384 THE POPULAR SCIENCE MONTHLY. 



THE LOGIC OF ORGANIC EVOLUTION. 



By FRANK CRAMEK. 



IN his work on the Principles of Science, Jevons described with, 

 great clearness the logical phases of scientific theories and 

 illustrated them by a wealth of instances drawn from the sciences 

 of mathematics, physics, astronomy, and chemistry. While he 

 accords to the theory of evolution an importance equal to that of 

 any other theory, he says but little about its evidence or logical 

 history. He practically leaves the biological sciences and geology 

 untouched, except in the chapter on classification, where he says, 

 in closing the subject : " Natural classification in the animal and 

 vegetable kingdoms is a special problem, and . . . the particular 

 methods and difficulties to which it gives rise are not those com- 

 mon to all cases of classification, as so many physicists have sup- 

 posed. Genealogical resemblances are only a special case of re- 

 semblances in general." * 



The sciences of chemistry, physics, and astronomy, based as 

 they are on mathematics, allow precise statement and accurate 

 experiment. In geology and biology, on the other hand, the fac- 

 tors are so complex that these sciences take on the nature of his- 

 torical sciences, with all the difficulties which such a statement 

 implies. The difference can be easily illustrated. Certain per- 

 turbations of the planets indicated the presence of another one as 

 yet unseen. The amount of the disturbances could be accurately 

 determined. Adams and Le Verrier almost simultaneously pre- 

 dicted the presence of Neptune at a definite point in the heavens, 

 and the prediction was verified by the immediate discovery of the 

 planet. The human race must have appeared at a definite time in 

 some definite part of the earth ; but biological science lacks all 

 the factors with which to parallel the case of Neptune by point- 

 ing out by a prediction the time and place of the appearance of 

 the race. It knows that the event occurred, but must wait for 

 accident to reveal the place and depend on the broadest generali- 

 zations to reveal even the relative age of man. 



The utter lack of rigidity in the relations of living things puts 

 quantitative statement almost entirely out of question; except 

 that in some cases it is possible to work out a valuable system of 

 averages. Individual beings can be measured, but the laws of 

 biology can not be put in mathematical form ; hence the lack of 

 mathematical precision, without which the history of a science 

 does not lend itself very easily to logical treatment. Evolution 

 is pre-eminently a historical law, but its relations to the evidence 



* Jevons, Principles of Science, p. 727. 



