Yet a still more wonderful fact is revealed by the study of the very earliest stages of individual development.  The human embryo begins its very existence as a single cell,—­nothing more and nothing less; in general structure the human egg, like the eggs of all other many-celled organisms, is just one of the unitary building blocks of the entire organic world.  And yet the egg may ultimately become the adult man.  Does this mean that man and all the other higher forms have evolved from protozoa in the course of long ages?  Science asks if it can mean anything else.  When the comparative anatomist bids us look upon the wide and varied series of adult animals lower than man as his relatives, because they display similar structural plans beneath their minor differences, it may be difficult at first to obey him.  But in the brief time necessary for the human egg to develop into an adult, the entire range is compassed from the single cell to the highest adult we know.  There are no breaks in the series of embryonic stages like those between the diverse adult animals of the comparative array.  I do not think we could ask nature for more complete proof that human beings have evolved from one-cell ancestors as simple as modern protozoa beyond the obvious facts of human transformation during development.  They at least are real and not the logical deductions of reason; yet their very reality and familiarity render us blind to the deeper meaning revealed to us only when science places the facts in intelligible order.

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And now, in the third place, we may look to nature for fossil evidence regarding the ancestry of our species.  Much is known about the remains of many kinds of men who lived in prehistoric times, but we need consider here only one form which lived long before the glacial period in the so-called Tertiary times.  In 1894 a scientist named Dubois discovered in Java some of the remains of an animal which was partly ape and partly man.  So well did these remains exhibit the characters of Haeckel’s hypothetical ape-man, Pithecanthropus, that the name fitted the creature like a glove.  Specifically, the cranium presents an arch which is intermediate between that of the average ape and of the lowest human beings.  It possessed protruding brows like those of the gorilla.  The estimated brain capacity was about one thousand cubic centimeters, four hundred more than that of any known ape, and much less than the average of the lower human races.  Even without other characters, these would indicate that the animal was actually a “missing link” in the scientific sense,—­that is, a form which is near the common progenitors of the modern species of apes and of man.  We would not expect to find a missing link that was actually intermediate in all respects between modern apes and modern men, any more than we should look for actual connecting bands of tissue between any two leaves upon a tree.  A missing link, in the true sense, is like a bud of earlier years which stood near the point from which two twigs of the present day now diverge.  So Pithecanthropus is a part of the chain leading to man, not far from the place where the human line sprang from a lower primate ancestor.