382 BihKographical Notice. 



sense is not a single phenomenon, but the result of a great number 

 of factors, which were developed with wonderful power by the great 

 naturalist whose works have revolutionized modern thought, even 

 in quarters which might seem to lie beyond the influence of the 

 student of natural history. 



On looking into Mr. Dixon's book to see what are the phenomena 

 which he places instead of the undoubted factors in Natural Selec- 

 tion, we find his work divided into five sections, treating in order of 

 Isolation, Climatic Influences, Use and Disuse of Organs, Sexual 

 Selection, and Interbreeding. Of these the first four are manifestly 

 factors in " Natural Selection " in the Darwinian sense ; in fact the 

 whole of them were referred to by Darwin at greater or less length 

 in his various writings. 



To take the case of Isolation, to which the author devotes the 

 longest chapter of his book. It is perfectly clear that in " methodical 

 selection '' the practical isolation of the individuals presenting the 

 particular characters which the breeder wishes to perpetuate or in- 

 tensify is absolutely indispensable ; and the same thing must also be 

 effected in nature, free interbreeding, as Mr. Dixon himself admits, 

 having the effect of preventing the production of sharply-defined 

 forms. Practical isolation is an important factor in Natural 

 Selection. 



Mr. Dixon, however, does not see this. For example, he says : — 

 '• Take, for instance, Catliarus c/riscicej)s and C phcp.opleuriis, only 

 distinguished by the shade of colour on the back. In the former 

 species it is russet-brown, in the latter it is olive-brown. C. firisn- 

 ceps has only been obtained on the highlands of Panama, whilst C. 

 phoioplenrus is apparently confined to the mountain-forests of Antio- 

 quia in Colombia. Undoubtedly these two species at no very 

 remote period were one. Circumstances arose that divided the area 

 of its distribution, sending one portion up the highlands of Panama, 

 the other up those of Colombia. Our single species is now isolated 

 into two colonies : perhaps C. griseiceps was exposed to a more 

 pluvial climate, causing it to gradually develop a russet-brown 

 back. The two colonies never intermarried ; the slight differences 

 woiild therefore soon become constant by Isolation, and finally the 

 result is as we see it at the present time — two nearly allied, but 

 a]iparently perfectly distinct, species inhabiting different areas of 

 distribution. No Natural Selection could possibly have been at 

 work in such a case " [but it is itself a case of Natural Selection] ; 

 " for it could have served no beneficial purpose. It was no advan- 

 tage for either colony to possess those differences that have finally 

 become constant characters " [but how do we know this ?] ; " they 

 gave their fortunate possessors no favour in the struggle for life — 

 a russet back was no more advantage to its possessor than an olive 

 back " [then why did the variation occur ?], " though both have been 

 preserved, not by the survival of the fittest, but simply by Iso- 

 lation." Evidently, however, there must have been a " survival of 

 the fittest," unless the changes involved be assumed to have taken 

 place pe7' saltum immediately after the change of conditions. 



