EVOLUTION 747 stage, is termed a gastnda. 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 canal ; 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 pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echino- denus, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief, that all animals of higher organization 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 passes through a condition which represents the gastrula stage, before taking in the features distinctive of the group to which it belongs. Stated in this form, the " gastrsea theory " of llaeckel appears to the present writer to be one of the most important and best founded of recent generalizations. 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 epigcncsin procreatur, materiara simul attrahit, parat, concoquit, et eadem utitur ; formatur simul et augetur. . . . primum futuri corporis coiicrementum . . . prout augetur, dividitur sensim et distinguitur in partes, non simul omues, sed alias post alias natas, et ordine quasque suo emergentee." 1 In these words, by the divination of genius, Harvey, in the 17th 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 19th century. Nevertheless, though the doctrine of epigenesis, as un derstood by Harvey, has definitively triumphed over the doctrine of evolution as understood by his opponents of the 18th century, it is not impossible that, when the analysis 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 2 of another more or less modi fied 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 mass of molecules, the entire organism may be compared to a web of which the warp is derived from the female and the woof from the male. And each of these may constitute 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 1 Harvey, Exerciiationes de Generatione. Ex. 45, Quccnam sitpulli materia et qnomodo fiat in Oro. a Not yet actually demonstrated iu the case of tlie pliaeuogamous plants. play the part of Buffon s " moules organiques," and mould the assimilated nutriment, each according to its own type, into innumerable new molecules. From this point of view, the process, which, in its superficial aspect, is epigeuesis, 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. 2. Tlie Evolution of the Sum of Living Ueings. 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 further back than the 17th 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 analogy 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 tbe 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 distribu tion. 8. The observation of the facts of the geological succes sion 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 Prindpet de la Philosophic 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 pro cess of evolution, due to the continuous operation of purely physical causes, out of a primitive relatively formless matter. 3 The following passage is especially instructive : " Et tant s en faut que je veuille que Ton croie toutes les choses que j ecrirai, que meme je pretends en proposer ici quelques unes quo je crois absolument etre fausses ; a savoir, je lie doute point que le monde n ait etc cree au commencement avec autant de per fection qu il en a ; en sorte que le soleil, la.terre, la lune, et les etoiles out ete des lors ; et que la terre u a pas eu settlement en soi les semences des plantes, mais que les plantes meme en out couvert une partie ; et qu Adam et Eve n ont pas ete crees enfans mais eii age d hommes parfaits. La religion cnrctienne veut que nous le croyons ainsi, et la raison naturelle nous persuade entiemneut 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