Two great agencies, according to the Darwinian theory, have operated to develop the different species of animals from some low primordial type, through a long series which has culminated in man, who can not lay claim to be a special creation, but must trace his pedigree to some ape-like creature, and so on to the remote progenitor of all the _Vertebrata_. It is now needful to grasp, with as much precision as such a theory admits of, the nature and operation of these agencies, and to note the strength or weakness of the proof which they afford of the main hypothesis. First, we have what is called "the struggle for existence," which may be conceded as a fact, and to which more or less may be attributed. The term is used by Mr. Darwin in a metaphorical sense, to include all that any being has to encounter in maintaining its individual existence, and in leaving progeny, or perpetuating its kind. In the animal kingdom, the struggle for individual existence is chiefly a struggle for food among the different individuals which depend on the same food, or against a dearth of one kind of food which compels a resort to some other kind. The struggle for a continuation of its species is dependent on the success with which the individual animal maintains the contest for its own existence. Now, it is argued that in this great and complex battle for life it would occur that infinitely varied diversities of structure would be useful to the animals in helping them to carry on the battle under changing conditions. These useful diversities, consisting of the development of new organs and powers, would be preserved and perpetuated in the offspring, through many successive generations, while the variations that were injurious would be rigidly destroyed. The animals in whom these favorable individual differences and variations of structure were preserved would have the best chance of surviving and of procreating their kind. So that, by this "survival of the fittest," Nature is continually selecting those variations of structure which are useful, and continually rejecting or eliminating those which are injurious; the result being the gradual evolution of successive higher types of animals out of the lower ones, until we reach man, the highest animal organism that exists on this earth. In the next place, we have, as an auxiliary agency, in aid of natural selection, what is called "the sexual selection," by which the best endowed and most powerful males of a given species appropriate the females, and thus the progeny become possessed of those variations of structure and the superior qualities which have given to the male parent the victory over his competitors.