The changes in certain species such as roses, primroses, tomatoes, pigeons and dogs, are not new species, but only varieties, having none of the traits of species, easily intermingling, propagating, and readily reverting to their original forms, changes which true species are not susceptible of. Darwin admitted that the continued fertility of these varieties was one of his greatest difficulties. One of the definitions of species is that they will not interbreed and propagate. So that hybrids are sterile. "After its kind," is the primal law of nature, and as Dr. Jesse B. Thomas says, "The stubborn mule still blocks the way of Evolution." NO CAUSE OF EVOLUTION IS KNOWN. Evolution is not a force. There is no power or cause which is known as Evolution. The word simply describes the order in which things have been supposed to come. We must draw a clear line of distinction between Cause and Order of Appearance. There is a certain order in the succession of living things as they came, but what caused that order is the very question at issue. The Duke of Argyle warns against confusing these when he says, "Evolution puts forward a visible order of phenomena as a complete and all-sufficient account of its own origin and cause." (_Theories of Darwin._) The absence of an agreed cause is admitted by evolutionists. Huxley says, "The great need of Evolution is a theory of derivation." (_Man's Place in Nature._) Darwin admits, "Our ignorance of the laws of derivation is profound." (_Descent of Man._) "The laws governing inheritance are for the most part unknown." (_Origin of Species._) Prof. Conn in _Evolution of To-day_, says, "No two scientists are agreed as to what is the cause of the supposed changes of species." (p. 337.) Prof. Clodd traces it to the protoplasm which forms the germ and ends his exhaustive treatise by saying the cause is still unknown. (_Method of Evolution._) Darwin's theory was Natural Selection. It is this which is technically called "Darwinism," although some writers apply that name to the general subject of Evolution. Natural selection is the theory that inasmuch as minute variations occur in the struggle of living things for existence, the variations which would prove favorable to the welfare of the animal would be transmitted to its progeny and be increased and so, in many generations, the accumulating effects, aided by climate, food, sexual selection, and other causes, would amount to a new species. Prof. Conn says of this theory, "Natural selection is almost universally acknowledged as insufficient to meet the facts of nature, since many facts of life cannot be explained by it." (p. 243.)