As already stated, the two essential features of physical sex are ovulation and sperm-formation. These two important factors in the joint work of reproduction are governed by a different rule in human and in brute life. In man they exist under the rule of continuity and of self-adjustment--_i.e._, these functions are always existent--but at the same time they adapt themselves to the higher needs of the individual. These two laws under which the functions exist--viz., 1st, continuity of action; 2nd, power of self-adjustment--are distinctive marks of superior human sexual function. Both are necessitated by the growth of reason--_i.e._, by a progressive civilization. This will be understood clearly by dwelling more in detail on the way in which these two essential parts of reproduction--viz., sperm-formation and ovulation--are established in the human race. In reproduction, the ova which are constantly produced in the female require to be fertilized by contact with the semen, which is constantly produced by the male, before they can commence the remarkable series of changes and transformations which result in the formation of the embryo, the rudimentary human being. Semen is a highly vitalized fluid, slowly but constantly secreted or formed by the male. As is the case with all organized living fluids, it is filled with rapidly-moving particles (spermatozoa), and its vitality appears to be in direct ratio to the quantity and activity of such movement. Motion seems to be inseparably connected with life, and is distinctive of any highly vitalized fluid. Thus, in the important and highly organized fluid, the blood, we observe constant motion and change in the active little bodies with which it is filled. This quality of great and active vitality appears to be indispensable to the spermatozoon which in the work of procreation is obliged to traverse long and winding passages in order to come in contact with the ovum which is advancing to meet it. An intense energy in the special act of procreation is needed to overcome the difficulties which may prevent conception. It is here necessary to note a common but mischievous fallacy. This necessary energy on the part of the male, in order to overcome anatomical difference of structure in sexual congress, is commonly considered an indication or measurement of the superior force of sexual attraction or passion in the male. This superficial judgment is not unnatural, as facts which are patent to the senses suggest the first crude thought. The chief structures of the male are external, but they are internal in the female. This difference of structure first suggests to the boy the meaning of actions of the lower animals, whilst the girl may grow up to full womanhood in complete unconsciousness of their signification.