-sexual means. And if this is so, it is not clear how higher forms which sexually propagate can be derived from them, if the latter have originally to furnish the fundamental conditions of variation. The adherents of Neo-Darwinism will, accordingly, have to furnish many additional facts if they wish to invest their theory with any degree of probability.

CARUS STERNE.

FOOTNOTES:

[16] _Cenogenesis_, from κενός, empty, fruitless (and γένεσις, birth); not from κοινός, common, the derivatives of which are sometimes written “c_e_no.”—ED.