146 Proceedings of the Royal Physical Society. 



in pointing out that it fails to provide a complete explanation, 

 because it has hitherto failed to explain how progress is 

 possible in more than a single direction without some form 

 of isolation — without something to take the place of fences 

 and other barriers, regarded as indispensable in artificial 

 selection. In answer to this criticism, it may be replied that 

 even should some form of isolation be necessary, it by no 

 means follows that isolation is not a part, it may be an essen- 

 tial part, of natural selection, as essential as are barriers in 

 artificial selection. Whether isolation may be regarded as 

 an integral part of the doctrine of selection will obviously 

 depend on the nature of the isolation. 



Several naturalists who believe that new varieties, unless 

 sterile, or nearly sterile, with the parent species, must sooner 

 or later be swamped by intercrossing, have arrived at the 

 conclusion that all varieties which happen to be fertile with 

 the parent species inevitably disappear, unless accidentally 

 isolated. Huxley tells us that in his earliest criticisms of 

 the " Origin," he ventured to point out that its logical 

 foundation was insecure, so long as experiments in selective 

 breeding had not produced varieties which were more or less 

 infertile. 



A similar attitude was subsequently adopted by Moritz, 

 Wagner, Eomanes, and others. Wagner believed that it 

 was impossible for new species to arise without the help of 

 physical isolation; while Eomaoes asserted that, owing to 

 the swamping effects of free intercrossing, the evolution of 

 species in divergent lines (i.e., polytypic evolution) was 

 impossible by natural selection alone. In his theory of 

 physiological selection, Eomanes believed he had discovered 

 the missing factor. The criticisms of Huxley, Eomanes, and 

 others are obviously based on the assumption that inter- 

 crossing inevitably results in the swamping of all new 

 varieties which happen to prove fertile with the parent 

 species. That all new varieties fertile with the parent 

 species and with other varieties are thus nipped in the bud 

 by intercrossing has not been proved, neither has it been 

 proved that the only chance of any given new variety 

 surviving and eventually forming a new species depends on 



