for and against it are such that its logical history is both unique and inspiring. If it has passed through a definite series of logical phases similar to those through which the mathematico-physical sciences have passed, if it has fulfilled the same conditions and led to similar and equally brilliant results, its logical status is permanently fixed.
The cell doctrine, which now lies at the foundation of biological science, illustrates an important principle in the growth of theories. The significant points in its history, for the present purpose, are: 1. It took its earliest shape as a botanical theory, arising from a very small part of the facts that it was destined to explain. 2. Those facts were the most obtrusive of the whole group of facts to which they belong. The most highly wrought products of the forces involved are always first discovered, and thus it comes about that the facts which are most difficult to explain and which are farthest away from the point where Nature began its work, at first form the foundation of a scientific theory. Knowledge increases by backward from the specialized to the generalized, and the theory is perfected only after a complete series of facts has been secured in this way.
One of the results of this principle is that all scientific doctrines in which a historical arrangement of phenomena is involved must pass through what is aptly called the catastrophic stage. The geological doctrine current in the early part of the century, that there have been successive world-wide catastrophes followed by recreations, was perfectly natural in that stage of the science. Only the most obtrusive facts were known. The mountains were regarded as simple products instead of very complex accumulations of the effects of forces working steadily. When geology passed from catastrophe to continuity, it made its great permanent stride forward by providing itself with a key to all the facts that have since been discovered.
The theory of evolution has passed through all these developmental stages. The law itself was not recognized until long after formal relationships had been established, and its discovery was simply a recognition of the principle of continuity. Natural history began with species—the mountain-ranges of biology—and regarded them as simple facts instead of last terms in a long series. The breaches between species, as between mountains, was what made them striking. The evidence that has been destroyed played an important part in the early stages of this as of all other lines of scientific reasoning which are dependent on historical evidence. The more hidden and comparatively insignificant facts, the residuum which constitutes the difficulties of classification,
- Sachs, History of Botany.