While there is general agreement among biologists that species have been derived from one another by natural causes, there is a wide diversity of opinion as to the method by which this result has been brought about. Darwin's theory of natural selection has a struggle for existence of its own, a fight for life with other evolutionary theories. Emphatic protests are made from the side of experimental biology (de Vries), of paleontology (Osborn), and of philosophical evolution (Bergson) against the Darwinian hypothesis that the selection of minute fortuitous variations can account for the rise of new species or explain the great lines of development. It is only necessary to read the two volumes published in England and America[55] in honour of the hundredth anniversary of Darwin's birth and the fiftieth anniversary of the publication of the "Origin of Species" to see that scientific opinion upon the question of the method of evolution is widely divided. 55: "Darwin and Modern Science," Cambridge, 1909; "Fifty Years of Darwinism," New York, 1909. In Biblical language, the question of the hour in biology is, Who (or what) made thee to differ? "It is the question," in the words of C. H. Eigenmann, "of how the straight line of exact hereditary repetition may be caused to swerve in a definite direction to reach an adaptive point. This is the question of the present generation, perhaps of the entire twentieth century."[56] 56: "Fifty Years of Darwinism," p. 191. The Newton of biology, who will discover the laws of variation and heredity, has not, it is safe to say, yet appeared. Variation in a definite direction in virtue of an internal tendency in the organism (Nägeli); variation in response to the specific stimulus of the environment (Eimer); variation due, at least in animals, to the conscious effort of the individual (Lamarck); variation inciting a corresponding strengthening of parts of the individual organism, until time should be given for hereditary strengthening of these parts (organic selection as taught by Baldwin, Osborn and Lloyd Morgan); variation due to the preservation and accumulation of minute fluctuations by natural selection (Darwinism in its usual form); variation from unknown causes suddenly and discontinuously (the mutationism of de Vries); variation due to a mystical vital impulse in organic life as a whole (the creative evolution of Bergson):--no one of these views, if we take scientific opinion as a whole, can be said to have torn aside the veil behind which nature carries on her creative works.