geographic distribution, are looked upon as the result of evolutionary processes, they take on orderliness, reasonableness, unity and coherency. Not only this, but each subscience becomes more closely linked with the others and all turn out to be but different aspects of the one great process. No other explanation of biological phenomena that in any sense rivals the evolution principle has ever been offered to the public. This principle cannot be abandoned until one more satisfactory comes forth to take its place. To revert to the thoroughly discredited and unscientific idea of special creation would be as utterly impossible as to revert to the ancient geocentric conception of the universe, according to which a flat earth was thought to occupy the center of the universe and the sun, moon and stars to revolve about it.
Let us reiterate that a theory or a principle is acceptable only so long as it accords with the facts already known and leads to the discovery of new facts and principles. Whether or not the principle of evolution meets these requirements the reader must judge for himself after a perusal of the facts that lie at the basis of the principle.
The evidences of evolution that we shall investigate are contained within the following fields of biology:
First—Comparative anatomy or morphology, the science of structure.
Second—Taxonomy, the science of classification.
Third—Serology, the science of blood tests.
Fourth—Embryology, the science of development.
Fifth—Paleontology, the science of extinct life.
Sixth—Geographic distribution, the study of the horizontal distribution of spcies upon the earth's surface.
Seventh—Genetics, the analytic and experimental study of evolutionary processes going on today.
A careful study of the situation reveals that the entire fabric of evolutionary evidences is woven about a single broad assumption: That fundamental structure resemblance signifies blood relationship; that, generally speaking, the closeness of structural resemblance runs essentially parallel with closeness of kinship. Most biologists would say that this may once have been only an assumption, but that it is now so amply supported by facts that it has become axiomatic. However obvious the validity of this assumption may be, it is the plain duty of one who attempts to justify the evolutionary principle to avoid taking steps that are in the least open to serious criticism. If we cannot rely upon this principle we can make no sure progress toward the proof of evolution. The assumption we are now discussing is tantamount to an affirmation of the principle of heredity; that like tends to produce like. We continually employ the principle in every day life. We fully expect the offspring of sparrows to be sparrows, of robins to be robins; and if we should ever find an instance to the contrary, we would be greatly surprised and shocked. Futhermore, we have learned by experience that offspring not only belong to the same species as the parents, but resemble the parents more closely than they do other people. Whenever we see two people whose resemblance is closer than usual we immediately come to the conclusion that such persons are relations, probably offspring of the same parents. Every one has had the experience of meeting two persons so strikingly alike that it is almost impossible to distinguish them apart, and the natural assumption is that such persons are duplicate or identical twins. Twins of this sort are vastly more closely related than are brothers or sisters, or even than ate fraternal twins who are usually no more alike than are brothers and sisters of closely similar ages. It is practically established that duplicate twins are products of the early division of a single germ cell. No closer degree of kinship can well be imagined than this, for the two individuals bear the same relationship to each other as do the two bilateral halves of one body.
