The theory of spontaneous generation rests almost entirely upon assumptions. Its only semblance of support from facts is derived from certain experiments of a very unsatisfactory character, which are said to have resulted in the production of some of the lowest forms of animal life. These experiments have been by no means uniformly successful. One or two experimenters have thought that they have succeeded, but not uniformly, while the same process, repeated by men whose scientific and manipulative powers are universally recognized, has never once resulted in any seeming development of life. Even if, however, they had been uniformly successful, there would have been great reason to doubt whether the apparent success was not really a failure--a failure in the precautions necessary to exclude all germs of life from the matter experimented upon. For the lower forms of life are excessively minute; and their germs--eggs, seeds, or spores--must be far smaller. It is known that these are constantly floating in the atmosphere, though, owing to their extreme minuteness, the fact can only be ascertained by the most skilful investigation. And the lower forms of animalcules have a singular tenacity of life; they can pass unharmed through processes which would be fatal to creatures of higher organization. One variety is known to survive entire desiccation; another lives upon strychnine; others bear without injury great extremes of heat and cold; and if this is the case with the mature creatures, it is probable that the germ possesses still stronger powers of vitality. If one acarus can live upon strychnine, then it is not impossible that mineral acids should be harmless to others; the germs might be carried through sulphuric acid in air without coming into contact with the acid, as air would pass through in bubbles, in the centre of which they might be suspended; or if like the diatomaceae, they were coated with silex, they might come into contact with it and resist its action. Thus one of the precautions commonly taken is not certain in its action, and the same might be shown to be true of the others. The theory of spontaneous generation is, in fact, generally repudiated by Evolutionists, and cannot therefore be taken as a starting-point. We come then to the theory of Evolution with which Mr. Darwin's name is associated. This theory asserts that all the varieties of animal life now existing on the earth, however widely they may differ from each other, are in reality derived from one, or a very few original types; and that in this general statement the human race is to be included. This theory rests upon the following admitted facts.