From whatever point of view we regard the problem of life we are in the presence of a group of of related phenomena which are peculiar to and characteristic of protoplasm. In the simplest living organism the organic characters enact a drama nowhere played in just this way on any inorganic stage. There is a continuous give-and-take both of matter and energy which is scarcely so much as hinted elsewhere; there is a unified sequence of changes constituting a continuous life-history, which may reach great complexity, and affording the basis of our conception of development; and there is through the process of reproduction an extended continuity which is now interpreted in terms of evolution. Is it a matter for wonder that the origin and significance of these vital phenomena, seen alike in a microscopic speck of amoeboid protoplasm, and in the quickened body of man, with its multitude of closely-related living cells, have long been regarded as fraught with "a mystery transcending naturalistic conception, as an alien influx into nature baffling scientific interpretation?"
The problem before the biologist, who seeks to contribute within his special domain to the interpretation of nature, is this: Given life and its environment, to describe in particular cases the sequence of events presented by individual development and racial evolution, and by generalising the results so reached to afford a scientific explanation of the phenomena. This is a difficult problem, and one well worthy of the intellect of Darwin and his followers. The results so far obtained call forth our sincere admiration, and augur well for yet further advances in biological knowledge. Accepting, as facts based upon observation, first that variations occur among offspring, secondly that more young are born than can or do survive to propagate their kind and transmit their peculiar idiosyncrasies, and thirdly that there is a struggle for existence leading to the elimination of the variants less fitted to their place in a complex scheme, Darwin was led to the conclusion that the survivors were those best adapted to their environment, and that herein lay the conditions of progress and of varied adaptation such as is seen in the multifarious species of the organic world.
Whether natural selection is all-sufficing as a principle of interpretation, or whether it must be supplemented by other factors, are questions which are keenly discussed; but that all the factors are of the scientific order and susceptible of interpretation in a scheme of naturally-conditioned sequence is the contention of naturalism. Darwin accepted the existence of variations on the ground of observation; the conditions of their occurrence and origin have occupied the attention of his successors. Their nature and distribution have been discussed by the application of more and more refined statistical methods. Weismann and Mendel have suggested hypotheses based upon the intimate structure and properties of the germinal substance and the properties of its constituent units. Much still remains to be done. But the naturalistic conviction gains daily in strength and cogency that the totality of life and its conditions to-day is the outcome of the life and its conditions of yesterday, and will surely give rise to the life and its conditions of to-morrow. This is the creed of evolution. The ideal towards which biological science is slowly but surely advancing is the description of the assemblage of antecedents which constitute the cause and the assemblage of consequents which we name the effect, and the complete formulation of the relations of the one assemblage to the other. But, it is urged, when the riddle of development and of evolution shall have been answered in terms of the ideal constructions of biological science, the riddle of life will still remain unsolved and insoluble in these terms.
What is life? As used by the man of science the term comprises an observable sequence of phenomena. We can from his standpoint neither say that life is due to the phenomena nor that the phenomena are due to life. The sequence itself, as it actually occurs, is just that which characterises what the biologist terms life. And though he may speak of the phenomena as those characteristic of life, all that is meant by this expression is that this or that particular phenomenon falls within the group to which the term "vital" is properly applicable. If, then, by life we mean the underlying cause of the sequence itself, then the question, What is life? is one with which the biologist, as such, has no concern. Life in this sense in biology, like force in an analogous sense in physics, is altogether outside the scientific universe of discourse. Whether it is or is not, or if it be, in what sense it is "an alien influx into nature," are questions which must be tried before a different court of appeal.
It is true, and should be frankly admitted, that in the present state of natural knowledge the antecedent conditions of the genesis of protoplasms are unknown. Some of the products of protoplasm, the so-called organic compounds, can be manufactured in the laboratory. In the case of the more complex products the difficulty is to discover the long sequence of progressive stages which lead up to the final synthesis. And we may well suppose that the complete sequence of all the appropriate stages of the synthesis of living protoplasm is of the rarest occurrence, may even have occurred only at a certain stage of the earth's history. That it has occurred is part of the faith of the evolutionist; it is accepted as a corollary from the ideal construction of naturalism taken as a whole. And granting, as we should grant with befitting frankness, that the antecedent conditions of its genesis are unknown, what then? With so much that is, and is likely long to remain, unknown to science it surely ill beseems us to build too much upon this. It is but our familiarity with the genesis of the crystal that affords any justification for the supposition that this is the outcome of a natural evolution while the genesis of protoplasm is not so. Science can tell us in this case no more than in that of protoplasm the why of its existence; while even of the how of crystalline architecture science can only say that, given such and such conditions, it appears. Of protoplasm we may likewise say that under certain conditions, at present unknown, it appeared. Those who would concentrate the mystery of existence on the pin-point of the genesis of protoplasm do violence alike to philosophy and to religion. Those who would single out from among the multitudinous differentiations of an evolving universe this alone for special interposition would seem to do little honour to the Divinity they profess to serve. Theodore Parker gave expression to a broader and more reverent theology when he said: "The universe, broad and deep and high, is a handful of dust which God enchants. He is the mysterious magic which possesses" not protoplasm merely, but "the world".