As displayed in individual organisms, the effects of structures on functions are to be studied, not only in the broad fact that the general kind of life an organism leads is necessitated by the main characters of its organization, but in the more special and less conspicuous fact, that between members of the same species, minor differences of structure lead to minor differences of power to perform certain actions, and of tendencies to perform such actions. Conversely, under the reactions of functions on structures in individual organisms, come the facts showing that functions, when fulfilled to their normal extents, maintain integrity of structure in their respective organs; and that within certain limits increases of functions are followed by such structural changes in their respective organs, as enable them to discharge better their extra functions. Inquiry into the influence of structure on function as seen in successions of organisms, introduces us to such phenomena as Mr. Darwin's _Origin of Species_ deals with. In this category come all proofs of the general truth, that when an individual is enabled by a certain structural peculiarity to perform better than others of its species some advantageous action; and when it bequeaths more or less of its structural peculiarity to descendants, among whom those which have it most markedly are best able to thrive and propagate; there arises a visibly modified type of structure, having a more or less distinct function. In the correlative class of facts (by some asserted and by others denied), which come under the category of reactions of function on structure as exhibited in successions of organisms, are to be placed all those modifications of structure which arise in races, when changes of conditions entail changes in the balance of their functions--when altered function externally necessitated, produces altered structure, and continues doing this through successive generations. § 41. The fourth great division of Biology, comprehending the phenomena of Genesis, may be conveniently separated into three subdivisions. Under the first, comes a description of all the special modes whereby the multiplication of organisms is carried on; which modes range themselves under the two chief heads of sexual and asexual. An account of Sexual Multiplication includes the various processes by which germs and ova are fertilized, and by which, after fertilization, they are furnished with the materials, and maintained in the conditions, needful for their development. An account of Asexual Multiplication includes the various processes by which, from the same fertilized germ or ovum, there are produced many organisms partially or totally independent of one another.