THE LOGIC OF ORGANIC EVOLUTION. 385 



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 matheniatico-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 biologi- 

 cal 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 ex- 

 plain 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 traveling backward from the specialized 

 to the generalized, and the theory is perfected only after a com- 

 plete series of facts has been secured in this way. 



One of the results of this principle is -that all scientific doc- 

 trines in which a historical arrangement of phenomena is in- 

 volved 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 moun- 

 tains 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 develop- 

 mental 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 his- 

 tory 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 evi- 

 dence. The more hidden and comparatively insignificant facts, 

 the residuum which constitutes the difficulties of classification, 



* Sachs, History of Botany. 

 vol. xlii. — 26 



