But while there was much scientific progress from Plato to Galen, and while Galen's physiological ideas of the functions of the brain, the heart, and the liver held their place until Harvey's discovery of the circulation of the blood in the seventeenth century, that discovery and the subsequent investigations proved that Galen, although not far wrong as to the brain, was wholly wrong as to the liver, and partially wrong as to the heart. Yet Galen's physiological theories concerning these organs were founded on many anatomical facts and results of experiments, such as could then be made.

There is another fact which marks the state of anatomical knowledge among the Greeks in the time of Plato, and of Aristotle, who belonged to the same century. The "Timæus" of Plato shows that there were physicians at that period, and that he was acquainted with the writings of Hippocrates. The important fact is, as stated by Mr. Grote, that "the study and practice of medicine was at that time greatly affected by the current speculations respecting Nature as a whole; accomplished physicians combined both lines of study, implicating cosmical and biological theories."[10]

It is now only needful to say that modern anatomy and physiology afford aids to sound deductions in natural theology in reference to the structure of the human body as an animal organism, and all the functions of its different organs, which immeasurably transcend all that was known or assumed among the early Greeks, or in the time of Plato and Aristotle, or in the time of Galen. Notwithstanding the dispute whether the origin of man as an animal is to be referred to a special act of creation, or to the process of what has been called evolution, there can be no controversy on one point, namely, that modern anatomy and physiology have vastly increased our knowledge of the structure of the human frame, and the means of rational speculation upon the nature of intellect, as compared with any means that were possessed by the most accomplished and learned of the Greeks of antiquity. It matters little on which side of the controversy, between creation and evolution, the great anatomists of the present day range themselves. It is upon the facts which their investigations have revealed that we have to judge of the probable truth of the one hypothesis or the other. The probable destiny of man as an immortal being is an inquiry that has certainly lost nothing by our increased knowledge of the facts in his animal structure which tend to support the hypothesis of design in his creation.

Lord Macaulay attributes an utter failure to the efforts of the philosophers, from Plato to Franklin, to "prove" the immortality of the soul without the help of revelation. What did he mean by proof? Revelation is, of course, the only direct proof. It is so, because it is direct testimony of a fact, proceeding from the only source that can have direct and certain knowledge of that fact. When the evidences which are supposed to establish the existence and authority of the witness have become satisfactory to us, we are possessed of proof of our immortality, and this proof is the only direct evidence of which the fact admits, and it constitutes all that should be spoken of as proof. But there is collateral although inferior evidence—inferior, because it consists in facts which show a high degree of probability that the soul of man is immortal, although this kind of evidence is not like the direct testimony of a competent witness. Is all this presumptive evidence, with its weighty tendency to establish the probable truth of immortality, to be pronounced of no value, because it belongs to a different order of proof from that derived from the assertion of a competent witness to the fact? It is one of the advantages of our situation in this life, that the collateral evidence which tends to show the high probability of a future state of existence is not withheld from us. As a supplemental aid to the direct teaching of revelation, it is of inestimable importance if we do not obscure it by theories which pervert its force, and if we reason upon it on sound philosophical principles. What we have to do in estimating the probable truth of our immortality, as shown by the science of natural religion, is to give the same force to moral evidence in this particular department of belief, that we give to the moral evidence which convinces us of many things of which we have no direct proof, or of which the direct proof lies in evidence of another kind.

"He knew as much about it," said Voltaire, "as has been known in all ages—that is to say, very little indeed." This, like many of the witticisms of Voltaire, pressed into the service of an argument against the value of natural religion at the present day when studied by mature and disciplined minds, is quite out of place. What human reason has done in the course of three thousand years is not to be put on a par with the speculations of intelligent children or half-civilized men; and although some of the riddles which perplexed Eliphaz and Zophar have not had a perfectly satisfactory solution, it is quite wide from the truth to assert that there has been no approximation to a satisfactory solution, or that some of the riddles have not ceased to be the riddles which they were three thousand years ago. In that period there has been an accumulation of evidence concerning the phenomena of Nature, and the phenomena of mind, vast beyond comparison when placed in contrast with what was known in the tents of the Idumean emirs, and the importance of this accumulation of evidence is proved by the fact that theories have been built upon it which undertake to explain it by hypotheses that were never heard of before, and which may possibly leave the "riddles" in a far less satisfactory state than they were in the time of Job. On the other hand, while the companions of Job may have been unable to suggest to him any solution of the problems of life, it does not at all follow that we are as helpless as they were, even if we avail ourselves of nothing but what the science of natural theology can now teach us.[11]

It will be seen that I attach great importance to natural theology. But I do not propose to write for the confirmed believers in revelation, on the one hand, who have become convinced by the evidence which supports revelation; or for those, on the other hand, who believe nothing, and who have become confirmed in habits of thinking which unfit them for judging of the weight of evidence on such subjects as the existence of God and the creation of man. I write for that great mass of people of average intelligence, who do not understand accurately what the doctrine of evolution is as expounded by its leading representatives, and who do not know to what it leads. It will be found that in some respects there is a distinction between the school of which Darwin is the representative and the school which follows Spencer. To point out this distinction, and yet to show that both systems result in negatives which put an end to the idea of immortality, and that the weight of evidence is against both of them, is what I propose to do.