5i2 THE POPULAR SCIENCE MONTHLY 



transformed, through the accumulation of variations, into new species 

 differing from their progenitors by the final test of specificity of char- 

 acter. In the 1850's such a radical distinction seemed to hold; even by 

 the sixth edition of the "Origin," dated 1872, Darwin was able to 

 point to only four somewhat debatable instances, in plants, of the 

 infertility of varieties when intercrossed. If this difficulty appeared 

 to Huxley and other zoologists an insuperable objection to evolutionism 

 before 1858, it was not, in Huxley's opinion, removed after that date. 

 Yet he no longer found the difficulty insuperable ; it was purely a nega- 

 tive argument, e silentio, and he had faith to believe that by further 

 investigation it would be removed. In his Edinburgh lectures of 1862, 

 " he warned his audience of the one missing link in the chain of evi- 

 dence — the fact that selective breeding has not yet produced species 

 sterile to one another. But it is to be accepted as a working hypothesis, 

 like other scientific generalizations, ' subject to the production of proof 

 that physiological species may be produced by selective breeding.' " 

 In the same year Huxley wrote Darwin : 



I have told my students that I entertain no doubt whatever that twenty 

 years' experiments on pigeons, conducted by a skilled physiologist, instead of by 

 a mere breeder, would give us physiological species sterile inter se from a com- 

 mon stock, . . . and I have told them that when these experiments have been 

 performed I shall consider your views to have a complete physical basis. 20 



It is certainly interesting thus to observe that, as Huxley, before his 

 conversion, saw no potency in arguments which afterwards seemed to 

 him conclusive, so also he was able, in his second phase, to pass over 

 by an act of faith one of the most serious of the pre-Darwinian objec- 

 tions to evolutionism. This provisional disregard of the " missing 

 link" in an argument otherwise impressively well concatenated was, 

 under the circumstances, far from unreasonable. But it would have 

 been equally reasonable in 1846 or in 1851. 



Leaving these antecedent considerations in favor of evolutionism 

 drawn from the general principles of scientific method, I turn to the 

 more specific facts which — when illumined by those principles — pro- 

 vide the now usual and familiar arguments for the theory. All the 

 more essential of these facts were known before 1844; and attention 

 was duly called to their bearings by the neglected prophets of evo- 

 lutionism during the fifteen years preceding the publication of the 

 " Origin of Species." In speaking of these " facts," it is well to ex- 

 plain what is meant, in this connection, by the expression. The theory 

 of evolution does not rest immediately upon an induction of individual 

 phenomena; and the evidence for it did not increase by a slow arith- 

 metical progression, through the accumulation of observations of indi- 

 vidual phenomena. It is a generalization established inferentially, by 



20 Huxley's " Life and Letters," I., 193, 195. 



