changing today are quite direct in character, for more or less radical hereditary changes have been seen in the act of taking place, though, as yet, we have little knowledge of the causes responsible for them. The discovery that species are changing at a noticeable rate at the present time is in itself strong evidence that they have changed in the past, and doubtless in the same ways and at the same rates of speed as those observable today; for even the convinced special creationist would hardly claim that species have remained immutable since their creation only to begin change during the present era. Little can be learned about the large changes involved in organic evolution by observing relatively small changes of the present, for it takes immense periods of time for the larger waves of change to run their course and reach their culmination. For the study of past evolutionary events we use the historical method so successfully employed in archaeology and ancient history; for the study of present evolution we make use of the methods of direct observation and experiment. The findings in one field strongly support and supplement the other. When we admit that the evidences of past evolution are indirect and circumstantial, we should hasten to add that the same is true of all other great scientific generalizations. The evidences upon which the law of gravity are based are no less indirect than are those supporting the principle of evolution. Like all other great scientific generalizations, the law of gravity has acquired its validity through its ability to explain, unify and rationalize many observed facts of physical nature. If certain facts entirely out of accord with the law of gravity were to come to light, physicists would be forced either to modify the statement of the law so as to bring it into harmony with the newly-discovered facts, or else to substitute a new law capable of meeting the situation. Laws of nature are no more or less than condensed statements about the facts of nature and therefore are valid only in so far as they agree with the facts. The nebular hypothesis and its modern rival, the planetesimal hypothesis, are both deductions from facts; they both seem to agree with many of the obscured data, but neither of them is as yet fully adequate for all. In the field of physical chemistry we had first the molecular theory, then the atomic theory, then the ionic theory and now the electron theory; each of those has appeared in direct response to the necessity of explaining new sets of facts, and none of them is so well founded as is the theory of evolution. No one has ever seen a molecule, an atom, an ion or an electron; the existence of and the properties of these entities have been deduced from the behaviors of various chemical substances when subjected to experimental conditions.
The principle of evolution stands in the first rank among natural laws not only in its range of applicability, but in the degree of its validity, the extent to which it may lay claim to rank as an established law. It is the one great law of life. It depends for its validity, not upon conjecture or philosophy, but upon exactly the same sorts of evidence as do other laws of nature.
Evolution has been tried and tested in every conceivable way for considerably over half a century. Vast numbers of biological facts have been examined in the light of this principle and without a single exception they have been entirely compatible with it. Think what a sensation in the scientific world might be created if some one were to discover even one well-authenticated fact that could not be reconciled with the principle of evolution. If the enemies of evolution ever expect to make any real headway in their campaign they should devote their energies toward the discovery of such a fact.
The exact nature of the proof of the principle of evolution is that when great masses of scientific data such as are involved in those branches of biology known as taxonomy, comparative anatomy, embryology, serology, paleontology and
