746 EVOLUTION " Entwickelung," and " Evolutio " are now indiscriminately used for the series of genetic changes exhibited by living beings, by writers who would emphatically deny that "Development" or "Entwickelung" or "Evolutio," in the sense in which these words were usually employed by Bonnet or Haller, ever occurs. Evolution, or development, is, in fact, at present employed in biology as a general name for the history of the steps by which any living being has acquired the morphological and the physiological characters which dis tinguish it. As civil history may be divided into bio graphy, which is the history of individuals, and universal history, which is the history of the human race, so evolu tion falls naturally into two categories, the evolution of the individual, and the evolution of the sum of living beings. It will be convenient to deal with the modern doctrine of evolution under these two heads. 1. The Evolution of the Individual. No exception is, at this time, known to the general law, established upon an immense multitude of direct observa tions, 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 1 says " Omnibus viventibus primordium insit, ex quo et a quo pro- veniant. Liceat hoc nobis primordium vcgctale nominare ; nempe substantial!! quandain corporeaiu vitam habentem poteutia ; vel quoddam per se existens, quod aptum sit, in vegetativam formam, ab interne principle operante, nmtari. Quale nempe primordium, ovum est et plautarum semen ; 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 assum i i 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 remind ing us that the great characteristic of the germ is not so much what it is, but what it may, under suitable condi tions, 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 in deed a fair summary of his reiterated assertions, 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 dis prove 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 associated 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 con stituent 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. 2 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 passes, step by step, from an extreme simplicity, or relative homogeneity, of visible struc ture, to a greater or less degree of complexity or hetero geneity ; and the course of progressive differentiation is usually accompanied by growth, which is effected by intus susception. 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 constituents of the germ, as Bonnet imagined ; still less does it consist of the " molecules orgauiques " of Button. The new material is, in great measure, not only absorbed but assimilated, 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 constitutents of the germ, and that it is almost wholly made up of assimilated 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 plantte omues ex eodem communi nutri- mento (sive rore sen terras 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 in dividual 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 space, 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 invaginated, whereby it is con verted into a double walled sac with an opening, the Ilastos2)ore, which leads into the cavity lined by the inner wall. This cavity is the primitive alimentary cavity, or archenterom ; the inner, or invaginated, layer is the kt/pollast, the outer the cpiblast; and the embryo, in this 2 In some cases of sexless multipli cation the germ is a cell-aggre gate if we call germ only that which is already detached from the
parent organism.