212 SUMMARY 



practice the limitation of thought to data provided by experi- 

 ment, and the abandonment, from lack of material, of all attempts 

 to ascertain the correctness of hypotheses. The experimental 

 method can do no more than fill some lacunae in our already wide 

 knowledge of living beings. In the past men have sometimes 

 filled these gaps with imaginary facts and have speculated wildly. 

 But such lapses from scientific caution afford no excuse for treat- 

 ing the gaps when filled by experiment with authentic facts as the 

 whole of knowledge and then speculating still more wildly. 



350. Splendid examples of scientific method, of combined 

 induction and deduction, are the hypotheses of evolution and 

 Natural Selection. The actuality of evolution was suspected even 

 by the Greeks, but modern conviction, founded as it is on authenti- 

 cated facts and carefully tested thinking, is more surely based. 

 By linking together particular facts we infer (i) that it is possible 

 to create new varieties of animals and plants by careful artificial 

 selection, (2) that varieties, species, orders, and genera tend to 

 shade into one another, (3) that species inhabiting the same or adja- 

 cent areas tend to have features more alike than those more widely 

 separated, (4) that the remains of animals and plants preserved 

 in geological strata indicate that related species, which were 

 widely separated in time, were as a rule more unlike than those 

 that were more nearly contemporary, and (5) that the structures of 

 embryos are often more suggestive of lower types than of their 

 own adult progenitors. Linking together all these inductions, as 

 in reaching them we linked together particular facts, we are able 

 to infer the hypothesis of evolution. In this way, not only is a 

 deeper knowledge and a larger synthesis achieved, but we are 

 able to perceive a new and very valuable aspect of truth which 

 induction could never have established unaided by deduction, and 

 which suggests many fresh inductions and deductions which, again, 

 not only reveal still newer truths, but tend to test the aspect of 

 reality, which was reached in part by deduction. 



351. Darwin accepted the theory of evolution and linked it 

 with the inductions which had previously been more or less 

 familiar to all men (i) that offspring tend to vary from their 

 parents, (2) that some of them are better adapted to achieve 

 survival than others, (3) that they tend to outnumber their parents, 

 (4) that, nevertheless, the number of individuals in a species 

 does not, as a rule, increase, and (5) that species are adaptational 

 forms. Noting that all these inductions were in harmony and 

 testing them rigorously, he linked them together in the hypothesis 



