The third hypothesis, derived mainly from the writings of Lamarck, Darwin, and Huxley, is based upon the principle of evolution. All existing species are developments of some preëxisting form, which in like manner descended by true generation from a form still lower. Man, say they, bears no impress of a divine original that is not common to brutes; he is but an animal, more perfectly developed through natural and sexual selection. Commencing with the spontaneous generation of the lowest types of vegetable and animal life,--as the accumulation of mold upon food, the swarming of maggots in meat, the infusorial animalcules in water, the generation of insect life in decaying vegetable substances,--the birth of one form arising out of the decay of another, the slow and gradual unfolding from a lower to a higher sphere, acting through a long succession of ages, culminate in the grandeur of intellectual manhood. Thus much for this life, while the hope of a like continued progress is entertained for the life to come. While the tendency of variety in organic forms is to decrease, argue these latter-day naturalists, individuals increase in a proportion greater than the provisional means of support. A predominating species, under favorable circumstances, rapidly multiplies, crowding out and annihilating opposing species. There is therefore a constant struggle for existence in nature, in which the strongest, those best fitted to live and improve their species, prevail; while the deformed and ill-favored are destroyed. In courtship and sexual selection the war for precedence continues. Throughout nature the male is the wooer; he it is who is armed for fight, and provided with musical organs and ornamental appendages, with which to charm the fair one. The savage and the wild beast alike secure their mate over the mangled form of a vanquished rival. In this manner the more highly favored of either sex are mated, and natural selections made, by which, better ever producing better, the species in its constant variation is constantly improved. Many remarkable resemblances may be seen between man and the inferior animals. In embryonic development, in physical structure, in material composition and the function of organs, man and animals are strikingly alike. And, in the possession of that immaterial nature which more widely separates the human from the brute creation, the 'reasonable soul' of man is but an evolution from brute instincts. The difference in the mental faculties of man and animals is immense; but the high culture which belongs to man has been slowly developed, and there is plainly a wider separation between the mental power of the lowest zoöphyte and the highest ape, than between the most intellectual ape and the least intellectual man. Physically and mentally, the man-like ape and the ape-like man sustain to each other a near relationship; while between the mammal and the mollusk there exists the greatest possible dissimilarity.