I. _The Evolution of the Individual_.

No exception is at this time, known to the general law, established upon an immense mult.i.tude of direct observations, that every living thing is evolved from a particle of matter in which no trace of the distinctive characters of the adult form of that living thing is discernible. This particle is termed a _germ_. Harvey [Footnote: _Execitationes de Generatione_. Ex. 62, "Ovum esse primordium commune omnibus animalibus."] says--

"Omnibus viventibus primordium insit, ex quo et a quo proveniant. Liceat hoc n.o.bis _primordium vegetale_ nominare; nempe substantiam quandam corpoream vitam habentem potentia; vel quoddam per se existens, quod aptum sit, in vegetativam formam, ab interno principio operante, mutari. Quale nempe primordium, ovum est et plantarum s.e.m.e.n; tale etiam viviparorum conceptus, et insectorum _vermis_ ab Aristotele dictus: diversa scilicet diversorum viventium primordia."

The definition of a germ as "matter potentially alive, and having within itself the tendency to a.s.sume a definite living form," appears to meet all the requirements of modern science. For, notwithstanding it might be justly questioned whether a germ is not merely potentially, but rather actually, alive, though its vital manifestations are reduced to a minimum, the term "potential" may fairly be used in a sense broad enough to escape the objection. And the qualification of "potential" has the advantage of reminding us that the great characteristic of the germ is not so much what it is, but what it may, under suitable conditions, become. Harvey shared the belief of Aristotle--whose writings he so often quotes and of whom he speaks as his precursor and model, with the generous respect with which one genuine worker should regard another--that such germs may arise by a process of "equivocal generation" out of not-living matter; and the aphorism so commonly ascribed to him, "_omne vivum ex ovo_" and which is indeed a fair summary of his reiterated a.s.sertions, though incessantly employed against the modern advocates of spontaneous generation, can be honestly so used only by those who have never read a score of pages of the "Exercitationes." Harvey, in fact, believed as implicitly as Aristotle did in the equivocal generation of the lower animals. But, while the course of modern investigation has only brought out into greater prominence the accuracy of Harvey's conception of the nature and mode of development of germs, it has as distinctly tended to disprove the occurrence of equivocal generation, or abiogenesis, in the present course of nature. In the immense majority of both plants and animals, it is certain that the germ is not merely a body in which life is dormant or potential, but that it is itself simply a detached portion of the substance of a pre-existing living body; and the evidence has yet to be adduced which will satisfy any cautious reasoner that "omne vivum ex vivo" is not as well-established a law of the existing course of nature as "omne vivum ex ovo."

In all instances which have yet been investigated, the substance of this germ has a peculiar chemical composition, consisting of at fewest four elementary bodies, viz., carbon, hydrogen, oxygen, and nitrogen, united into the ill-defined compound known as protein, and a.s.sociated with much water, and very generally, if not always, with sulphur and phosphorus in minute proportions. Moreover, up to the present time, protein is known only as a product and const.i.tuent of living matter. Again, a true germ is either devoid of any structure discernible by optical means, or, at most, it is a simple nucleated cell. [Footnote: In some cases of s.e.xless multiplication the germ is a cell-aggregate--if we call germ only that which is already detached from the parent organism.]

In all cases the process of evolution consists in a succession of changes of the form, structure, and functions of the germ, by which it pa.s.ses, step by step, from an extreme simplicity, or relative h.o.m.ogeneity, of visible structure, to a greater or less degree of complexity or heterogeneity; and the course of progressive differentiation is usually accompanied by growth, which is effected by intussusception. This intussusception, however, is a very different process from that imagined either by Buffon or by Bonnet.

The substance by the addition of which the germ is enlarged is in no case simply absorbed, ready-made, from the not-living world and packed between the elementary const.i.tuents of the germ, as Bonnet imagined; still less does it consist of the "molecules organiques" of Buffon. The new material is, in great measure, not only absorbed but a.s.similated, so that it becomes part and parcel of the molecular structure of the living body into which it enters. And, so far from the fully developed organism being simply the germ _plus_ the nutriment which it has absorbed, it is probable that the adult contains neither in form, nor in substance, more than an inappreciable fraction of the const.i.tuents of the germ, and that it is almost, if not wholly, made up of a.s.similated and metamorphosed nutriment.

In the great majority of cases, at any rate, the full-grown organism becomes what it is by the absorption of not-living matter, and its conversion into living matter of a specific type. As Harvey says (Ex. 45), all parts of the body are nourished "ab eodem succo alibili, aliter aliterque cambiato," "ut plantae omnes ex eodem communi nutrimento (sive rore seu terrae humore)."

In all animals and plants above the lowest the germ is a nucleated cell, using that term in its broadest sense; and the first step in the process of the evolution of the individual is the division of this cell into two or more portions. The process of division is repeated, until the organism, from being unicellular, becomes multicellular. The single cell becomes a cell-aggregate; and it is to the growth and metamorphosis of the cells of the cell-aggregate thus produced, that all the organs and tissues of the adult owe their origin.

In certain animals belonging to every one of the chief groups into which the _Metazoa_ are divisible, the cells of the cell-aggregate which results from the process of yelk-division, and which is termed a _morula_, diverge from one another in such a manner as to give rise to a central s.p.a.ce, around which they dispose themselves as a coat or envelope; and thus the morula becomes a vesicle filled with fluid, the _planula_. The wall of the planula is next pushed in on one side, or inv.a.g.i.n.ated, whereby it is converted into a double-walled sac with an opening, the _blastopore_, which leads into the cavity lined by the inner wall. This cavity is the primitive alimentary cavity or _archenteron_; the inner or inv.a.g.i.n.ated layer is the _hypoblast_; the outer the _epiblast_; and the embryo, in this stage, is termed a _gastrula_. In all the higher animals a layer of cells makes its appearance between the hypoblast and the epiblast, and is termed the _mesoblast_. In the further course of development the epiblast becomes the ectoderm or epidermic layer of the body; the hypoblast becomes the epithelium of the middle portion of the alimentary ca.n.a.l; and the mesoblast gives rise to all the other tissues, except the central nervous system, which originates from an ingrowth of the epiblast.

With more or less modification in detail, the embryo has been observed to pa.s.s through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief that all animals of higher organisation than the _Protozoa_, agree in the general character of the early stages of their individual evolution. Each, starting from the condition of a simple nucleated cell, becomes a cell-aggregate; and this pa.s.ses through a condition which represents the gastrula stage, before taking on the features distinctive of the group to which it belongs.

Stated in this form, the "gastraea theory" of Haeckel appears to the present writer to be one of most important and best founded of recent generalisations. So far as individual plants and animals are concerned, therefore, evolution is not a speculation but a fact; and it takes place by epigenesis.

"Animal...per _epigenesin_ procreatur, materiam simul attrahit, parat, concoquit, et eadem ut.i.tur; formatur simul et augetur ... primum futuri corporis concrementum ... prout augetur, dividitur sensim et distinguitur in partes, non simul omnes, sed alias post alias natas, et ordine quasque suo emergentes." [Footnote: Harvey, _Exercitationes de Generatione_.

Ex. 45, "Quaenam sit pulli materia et quomodo fiat in Ovo."] In these words, by the divination of genius, Harvey, in the seventeenth century, summed up the outcome of the work of all those who, with appliances he could not dream of, are continuing his labours in the nineteenth century.

Nevertheless, though the doctrine of epigenesis, as understood by Harvey, has definitively triumphed over the doctrine of evolution, as understood by his opponents of the eighteenth century, it is not impossible that, when the a.n.a.lysis of the process of development is carried still further, and the origin of the molecular components of the physically gross, though sensibly minute, bodies which we term germs is traced, the theory of development will approach more nearly to metamorphosis than to epigenesis.

Harvey thought that impregnation influenced the female organism as a contagion; and that the blood, which he conceived to be the first rudiment of the germ, arose in the clear fluid of the "colliquamentum" of the ovum by a process of concrescence, as a sort of living precipitate. We now know, on the contrary, that the female germ or ovum, in all the higher animals and plants, is a body which possesses the structure of a nucleated cell; that impregnation consists in the fusion of the substance [Footnote: [At any rate of the nuclei of the two germ-cells. 1893]] of another more or less modified nucleated cell, the male germ, with the ovum; and that the structural components of the body of the embryo are all derived, by a process of division, from the coalesced male and female germs. Hence it is conceivable, and indeed probable, that every part of the adult contains molecules, derived both from the male and from the female parent; and that, regarded as a ma.s.s of molecules, the entire organism may he compared to a web of which the warp is derived from the female and the woof from the male. And each of these may const.i.tute one individuality, in the same sense as the whole organism is one individual, although the matter of the organism has been constantly changing. The primitive male and female molecules may play the part of Buffon's "moules organiques," and mould the a.s.similated nutriment, each according to its own type, into innumerable new molecules. From this point of view the process, which, in its superficial aspect, is epigenesis, appears in essence, to be evolution, in the modified sense adopted in Bonnet's later writings; and development is merely the expansion of a potential organism or "original preformation" according to fixed laws.

II. _The Evolution of the Sum of Living Beings_.

The notion that all the kinds of animals and plants may have come into existence by the growth and modification of primordial germs is as old as speculative thought; but the modern scientific form of the doctrine can be traced historically to the influence of several converging lines of philosophical speculation and of physical observation, none of which go farther back than the seventeenth century. These are:--

1. The enunciation by Descartes of the conception that the physical universe, whether living or not living, is a mechanism, and that, as such, it is explicable on physical principles.

2. The observation of the gradations of structure, from extreme simplicity to very great complexity, presented by living things, and of the relation of these graduated forms to one another.

3. The observation of the existence of an a.n.a.logy between the series of gradations presented by the species which compose any great group of animals or plants, and the series of embryonic conditions of the highest members of that group.

4. The observation that large groups of species of widely different habits present the same fundamental plan of structure; and that parts of the same animal or plant, the functions of which are very different, likewise exhibit modifications of a common plan.

5. The observation of the existence of structures, in a rudimentary and apparently useless condition, in one species of a group, which are fully developed and have definite functions in other species of the same group.

6. The observation of the effects of varying conditions in modifying living organisms.

7. The observation of the facts of geographical distribution.

8. The observation of the facts of the geological succession of the forms of life.

1. Notwithstanding the elaborate disguise which fear of the powers that were led Descartes to throw over his real opinions, it is impossible to read the "Principes de la Philosophie" without acquiring the conviction that this great philosopher held that the physical world and all things in it, whether living or not living, have originated by a process of evolution, due to the continuous operation of purely physical causes, out of a primitive relatively formless matter. [Footnote: As Buffon has well said:--"L'idee de ramener l'explication de tous les phenomenes a des principes mecaniques est a.s.surement grande et belle, ce pas est le plus hardi qu'on peut faire en philosophie, et c'est Descartes qui l'a fait."--_l. c._ p. 50.]

The following pa.s.sage is especially instructive:--

"Et tant s'en faut que je veuille que l'on croie toutes les choses que j'ecrirai, que meme je pretends en proposer ici quelques unes que je crois absolument etre fausses; a savoir, je ne doute point quo le monde n'ait ete cree au commencement avec autant de perfection qu'il eu a; en sorte que le soleil, la terre, la lune, et les etoiles ont ete des lors; et que la terre n'a pas eu seulement en soi les s.e.m.e.nces des plantes, mais que les plantes meme en ont couvert une partie; et qu' Adam et Eve n'ont pas ete crees enfans mais en age d'hommes parfaits. La religion chretienne veut que nous le croyons ainsi, et la raison naturelle nous persuade entierement cette verite; car si nous considerons la toute puissance de Dieu, nous devons juger que tout ce qu'il a fait a eu des le commencement toute la perfection qu'il devoit avoir. Mais neanmoins, comme on connoitroit beaucoup mieux quelle a ete la nature d'Adam et celle des arbres de Paradis si on avoit examine comment les enfants se forment peu a peu dans le ventre de leurs meres et comment les plantes sortent de leurs s.e.m.e.nces, que si on avoit seulement considere quels ils ont ete quand Dieu les a crees: tout de meme, nous ferons mieux entendre quelle est generalement la nature de toutes les choses qui sont au monde si nous pouvons imaginer quelques principes qui soient fort intelligibles et fort simples, desquels nous puissions voir clairement que les astres et la terre et enfin tout ce monde visible auroit pu etre produit ainsi que de quelques s.e.m.e.nces (bien que, nous sachions qu'il n'a pas ete produit en cette facon) que si nous la decrivions seulement comme il est, ou bien comme nous croyons qu'il a ete cree. Et parceque je pense avoir trouve des principes qui sont tels, je tacherai ici de les expliquer." [Footnote: _Principes de la Philosophie_, Troisieme partie, -- 45.]

If we read between the lines of this singular exhibition of force of one kind and weakness of another, it is clear that Descartes believed that he had divined the mode in which the physical universe had been evolved; and the "Traite de l'Homme," and the essay "Sur les Pa.s.sions" afford abundant additional evidence that he sought for, and thought he had found, an explanation of the phenomena of physical life by deduction from purely physical laws.

Spinoza abounds in the same sense, and is as usual perfectly candid--

"Naturae leges et regulae, secundum quas omnia fiunt et ex unis formis in alias mutantur, sunt ubique et semper eadem." [Footnote: _Ethices_, Pars tertia, Praefatio.] Leibnitz's doctrine of continuity necessarily led him in the same direction; and, of the infinite mult.i.tude of monads with which he peopled the world, each is supposed to be the focus of an endless process of evolution and involution. In the "Protogaea," xxvi., Leibnitz distinctly suggests the mutability of species--

"Alii mirantur in saxis pa.s.sim species videri quas vel in orbe cognito, vel saltem in vicinis locis frustra quaeras. 'Ita Cornua Ammonis,' quae ex nautilorum numero habeantur, pa.s.sim et forma et magnitudine (nam et pedali diametro aliquando reperiuntur) ab omnibus illis naturis discrepare dic.u.n.t, quas praebet mare. Sed quis absconditos ejus recessus aut subterraneas abyssos pervestigavit? quam multa n.o.bis animalia antea ignota offert novus...o...b..s? Et credibile est per magnas illas conversiones etiam animalium species plurimum immutatas."

Thus, in the end of the seventeenth century, the seed was sown which has, at intervals, brought forth recurrent crops of evolutional hypotheses, based, more or less completely, on general reasonings.

Among the earliest of these speculations is that put forward by Benoit de Maillet in his "Telliamed," which, though printed in 1735, was not published until twenty-three years later. Considering that this book was written before the time of Haller, or Bonnet, or Linnaeus, or Hutton, it surely deserves more respectful consideration than it usually receives. For De Maillet not only has a definite conception of the plasticity of living things, and of the production of existing species by the modification of their predecessors; but he clearly apprehends the cardinal maxim of modern geological science, that the explanation of the structure of the globe is to be sought in the deductive application to geological phenomena of the principles established inductively by the study of the present course of nature. Somewhat later, Maupertuis [Footnote: _Systeme de la Nature_.

"Essai sur la Formation des Corps Organises," 1751, xiv.] suggested a curious hypothesis as to the causes of variation, which he thinks may be sufficient to account for the origin of all animals from a single pair.

Robinet [Footnote: _Considerations Philosophiques sur la gradation naturelle des formes de l'etre; ou les essais de la nature qui apprend a faire l'homme,_ 1768.] followed out much the same line of thought as De Maillet, but less soberly; and Bonnet's speculations in the "Palingenesie,"

which appeared in 1769, have already been mentioned. Buffon (1753-1778), at first a partisan of the absolute immutability of species, subsequently appears to have believed that larger or smaller groups of species have been produced by the modification of a primitive stock; but he contributed nothing to the general doctrine of evolution.

Erasmus Darwin ("Zoonomia," 1794), though a zealous evolutionist, can hardly be said to have made any real advance on his predecessors; and, notwithstanding that Goethe (1791-4) had the advantage of a wide knowledge of morphological facts, and a true insight into their signification, while he threw all the power of a great poet into the expression of his conceptions, it may be questioned whether he supplied the doctrine of evolution with a firmer scientific basis than it already possessed.

Moreover, whatever the value of Goethe's labours in that field, they were not published before 1820, long after evolutionism had taken a new departure from the works of Trevira.n.u.s and Lamarck--the first of its advocates who were equipped for their task with the needful large and accurate knowledge of the phenomena of life, as a whole. It is remarkable that each of these writers seems to have been led, independently and contemporaneously, to invent the same name of "Biology" for the science of the phenomena of life; and thus, following Buffon, to have recognised the essential unity of these phenomena, and their contradistinction from those of inanimate nature. And it is hard to say whether Lamarck or Trevira.n.u.s has the priority in propounding the main thesis of the doctrine of evolution; for though the first volume of Trevira.n.u.s's "Biologie" appeared only in 1802, he says, in the preface to his later work, the "Erscheinungen und Gesetze des organischen Lebens," dated 1831, that he wrote the first volume of the "Biologie" "nearly five-and-thirty years ago," or about 1796.

Now, in 1794, there is evidence that Lamarck held doctrines which present a striking contrast to those which are to be found in the "Philosophie Zoologique," as the following pa.s.sages show:--

"685. Quoique mon unique objet dans cet article n'ait ete que de traiter de la cause physique de l'entretien de la vie des etres organiques, malgre cela j'ai ose avancer en debutant, que l'existence de ces etres etonnants n'appartiennent nullement a la nature; que tout ce qu'on peut entendre par le mot _nature_, ne pouvoit donner la vie, c'est-a-dire, que toutes les qualites de la matiere, jointes a toutes les circonstances possibles, et meme a l'activite repandue dans l'univers, ne pouvaient point produire un etre muni du mouvement organique, capable de reproduire son semblable, et sujet a la mort.

"686. Tous les individus de cette nature, qui existent, proviennent d'individus semblables qui tous ensemble const.i.tuent l'espece entiere. Or, je crois qu'il est aussi impossible a l'homme de connoitre la cause physique du premier individu de chaque espece, que d'a.s.signer aussi physiquement la cause de l'existence de la matiere ou de l'univers entier.

C'est au moins ce que le resultat de mes connaissances et de mes reflexions me portent a penser. S'il existe beaucoup de varietes produites par l'effet des circonstances, ces varietes ne denaturent point les especes; mais on se trompe, sans doute souvent, en indiquant comme espece, ce qui n'est que variete; et alors je sens que cette erreur peut tirer a consequence dans les raisonnements que l'on fait sur cette matiere." [Footnote: _Recherches sur les causes des princ.i.p.aux faits physiques_, par J.B.

Lamarck. Paris. Seconde annee de la Republique. In the preface, Lamarck says that the work was written in 1776, and presented to the Academy in 1780; but it was not published before 17994, and, at that time, it presumably expressed Lamarck's mature views. It would be interesting to know what brought about the change of opinion manifested in the _Recherches sur l'organisation des corps vivants_, published only seven years later.]

The first three volumes of Trevira.n.u.s's "Biologie," which contain his general views of evolution, appeared between 1802 and 1805. The "Recherches sur l'organisation des corps vivants," in which the outlines of Lamarck's doctrines are given, was published in 1802, but the full development of his views, in the "Philosophie Zoologique," did not take place until 1809.

The "Biologie" and the "Philosophie Zoologique" are both very remarkable productions, and are still worthy of attentive study, but they fell upon evil times. The vast authority of Cuvier was employed in support of the traditionally respectable hypotheses of special creation and of catastrophism; and the wild speculations of the "Discours sur les Revolutions de la Surface du Globe" were held to be models of sound scientific thinking, while the really much more sober and philosophical hypotheses of the "Hydrogeologie" were scouted. For many years it was the fashion to speak of Lamarck with ridicule, while Trevira.n.u.s was altogether ignored.

Nevertheless, the work had been done. The conception of evolution was henceforward irrepressible, and it incessantly reappears, in one shape or another, [Footnote: See the "Historical Sketch" prefixed to the last edition of the _Origin of Species_.] up to the year 1858, when Mr.

Darwin and Mr. Wallace published their "Theory of Natural Selection." The "Origin of Species" appeared in 1859; and it is within the knowledge of all whose memories go back to that time, that, henceforward, the doctrine of evolution has a.s.sumed a position and acquired an importance which it never before possessed. In the "Origin of Species," and in his other numerous and important contributions to the solution of the problem of biological evolution, Mr. Darwin confines himself to the discussion of the causes which have brought about the present condition of living matter, a.s.suming such matter to have once come into existence. On the other hand, Mr.

Spencer [Footnote: _First Principles_. and _Principles of Biology_, 1860-1864.] and Professor Haeckel [Footnote: _Generelle Marphologie_, 1866.] have dealt with the whole problem of evolution. The profound and vigorous writings of Mr. Spencer embody the spirit of Descartes in the knowledge of our own day, and may be regarded as the "Principes de la Philosophie" of the nineteenth century; while, whatever hesitation may not unfrequently be felt by less daring minds, in following Haeckel in many of his speculations, his attempt to systematise the doctrine of evolution and to exhibit its influence as the central thought of modern biology, cannot fail to have a far-reaching influence on the progress of science.

If we seek for the reason of the difference between the scientific position of the doctrine of evolution a century ago, and that which it occupies now, we shall find it in the great acc.u.mulation of facts, the several cla.s.ses of which have been enumerated above, under the second to the eighth heads. For those which are grouped under the second to the seventh of these cla.s.ses, respectively, have a clear significance on the hypothesis of evolution, while they are unintelligible if that hypothesis be denied. And those of the eighth group are not only unintelligible without the a.s.sumption of evolution, but can be proved never to be discordant with that hypothesis, while, in some cases, they are exactly such as the hypothesis requires. The demonstration of these a.s.sertions would require a volume, but the general nature of the evidence on which they rest may be briefly indicated.

2. The accurate investigation of the lowest forms of animal life, commenced by Leeuwenhoek and Swammerdam, and continued by the remarkable labours of Reaumur, Trembley, Bonnet, and a host of other observers, in the latter part of the seventeenth and the first half of the eighteenth centuries, drew the attention of biologists to the gradation in the complexity of organisation which is presented by living beings, and culminated in the doctrine of the "ech.e.l.le des etres," so powerfully and clearly stated by Bonnet; and, before him, adumbrated by Locke and by Leibnitz. In the then state of knowledge, it appeared that all the species of animals and plants could be arranged in one series; in such a manner that, by insensible gradations, the mineral pa.s.sed into the plant, the plant into the polype, the polype into the worm, and so, through gradually higher forms of life, to man, at the summit of the animated world.

But, as knowledge advanced, this conception ceased to be tenable in the crude form in which it was first put forward. Taking into account existing animals and plants alone, it became obvious that they fell into groups which were more or less sharply separated from one another; and, moreover, that even the species of a genus can hardly ever be arranged in linear series. Their natural resemblances and differences are only to be expressed by disposing them as if they were branches springing from a common hypothetical centre.

Lamarck, while affirming the verbal proposition that animals form a single series, was forced by his vast acquaintance with the details of zoology to limit the a.s.sertion to such a series as may be formed out of the abstractions const.i.tuted by the common characters of each group. [Footnote: "Il s'agit donc de prouver que la serie qui const.i.tue l'ech.e.l.le animale reside essentiellement dans la distribution des ma.s.ses princ.i.p.ales qui la composent et non dans celle des especes ni meme toujours dans celle des genres."--_Philosophie Zoologique_. chap. v.]

Cuvier on anatomical, and Von Baer on embryological grounds, made the further step of proving that, even in this limited sense, animals cannot be arranged in a single series, but that there are several distinct plans of organisation to be observed among them, no one of which, in its highest and most complicated modification, leads to any of the others.

The conclusions enunciated by Cuvier and Von Baer have been confirmed, in principle, by all subsequent research into the structure of animals and plants. But the effect of the adoption of these conclusions has been rather to subst.i.tute a new metaphor for that of Bonnet than to abolish the conception expressed by it. Instead of regarding living things as capable of arrangement in one series like the steps of a ladder, the results of modern investigation compel us to dispose them as if they were the twigs and branches of a tree. The ends of the twigs represent individuals, the smallest groups of twigs species, larger groups genera, and so on, until we arrive at the source of all these ramifications of the main branch, which is represented by a common plan of structure. At the present moment, it is impossible to draw up any definition, based on broad anatomical or developmental characters, by which any one of Cuvier's great groups shall be separated from all the rest. On the contrary, the lower members of each tend to converge towards the lower members of all the others. The same may be said of the vegetable world. The apparently clear distinction between flowering and flowerless plants has been broken down by the series of gradations between the two exhibited by the _Lycopodiaceae, Rhizocarpeae_, and _Gymnospermeae_. The groups of _Fungi_, _Lichenes_, and _Algae_ have completely run into one another, and, when the lowest forms of each are alone considered, even the animal and vegetable kingdoms cease to have a definite frontier.