Thus the somewhat vague discussion as to the relative probability of Epigenesis and Evo lution has led to the laborious and accurate investigation of the various steps of the forma tive process or developement of the fcetus, and the conjectures as to the forces or causes which give rise to the growth of the new animal have fallen into comparative neglect; the erroneous notions respecting the source of the germs of male or female offsprings from one or other ovary or testicle have been replaced by a more satisfactory examination of the mode of deve lopment of the sexual organs in the early stages of their advancement; and the inquiry as to the share taken by one or other parent in the process of generation has been pursued in more modern times by the attentive investigation of the functions of the male and female organs of reproduction, upon the same principles that guide the physiologist in his attempts to explain any other class of functions of the economy.
Recent writings on our subject are not, how ever, altogether free from vague hypotheses of the same nature as the older theories of gene ration above mentioned. The mechanical explanation of fecundation by the entrance of the seminal animalcule into the egg has been revived by one author; a second considers all the changes of development as under the influence of electromagnetic currents ; and a third explains the same changes by attributing them to a spontaneous motive power and organic affinitive properties of the molecules of the ovum.
It has been well remarked by Professor Burdach that the generative function, com prising the production of a fruitful egg and the formation of the young animal from it, are natural phenomena not more secret in their essence than others occurring in organized bodies, and which, therefore, ought to be investigated by obtaining a knowledge of the conditions in which they take place, and of the operations and changes in which they consist.
The illustrious Harvey in his 51st Exercita tion expresses himself thus decidedly a sup porter of the theory of Epigenesis,—" it is plain that the chicken is built up by Epigencsis or the additament of parts budding one out of another;" but he does not admit that separate powers, such as the "alterative or immutative, formative, attractive, retentive, digestive, and expulsive faculties, or those of apposition, agglutination, and assimilative nutrition de scribed by Fabricius," can be distinguished in the production of the chicken. Ile thus limits our knowledge of the subject in the 54th Exercitation " But as in the greater world we say Jovis omnia plena, all things are full of the Deity, so also in the little edifice of a chicken, and all its actions and operations, digitus Dci, the finger of God or the God of nature cloth reveale himself." " A more sub lime and diviner artificer (than Man is) seems to make and preserve man ; and a nobler agent than a cock Both produce a chicken out of the egge. For we acknowledge our omnipotent God and most high Creator to be every where present in the structure of all creatures living, and to point himself out by his workes; whose instruments the cock and hen are in the gene ration of the chicken. For it is most apparent,
that in the generation of the chicken out of the egge, all things are set up and formed, with a most singular providence, divine wisdom, and an admirable and incomprehensible artifice." " Nor can these attributes appertain to any but to the Omnipotent Maker of all things, under what name soever we cloud him; whether it be the mens diving, the divine mind with Aristotle, or anima mundi, the soul of the universe with Plato; or with others natura ?autumns, Nature of nature herself; or else Saturnus or Jupiter with the heathen, or rather as befits us, the Creatour and Father of all things in heaven and earth ; upon whom all animals and their births depend : and at whose beck or mandat, all things are created and a. Spontaneous generation of animals.—In this introductory view of the function of gene ration, it may be proper shortly to inquire whether a regular affiliation from parent to offspring be an indispensable condition for the continuation of the species of every kind of animal,—a question somewhat speculative in its nature, but of considerable interest in rela tion to some of the general doctrines of physio logy, as well as closely connected with our present subject. It has already been stated in general terms that origin by generation and the power of reproduction arc characteristics be longing to all organized bodies whether of the vegetable or animal kingdoms. The existence of life implies the sequence of decay and death, or in other words, those varied operations' and changes which together constitute the living state continue to occur in each organized body for a limited period only : they sooner or later undergo a gradual alteration, are less regularly performed, and ultimately entirely cease in death. But although every individual belong ing to the organized kingdom of nature is necessarily subject to death, the species of each plant and animal never becomes extinct, but is continued upon earth in an undeviating suc cession of generations. The origin of a mineral, on the other hand, is wholly independent of any pre-existing body of its own kind ; and, in the mineral kingdom, all those bodies are held to belong to the same species which agree in external form, physical properties, and chemical constitution. The mineral owes its first origin, as its subsequent increase, to the simple union of its component particles; but the successive generations of every species of organized bodies constitute an uninterrupted chain extending from the time of their first creation, and in which the formation of every new link that is added depends on its tem porary attachment to that which preceded it. So fixed, indeed, is the law of continued reproduction of organized bodies, that many naturalists have, in the absence of more definite distinctive characters, adopted the circumstance of reproduction as the only certain means of determining what individuals ought to be regarded as belonging to one species.