possibly this may ultimately be the fate of the natural-selection theory. In approaching the problem our aim is not to "get positive results," but to find out the truth. Our object is not to bolster up a venerable and out-of-fashion hypothesis, but to test conscientiously that hypothesis against concrete data.
Like other theories, the Darwinian theory must stand or fall according as the evidence of quantitative biology shall be for or against it. If the micrometer scale and the calculating machine show that any given character has no influence in determining whether an individual shall survive, then for that organ, in that species at the time under consideration, evidence for the potency of selection is wanting.
The problem is a difficult one; a priori one would expect most generally to find no changes taking place in the characters of a species because of a selective death rate. If natural selection be actually at work in nature, it is likely that the ancestors of individuals collected in the open will have been subjected to the selective factors which one is trying to measure, and that the race will be held pretty close to the attainable limit of perfection. It is more likely that a selective elimination which recurs every generation will be observed than one of the kind that brings about changes in specific characters. Only in rare cases when a new territory is opened to organisms or some special modification of environment (inorganic or organic) has taken place can we reasonably expect to see the changing of types going forward. Possibly the very difficulties of demonstrating a selective death rate bear witness to its reality!
Taking all this for granted, biologists must, it seems to me, face the duty of determining whether natural selection is a fundamental factor in evolution—in short of actually measuring the intensity of the selective death rate. The calipers are ready and their efficiency has been proved.
The duty to use them is imposed by ideals of good workmanship. "Measure that which is measurable and render measurable that which is not," is the ideal which has hitherto separated the precise from the descriptive sciences. It is the duty as well as the opportunity of the biologist of to-day to break down this distinction.
The duty to use them is imposed by the history of our science. For nearly half a century natural selection has been one of the chief problems of biology, and it would be cowardly for naturalists of this generation to leave the problem until a definite solution has been secured.
The duty to use them is imposed by the inability of the biologist to construct for himself a philosophical theory of evolution without natural selection as one of the factors. Yet the philosophical necessity of a given factor does not relieve the scientist from the duty of finding out whether that factor be a reality, and of measuring its intensity.