We have most of us in the past looked upon health as a matter of inheritance, or temperance and moderation in working, in eating and drinking; or as depending on climate; or exercise, or plenty of sleep, pure water and a morning bath, or some other secret, one or more of which is pretty sure to be in the possession of most persons who have lived long enough to have had some experience with those things that do them good or harm. All these agencies have great value; but I think few of us realize that nature, through the laws of evolution, has long been working to produce a brave and strong, healthy and hardy race of men and women by other methods than those health habits which most of us value so highly. Nature has been doing this chiefly by two methods, and it seems necessary that I should say something about them in order to present my subject as I wish to present it. The methods to which I refer are those of sexual and natural selection. It is to these two processes that we are largely indebted for race improvements--more perfect bodies, more active brains, and the high degree of health which a considerable portion of the race enjoys. SEXUAL SELECTION.--By sexual selection is meant that preference which the male or the female has for certain characteristics of the other sex. It also includes the advantages which the stronger and more capable male has over the weaker one in obtaining a choice, or, among polygamous animals, a larger number of females, thus allowing offspring to be generated by the most capable, and preventing the most incapable from procuring mates. The first principle of sexual selection, that of preference, would imply a considerable development of the intellect, and some taste, but I do not think it has had great influence on the lower forms of life. It is difficult to study the preferences of insects, for instance; but I have studied the moth of the silkworm, and could never observe that either male or female had a choice for any particular mate. They always appear to take the first one that comes along. I think this is the conclusion come to by those entomologists who have had opportunities for studying other insects. The spider might perhaps be studied in this relation to advantage, as the female is ferocious, often eating her male suitors while they are trying to woo her. Nor do I believe that it is a very important matter in many other animals. Certainly among the domestic ones--the sheep, the horse, the bull and the cow--a superior male and female will mate with inferior ones of the opposite sex, apparently without the slightest objection. I have sometimes thought I had observed in pigeons a preference, having occasionally seen a male leave his mate for a more attractive female; at least one that seemed more attractive to me.