But such thoughts and visions did not prevent him from following Professor Caldwell closely.  And as he followed, comprehendingly and critically, he noted the unbroken field of the other’s knowledge.  As for himself, from moment to moment the conversation showed him gaps and open stretches, whole subjects with which he was unfamiliar.  Nevertheless, thanks to his Spencer, he saw that he possessed the outlines of the field of knowledge.  It was a matter only of time, when he would fill in the outline.  Then watch out, he thought—­’ware shoal, everybody!  He felt like sitting at the feet of the professor, worshipful and absorbent; but, as he listened, he began to discern a weakness in the other’s judgments—­a weakness so stray and elusive that he might not have caught it had it not been ever present.  And when he did catch it, he leapt to equality at once.

Ruth came up to them a second time, just as Martin began to speak.

“I’ll tell you where you are wrong, or, rather, what weakens your judgments,” he said.  “You lack biology.  It has no place in your scheme of things.—­Oh, I mean the real interpretative biology, from the ground up, from the laboratory and the test-tube and the vitalized inorganic right on up to the widest aesthetic and sociological generalizations.”

Ruth was appalled.  She had sat two lecture courses under Professor Caldwell and looked up to him as the living repository of all knowledge.

“I scarcely follow you,” he said dubiously.

Martin was not so sure but what he had followed him.

“Then I’ll try to explain,” he said.  “I remember reading in Egyptian history something to the effect that understanding could not be had of Egyptian art without first studying the land question.”

“Quite right,” the professor nodded.

“And it seems to me,” Martin continued, “that knowledge of the land question, in turn, of all questions, for that matter, cannot be had without previous knowledge of the stuff and the constitution of life.  How can we understand laws and institutions, religions and customs, without understanding, not merely the nature of the creatures that made them, but the nature of the stuff out of which the creatures are made?  Is literature less human than the architecture and sculpture of Egypt?  Is there one thing in the known universe that is not subject to the law of evolution?—­Oh, I know there is an elaborate evolution of the various arts laid down, but it seems to me to be too mechanical.  The human himself is left out.  The evolution of the tool, of the harp, of music and song and dance, are all beautifully elaborated; but how about the evolution of the human himself, the development of the basic and intrinsic parts that were in him before he made his first tool or gibbered his first chant?  It is that which you do not consider, and which I call biology.  It is biology in its largest aspects.