knowledge regarding the more obvious operations of the human body, if it would seek a real explanation of the fundamental phenomena of life, it can only attain its end through the study of cell Great as was the impulse which the cell-theory gave to anatomical and physiological investigation, it did not for many years measurably affect the more speculative side of biological inquiry. The Origin of Species, published in 1859, scarcely mentions it ; nor, if we except the theory of pangenesis, did Darwin attempt at any later period to bring it into any very definite relation to his views. The cell-theory first came in contact with the evolution-theory nearly twenty years later through researches on the early history of the germ-cells and the fertilization of the ovum. Begun in 1873-74 by Auerbach, Fol, and Butschli, and eagerly followed up by Oscar Hertwig, Van Beneden, Strasburger, and a host of later workers, these investigations raised wholly new questions regarding the mechanism of development and the role of the cell in hereditary transmission. The identification of the cell-nucleus as the vehicle of inheritance, made independently and almost simultaneously in 1884-85 by Oscar Hertwig, Strasburger, Mincer, and Weismann, must be recognized as the first definite advance t towards the internal problems of inheritance through the cell-theory ; and the discussions to which it gave rise, in which Weismann has taken the foremost place, must be reckoned as the most interesting and significant of the post-Darwinian period.
These discussions have set forth in strong relief the truth that the general problems of evolution and heredity are indissolubly bound up with those of cell-structure and cell-action. This can best be appreciated from an historical point of view. The views of the early embryologists in regard to inheritance were vitiated by their acceptance of the Greek doctrine of the equivocal or spontaneous generation of life ; and even Harvey did not escape this pitfall, near as he came to the modern point of view. " The egg," he says, " is the mid-passage or transition stage between parents and offspring, between those who are, or were, and those who are about to be ; it is the hinge or pivot upon which the whole generation of the bird revolves. The egg is the terminus from which all fowls, male and female, have sprung, and to which all their lives tend — it is the result which nature has proposed to herself in their being. And thus it comes that individuals in procreating their like for the sake of their species, endure forever. The egg, I say, is a period or portion of this eternity." 2 This passage appears at first sight to be a close approximation to the modern doctrine of germinal continuity about which all theories of heredity are revolving. To the modern student the germ is, in Huxley's words, simply a detached living portion of the substance of a pre-existing living carrying with it a definite structural organization characteristic of the species. Harvey's view is only superficially similar to this ; for, as Huxley pointed out, it was obscured by his belief that the germ might arise " spontaneously," or through 1 It must not be forgotten that Haeckel expressed the same view in 1866—only, however, as a speculation, since the data necessary to an inductive conclusion were not obtained until long afterwards. "The internal nucleus provides for the transmission of hereditary
characters, the external plasma on the other hand for accommodation or adaptation to the external world" (Gen. Morph., p. 287-9).
2 De Generation, 1651; Trans., p. 271.
the influence of a mysterious "calidum innatum," out of not-living matter. Whitman, too, in a recent brilliant essay,' has shown how far Harvey was from any real grasp of the law of genetic continuity, which is well characterized as the central fact of modern biology. Neither could the great physiologist of the seventeenth century have had the remotest conception of the actual structure of the egg. The cellular structure of living things was not comprehended until nearly two centuries later. The spermatozoon was still undiscovered, and the nature of fertilization was a subject of fantastic and baseless speculation. For a hundred years after Harvey's time embryologists sought in vain to penetrate the mysteries enveloping the beginning of the individual life, and despite their failure the controversial writings of this period form one of the most interesting chapters in the history of biology. By the extreme " evolutionists " or " praeformationists " the egg was believed to contain an embryo fully formed in miniature, as the bud contains the flower or the chrysalis the butterfly. Development was to them merely the unfolding of that which already existed ; inheritance, the handing down from parent to child of an infinitesimal reproduction of its own body. It was the service of Bonnet to push this conception to its logical consequence, the theory of emboltement or encasement, and thus to demonstrate the absurdity of its grosser forms ; for if the egg contains a complete embryo, this must itself contain eggs for the next generation, these other eggs in their turn, and so ad infinitum, like an infinite series of boxes, one within another — hence the term " emboitement." Bonnet himself renounced this doctrine in his later writings, and Caspar Frederich Wolff (1759) led the way in a return to the teachings of Harvey, showing by precise actual observation that the egg does not at first contain any formed embryo whatever ; that the structure is wholly different from that of the adult ; that development is not a mere process of unfolding, but a progressive process, involving the continual formation, one after another, of new parts, previously nonexistent as such. This is somewhat as Harvey, himself following Aristotle, had conceived it — a process of epigenesis as opposed to evolution. Later researches established this conclusion as the very foundation of embryological science.