The Biological Problem of To-day.

by Oscar Hertwig.

PREFACE

Shortly after the appearance of Dr. Oscar Hertwig's treatise 'Praformation oder Epigenese?' I published in _Natural Science_ (1894) a detailed abstract of it. But the momentous issues involved in the problem of heredity, and the great interest excited by Dr. Weismann's theories, make it desirable that a full translation should appear. By the kindness of Dr.

Hertwig and his German publisher, this is now possible. I have prefixed an introduction, written for those who are interested in the general problem, but who have little acquaintance with the technical matters on which the argument turns. In the actual translation I have tried no more than to give a faithful rendering of the German. After no little perplexity, I have rendered the German word _Anlage_ as 'rudiment.' It is true, a double meaning has been grafted upon the English word, and it is widely employed to mean an undeveloped structure, without discrimination between incipient and vestigial character. I use it in the etymological sense, as an incipient structure. For the difficult words, _Erbgleich_ and _Erbungleich_, a succession of new terms have been suggested. Here I use for the first term the word 'doubling,' for the second 'differentiating.'

P. C. M.

INTRODUCTION

Inquiry into the problems of heredity is beset with many difficulties, of which not the least is the temptation to argue about the possible, or the probable, rather than to keep in the lines of observation. Setting out from a laborious and beautiful series of investigations into the anatomy of the Hydromedusae, Weismann came to think that the organic material from which the s.e.xual cells of these animals arose was not the common protoplasm of their tissues, but a peculiar plasm, distinct in its nature and possibilities. In the course of several years, Weismann not only continued his own investigations in the many directions that his conception suggested, but made abundant use of that new knowledge of the nature and properties of cells which has been the feature of the microscopy of the last decade. His theory of the germplasm gradually grew, undergoing many alterations, so that even in its present form he regards it as tentative.

Neglecting the numerous modifications and accessory hypotheses by which he has sought to adapt the theory to the phantasmagorial complexity of organic nature, the main outline of the theory is as follows: A living being takes its individual origin only where there is separated from the stock of the parent a little piece of the peculiar reproductive plasm, the so-called germplasm. In s.e.xless reproduction one parent is enough; in s.e.xual reproduction equal ma.s.ses of germplasm from each parent combine to form the new individual. The germplasm resides in the nucleus of cells, and Weismann identifies it with the nuclear material which microscopists have named chromatin, on account of the avidity with which it absorbs certain dyes.

Like ordinary protoplasm, of which the bulk of cell-bodies is composed, the germplasm is a living material, capable of growing in bulk without alteration of structure, when it has access to appropriate food. But it is a living material much more complex than protoplasm. In the first place, the ma.s.s of germplasm which is the starting-point of a new individual consists of several, sometimes of many, pieces termed _ids_, each of which contains all the possibilities--generic, specific, individual--of a new organism. Each _id_ is a veritable microcosm, possessed of a historic architecture that has been slowly elaborated during the mult.i.tudinous series of generations that stretch backwards in time from every living individual. This microcosm, again, consists of a number of minor vital units called _determinants_, which cohere according to an orderly plan. A _determinant_ exists for every part of the adult organism which is capable of being different in different individuals. And, lastly, each _determinant_ consists of a number of ultimate particles called _bioph.o.r.es_, which eventually pa.s.s into the protoplasm of the cells in which they come to lie and direct the vital activities of these cells. A most important part of the theory is what it supposes to occur during the embryological development of the individual. The ma.s.s of germplasm derived from the germplasm of the parent lies in a ma.s.s of ordinary protoplasm.

Both the protoplasm and the germplasm, by the a.s.similation of food, gradually increase in bulk until the adult size of the organism is reached.

Along with the increase of size there occurs a gradual specialisation, during which the tissues, organs, and structure of the creature are attained. The simplest conception of this process is to regard the initial ma.s.s as a single cell, the nucleus of which is composed of the parental germplasm. The nucleus and the protoplasm increase in size, and then, first the nucleus and next the protoplasm divide, so that there are formed two cells, each with a nucleus. Each of these again divides, and the process goes on continuously, the new-formed cells gradually being marshalled into their places to form the adult tissues and organs, and they gradually a.s.sume the special characters of these tissues and organs. Now, Weismann's theory supposes that the first division of the germplasm is what is called in this translation a _doubling_ division (_Erbgleiche Theilung_). The ma.s.s has grown in bulk, without altering its character, so that each resulting ma.s.s is precisely like the other. One of the two portions subsequently increases in bulk, and may again divide repeatedly, but always by doubling division. It therefore remains unaltered germplasm, and eventually is marshalled to the part of the adult from which new organisms are to arise, becoming, for instance, in the case of a woman, the nuclear matter of the ovary. Thus, the germplasm is handed on continuously from generation to generation, forming an unbroken chain, through each individual, from grandparent to grandchild. This is the immortality of the germ-cells, the part of the theory which has laid so strong a hold on the popular imagination. And with this also is connected the equally celebrated denial of the inheritance of acquired characters. For, at first, it seemed a clear inference that, if the hereditary ma.s.s for the daughters were separated off from the hereditary ma.s.s that was to form the mother, at the very first, before the body of the mother was formed, the daughters were in all essentials the sisters of their mother, and could take from her nothing of any characters that might be impressed upon her body in subsequent development. As this treatise touches only indirectly on the question of acquired characters, it is necessary only to mention that while his early sharp denial of the possibility of inheritance of acquired characters has led to a damaging criticism of supposed cases, Weismann, in the riper development of his theory, has found a possibility for the partial transference of influences that affect the mother to the germplasm contained within her.

It is with the fate of the other portion coming from the first division of the germplasm that we are concerned here. It is set apart to form the nuclear matter, and so to control the building up of the actual individual.

Weismann supposes that the subsequent divisions it undergoes are what I call in this translation differentiating divisions (_Erbungleiche Theilung_). According to his theory, in each of these divisions the microcosms of the germplasm are not doubled, but are slowly disintegrated, the division differentiating among the determinants, and marshalling one set into one portion, the other set into the other portion. The differentiating process occurs in an order determined by the historic architecture of the microcosms, so that the proper determinants are liberated at the proper time for the modelling of the tissues and organs.

Ultimately, when the whole body is formed, the cells contain only their own kind of determinants. It follows, of course, from this that the cells of the tissues cannot give rise to structures containing less disintegrated nuclear material than their own nuclear material, and least of all to reproductive cells, which must contain the undisintegrated microcosms of the germplasm. As special adaptations for the formation of buds and for the reconstruction of lost parts, cells may be provided with latent groups of determinants to become active only on emergency. But with these exceptions, the nuclear matter of the cells of the body contains only what is called _idioplasm_, a differentiated portion of the germplasm peculiar to cells of their own order, and it can give rise only to idioplasm of the same or of a lower order. And here we come round again to the original observations from which Weismann set out. For he found that among the Hydromedusae, although the s.e.xual cells seemed to arise in very different topographical positions, there had always been a migration to these localities of a material which he would now call the germplasm. And here also, that the point may be made plain, there may be mentioned the observations of surgeons and physicians, who insist that the growths of disease always conform strictly, in their cellular nature, to the tissues from which they arose, and that in the healing of wounds like only grows from cellular like.

Dr. Oscar Hertwig is a scientific naturalist of the very first rank, and his name is peculiarly a.s.sociated with many of the most important advances in our knowledge of cells and of embryology. To him chiefly, for instance, is due the discovery of the intimate nature of fertilisation--that it consists in the union of the nuclear matter of a cell from the male with the nuclear matter of a cell from the female. With the exception of Francis Balfour, no man has laboured more patiently, or achieved more wonderful results, in the investigation of the origin and marshalling of cells by which the egg changes into the adult. From his own experience, and from his study of the observations made by others, he has been led to doubt the validity of apparently fundamental parts of Weismann's conception. In the first place, he thinks that there is no evidence for the existence of differentiating as opposed to doubling divisions, and that there is evidence that divisions always are doubling divisions. He thinks, in fact, that when a portion of germplasm divides, the daughter-cells receive portions of germplasm exactly alike and exactly like the original portion in the parent-cell. The cells, indeed, become different from each other as the organism grows, some becoming muscle-cells, others nerve-cells, others digestive-cells, and so forth. Weismann thinks that the differences occur because, in the disintegration of the germplasm-microcosms, according to a prearranged plan, only the determinants for nerve-cells are marshalled into nerve-cells, only those for muscle-cells into muscle-cells, and so forth.

The development is an evolution, an unfolding or unwrapping of little rudiments that lie in the germplasm. Hertwig insists that every cell receives the same kind of germplasm, but that, according to the situations in which they come to lie, different characters are impressed upon them.

The development is an epigenesis, or impressing on identical material of different characters by different surrounding forces. His second line of argument against Weismann leads to a similar conclusion. A large number of the characters that arise in an organism during its development are due to the combination of many cells. They cannot come into existence until the multiplication of cells has made their existence possible, and he thinks, therefore, that they cannot have rudiments inside a single cell as their determining cause.

It is no part of my present purpose to insist, even to the extent that in this treatise Hertwig himself insists, upon the points of agreement between the two views. We are only at the beginning of inquiry into the problems of heredity, and the protagonists of the opposing views, like all those who care more for knowledge than for argument, are concerned more for truth than for the establishment of a _modus vivendi_. Reconciliation is the parent of slothful thinking and of glosses; it is by sharp contrasting of the opposing views that we are like to have new facts elicited, and new lines of inquiry suggested.

As many are interested in the problems who have little acquaintance with the technical facts of embryology, a simple account of the early stages in the development of an animal may be useful for reference. I shall choose back-boned animals, as, from the inclusion of man among them, they are of more general interest. The process begins with the fertilisation of the egg-cell by the fusion with its nucleus of the nucleus or head of a male-cell or spermatozoon. At their first origin the nuclei of the sperm and of the egg may be of very different appearance, while that of the sperm is invariably smaller than that of the egg. But before or during the process of fertilisation, changes take place, the result of which is that the fusing nuclei are exactly alike in morphological character. The chromatin, or peculiar substance of the nuclei, is transformed into a number of bodies known as chromosomes, which are of the same number, form, and size, in the two s.e.xes. Form, size, and number are different in different animals, but there is reason to believe that they are normally the same in all the individuals of a species. The fertilised nucleus, thus consisting of chromosomes from male and female, then divides by a complicated process known as karyokinesis, in which each chromosome splits longitudinally, one half pa.s.sing to each daughter-nucleus. Throughout the whole process of embryonic and post-embryonic growth, the chromatin is gradually increasing in bulk, and being distributed by karyokinesis. The normal character of these divisions is as follows: A daughter-nucleus, after separation, pa.s.ses through a resting phase, in which the chromosomes, as definite structures, disappear, and in which growth of the nuclear matter occurs. Then chromosomes of definite size and form, and corresponding in number to those present in the fertilised egg-cell, again appear. These split longitudinally, and a half of each pa.s.ses to each daughter-nucleus. The similarity of these processes among all living creatures, vegetable and animal, and their extreme complication, suggests that karyokinesis is the chief factor in distributing the hereditary ma.s.s to the growing organism. Weismann and some others think that there is evidence for a difference in the nature of the process, which may in some cases correspond to his distinction between doubling and differentiating divisions, but it may be said at once that the record of observations is yet too conflicting for any such general interpretation.

Along with the increase in bulk and distribution of the nuclear matter, there goes an increase in bulk and segregation of the ordinary protoplasm.

The simplicity of the actual development of most back-boned animals is disguised by provision for the nutrition of the growing embryo. In a large number of cases, as, for instance, in birds and reptiles, the egg-cell, a microscopic structure at its first formation, is bloated out into the large eggs with which we are familiar, by the addition of quant.i.ties of food-yolk. These eggs, although morphologically single cells, do not divide as cells. A small disc of protoplasm, surrounding the nucleus, floats upon the surface of the yellow yolk, and, when the nucleus divides, furrows appear in this between the daughter-nuclei, but stretch very little way into the inert food-yolk. The subsequent marshalling of the cells is disguised by their a.s.sociation with a preponderating ma.s.s of inert material. In a far-distant period in the history of evolution, the eggs of mammals like man were large, and contained, as in the lowest existing mammals, a store of food-yolk. Now the food-yolk is not formed, as the developing embryo obtains its nourishment from the blood of the mother. But the course of development is distorted, partly as a legacy from the old large-yolked condition, and still more to suit the new method of nutrition.

Some of the simpler animals even among existing vertebrates still exhibit a marshalling of cells common among invertebrates, and to be traced under the complications of higher forms. In these, now, as in the marine ancestors of all the vertebrates, the fertilised egg is a tiny cell provided with very little yolk, and set adrift in the sea-water. The first division of the nucleus, and each subsequent division of the daughter-nuclei, is at once followed by division or _segmentation_ of the whole cell. The plane between the two cells thus formed is called the first cleavage-plane, and is regarded as vertical. The second cleavage-plane is at right angles to the first, and is also vertical, so that the little embryo consists of four cells, all on the same horizontal plane. The third cleavage-plane is horizontal, and divides the four cells into an upper and lower tier of four cells. In the course of a series of divisions the eight cells come to form a hollow sphere--the blastosphere--enclosing a cavity known as the cleavage or segmentation cavity.

The first great modelling then occurs. At one side the single layer of cells, of which the wall of the blastosphere is composed, begins to bend inwards, just as a dimple forms in a hollow india-rubber ball if a pin-p.r.i.c.k allow some of the contained air to escape. Further cell-divisions occur, and the inv.a.g.i.n.ation becomes deeper, until the inv.a.g.i.n.ating wall nearly touches the wall which has retained its primitive position. The embryo has thus become a hollow cup, the walls of which are double. The cup elongates, and its mouth, originally wide open, becomes more and more narrow, until it forms a small pore opening into an elongated blind sack.

The embryo in this stage is known as a gastrula. The central cavity becomes the cavity of the gut; the pore leading into it marks the hind end of the future animal, in the case of vertebrates, and is known as the blastopore. The layer of cells lining the cavity of the sack is known as the hypoblast, and gives rise chiefly to the cells lining the alimentary ca.n.a.l of the future animal. The outer layer of cells is known as the epiblast, and forms the outer layer of the skin, and, along the future dorsal line, gives rise to the nervous system. The muscles and skeleton and the reproductive cells arise from a set of cells known as the mesoblast, that are formed chiefly from the hypoblast, and that push their way in between the hypoblast and epiblast.

This general course of development may be traced in all members of the vertebrate group, and, with slight modifications, may be applied to a large number of invertebrates. As the modelling of the general contour of the whole body and of the separate organs proceeds, the protoplasm of the cells gradually a.s.sumes the characters of the substance of muscle-cells, liver-cells, nerve-cells, blood-cells, and so forth. The problem of this book will become clearer if it be considered with special reference to what goes on in these early stages. Hertwig says that all the cells of the epiblast, hypoblast, mesoblast, and of the later derivatives of these primary layers, receive identical portions of germplasm by means of doubling nuclear divisions. The different positions, relations to each other and to the whole organism, and to the environment in the widest sense of the term, cause different sides of the capacities of the cells to be developed, but they retain in a latent form all the capacities of the species. Weismann says that the nuclear divisions are differentiating, and that the microcosms of the germplasm, in accordance with their inherited architecture, gradually liberate different kinds of determinants into the different cells, and that, therefore, the essential cause of the specialisation of the organism was contained from the beginning in the germplasm.

THE BIOLOGICAL PROBLEM OF TO-DAY

INTRODUCTION.

What is development? Does it imply preformation or epigenesis? This perplexing question of biology has reappeared recently as a problem of the day. Of late years there have been set forth contradictory doctrines, each seeking to explain the process by which the fertilised egg-cell, an apparently simple beginning, gives rise to the adult organism, which often is exceedingly complicated, and which has the capacity of producing new beginnings like that from which it itself arose.

The opposing views of to-day were in existence centuries ago, and they are known in the history of science as the theory of preformation or evolution, and the theory of epigenesis. That most of the great biologists of the seventeenth and eighteenth centuries were decided upholders of evolution was the natural result of the contemporary knowledge of facts. For they knew only the external signs of the process of development. All they saw was the embryo becoming adult, the bud growing out into a blossom, as the result of a process in which nutrition transformed smaller to greater parts. And so they regarded development as a simple process of growth resulting from nutrition. Their mental picture of the germ or beginning of an organism was an exceedingly reduced image of the organism, an image requiring for its development nothing but nutrition and growth. That the material eye failed to recognise the miniature they attributed to the imperfection of our senses, and to the extreme minuteness and resulting opacity of the object.

That it might satisfy our human craving for final causes, the theory of preformation had to be accompanied by a corresponding explanation of the origin of the miniatures. Biologists had already abandoned the error of such spontaneous generation as the origin of flies from decaying meat, and, in its place, had accepted the doctrine of the continuity of life, formulating it in the phrase, _Omne vivum e vivo_ (Each life from a life), and in the similar phrase, _Omne vivum ex ovo_ (Each life from an egg). One creature issued from another, within which it had lain as a germ, and the series was continuous. Thus, the theory of preformation gave rise to the conception that living things were a series of cases or wrappings, germ folded within germ. The origin of life was relegated to the beginning, at the creation of the world: it became the work of a supernatural Creator, who, when He formed the first creatures, formed with them, and placed within them, the germs of all subsequent creatures.

To reckon at their proper value the theory of preformation, and, still more, the doctrine of enfolded germs, the standard of appreciation must not be the present range of our knowledge. They must be viewed historically, in the light of the knowledge of these days.

Nowadays it is not so much pure reason as a wider empirical knowledge of nature, with its consequent transformation of ideas, that makes the doctrine of enfoldment difficult. Abstract thought sets no limit to smallness or greatness; for mathematics deals with the infinitely small and with the infinitely great. So long as actual observation had not determined the limits of minuteness in the cases in question, there were no logical difficulties in the doctrine of enfolded germs. The biology of earlier centuries had not our empirical standard. What appeared then to be a simple organic material we have resolved into millions of cells, themselves consisting of different chemical materials. The chemical materials have been a.n.a.lysed into their elements, and chemistry and physics have determined the dimensions of the ultimate molecules of these. It is only because the minute const.i.tution of matter is no longer a secret to us that the theory of germ within germ now touches the absurd.

It was very different in earlier days; the acutest biologists and philosophers were evolutionists, and an epigenetic conception of the process of development could find no foothold alongside the apparent logical consistency of the theory of preformation.

Wolff's _Theoria Generationis_ (1759) failed to convince his contemporaries, because he could bring against the closed system of the evolutionists only isolated observations, and these doubtful of interpretation; and because, in his time, on account of the rudimentary state of the methods of research in biology, men attached more importance to abstract reasoning than to observation. His effort was the more praiseworthy in that it was observation bearing witness against abstract and dogmatic conceptions. By means of actual observation he tried to expose the fallacy in preformation, to show that the organism was not fully formed in the germ, but that all development proceeded by new formation, or epigenesis; that the germ consisted of unorganised organic material, which became formed or organised only little by little in the course of its development, and that Nature really was able to produce an organism from an unorganised material simply by her inherent forces.

It is interesting to display the essential contrast between preformation and epigenesis in the poetical words of Wolff himself. 'You must remember,'

so run his words in the second argument against the probability of preformation, 'that an evolution would be a phenomenon formed in its real essence by G.o.d at the Creation, but created in condition invisible, and so as to remain invisible for long before it would become visible. See, then, that a _phenomenon_ of enfolding is a miracle, differing from ordinary miracles only in these: first, it was at the creation of the world that G.o.d produced it; second, it remained invisible for long before it became visible. In truth, therefore, all organic bodies would be miracles. Would not this change for us the presence of Nature? Would it not spoil her of her beauty? Hitherto we had a living Nature, displaying endless changes by her own forces. Now it would be a fabric displaying change in seeming only, in truth and essence remaining unchanged and as it was constructed, save that it gradually becomes more and more used up. Formerly it was a Nature destroying herself and creating herself anew, only that endless changes might become visible and new sides be brought to light. Now it would be a lifeless ma.s.s shedding off piece after piece until the stock should come to an end.'

None the less, who seeks in Wolff's 'Theoria Generationis' an account of the means or forces by which Nature builds up organic forms will seek in vain. The _vis essentialis_ (inherent force) with which Wolff endowed his plastic organic material, or the _nisus formativus_ (formative force), afterwards suggested to science by Blumenbach--what are they but empty words by which men seek to grasp in thought what has eluded them? Wolff's epigenesis was not a complete explanation--indeed, from its fundamental conception it could not possibly be such. For investigation of the natural forces by which development proceeds can advance only slowly and step by step, and for long will const.i.tute the foremost task of biology. The prosecution of biological investigation will continuously endow the theory of epigenesis with a fuller and fuller meaning, but will never transform it into a solution final in the sense of the theory of preformation.

It seems to me that the significance of Wolff's doctrine lies in this: it rejected the purely formal theory of preformation because actual observations were against it. Thereby Wolff freed research from the straitened bonds of prejudice, and entered the only possible path by which science can advance--the path along which the biology of our century has made so great advances.

Biologists of to-day approach the problem of organic development equipped with incomparably greater knowledge and with more delicate methods of research. But in our thoughts to-day, as we discuss the essential nature of the process of organic development and the mutual causal relations between rudiments and their products, the same contradictory views are present, altered only as our methods of expression have altered.

In a striking fashion Roux[1] has contrasted the opposing ideas inherent in our modern conception of development, but yet identical with those which formerly found expression in the theories of preformation and epigenesis.

By the term "embryonic development," in its ordinary acceptation, we understand the appearance of visible complexity. But when we speak of the visibility of the resulting complexity, we use a subjective term, the value of which is relative to the human eye. Going further into the matter, we must break up the conception into two parts, and distinguish between the actual production of complexity and the mere transformation of complexity from a condition invisible to us into complexity visible to our senses.

'The two kinds of development I have indicated bear a relation to each other that recalls the old opposing doctrines of preformation and epigenesis, the alternatives of a time when it was a task--perhaps the only possible task--to record the completed results of the stages in development as they became complete--in fact, to record the externally visible changes of shape. In this descriptive investigation of the development of external form, epigenesis, the successive formation of new shapes, gained a complete victory over evolution, the mere becoming visible of pre-existing details of shape.

'The closer investigation of embryonic development that is necessary in a search for causes brings us once more against the old alternatives, and compels us to a closer scrutiny of them.

'In this, if we still retain the old terms, epigenesis would mean not merely the building up of complicated form through the agency of a substratum, apparently simple, but perhaps with an extraordinarily complicated, minute structure, but, in the strictest sense of the term, the new formation of complexity, an actual increase of complexity. Evolution, on the other hand, would imply the mere becoming visible of pre-existing latent differentiation. Clearly, according to these general definitions, occurrences which outwardly exhibit epigenesis may be in reality partial or complete evolution. In fact, the deepest consideration leads us again to the original question: Is embryonic development epigenesis or evolution? Is it the new formation of complexity, or is it the becoming visible of complexity previously invisible to us?'

Thus, in our own days, after the controversy has been at rest for long, biologists are a.s.sembled in opposing groups, one under the standard of epigenesis, another under that of preformation.

Weismann[2] leads the van for preformation; for the last ten years he has occupied himself with the theoretical discussion of the questions set forth above; and now, in a recent treatise, _The Germplasm_, he has combined his views, already many times modified, in a coherent theory. Now he explains candidly that he has been driven to the view that epigenetic development does not exist. 'In the first chapter of my book,' he remarks, 'will be found an actual proof of the reality of evolution, a proof so simple and obvious that I can scarcely understand to-day how it could have escaped my notice so long' (_Germplasm_, p. 14). Elsewhere he writes: 'I believe that I have established that ontogeny can be explained only by evolution, and not by epigenesis.'

A mental process, which consciously or unconsciously plays a great part with evolutionists, and helps to determine their conclusions, is characteristic of the direction of their inquiries. They set out from the fact that the characters of the parents, often to the smallest detail, are transmitted to children by means of the germ or rudiment; they conclude that the active causes of all the complexity that arises must be contained in the apparently h.o.m.ogeneous germ, embryological differentiation being a spontaneous process. It follows that the apparent h.o.m.ogeneity is, in reality, latent complexity which becomes patent during the progress of ontogeny. Latent complexity implies a material substratum, consisting of actual particles for which many different names have been found. As our senses can give us no experimental knowledge of these particles, which are so small as to be invisible, modern evolutionists attempt to picture them, in imagination, by reflecting all the visible characters of the perfected organism upon the undivided egg-cell, so peopling that globule of yolk with a system of minute particles corresponding in quality and in s.p.a.cial arrangement with the larger parts of the adult.

Weismann has practised this art in the true spirit of a virtuoso, and has elaborated it into a novel mode of biological investigation. Take an example;--'It would be impossible,' he says in _The Germplasm_ (p. 138), 'for any small portion of the human skin to undergo a hereditary and independent change from the germ onwards, unless a small vital element corresponding to this particular part of the skin existed in the germ substance, a variation in this element causing a corresponding variation in the part concerned. Were this not the case, birth-marks would not exist.'

Thus, in a slightly altered fashion, we come again to the position of the evolutionists of last century, for whom the germ was an extremely small miniature of the adult creature. The new evolution, as Weismann in especial has established it, seems to me to differ from the old doctrine only in two important points; and these must be placed to the credit of the greater scientific knowledge of our century. The first point concerns the relative positions of the parts in the patent and latent conditions. The older evolutionists a.s.sumed that these were identical, that the germ was a true miniature. It is true that Weismann regards his almost countless germinal particles as being held together in an architectural structure of almost inconceivable complexity. For him the germ is an exceedingly complicated living being, a microcosm in the truest sense, in which every independently variable part that ever appears throughout the whole life is represented by a living particle, and in which each of the living particles is endowed with a definite, inherited position, a const.i.tution, and the power of rapid multiplication. It is upon the qualities of these ultimate particles that he makes depend the qualities of the corresponding parts of the adult, the parts that are cells as well as the parts built of many cells. As, however, during visible development the parts of the embryo undergo many changes of position and metamorphoses, Weismann is compelled to make the a.s.sumption that the germ, as a micro-organism, is not simply a miniature of the adult, but that its minute particles have an arrangement totally different from that of the corresponding parts in the adult organism.