Bihliogra'phical Notice. 383 



It will be seen from the above extract that the author holds that 

 variations of which we cannot see the advantage are necessarily not 

 advantageous ; but until we know something more of the causes of 

 variation it is hardly safe to argue from such premisses. Further, 

 from this and other passages it is clear that he attaches too realistic 

 a meaning to the " struggle for life." He seems to regard it too 

 much as a literal struggle, whereas, in the Darwinian sense, while it 

 undoubtedly includes positive physical contests, it also iucludes that 

 more peaceful competition by which certain favourably endowed 

 individuals manage to come in for more than their share of the good 

 things of this life, leaving a scanty supply for their less favoured 

 brethren. The author in the very next chapter ascribes great im- 

 portance to " Climatic Influences " in the production of variation, 

 and justly, and in the above instance he hints that the differences 

 between the two species referred to are duo to such causes. But 

 how can any one, in the present state of our knowledge, venture to 

 declare that even the smallest colour-variation produced under the 

 influence of a change of climate is of no advantage to its possessor ? 



We have already devoted so much space to this book that the 

 next three chapters must be passed almost s»6 slUntio. The pheno- 

 mena referred to in them are all recognized factors in the process 

 called " Natural Selection," although the author, in his chapter on 

 " Sexual Selection," seems to regard the latter as a process distinct 

 from Natural or " Protective " Selection. 



In his last chapter, on "Interbreeding or Intercrossing," Mr. 

 Dixon enters upon a question of much interest, and one the inves- 

 tigation of which seems to promise very interesting results. He 

 distinguishes three kinds of interbreeding, namely : — 1, interbreed- 

 ing amongst the individuals of a species ; 2, interbreeding between 

 subspecies, local races, and representative forms ; and 3, interbreed- 

 ing " which, by absorbing a closely allied form, gradually works the 

 extinction of a species." The first kind of interbreeding, of course, 

 tends to the effacement of small variations and to keep the indivi- 

 duals true to the species (or variety) ; the second is of special im- 

 portance to the student of geographical distribution, as it leads to 

 the production of chains of intermediate forms uniting two or more 

 types which may have arrived at the dignity of true species, and its 

 consideration may serve to explain some difficult problems in the 

 distribution of species ; while the third is of still greater interest, 

 as it may furnish a clue to the explanation of other still more 

 obscure cases. The particular instance cited by the author is that 

 of the three species of Blue Titmice — Paras ccerideus, P. cyanus, and 

 P. Pli'sl'ii. The first of these species, the common Blue Titmouse, 

 is restricted to Europe; the second inhabits Eastern Kussia and 

 Siberia as far as the Pacific ; while the third is found in Central 

 Eussia ; this last interbreeds with both the others, and is gradually 

 being absorbed by them. " It would appear," says Mr. Dixon, 

 " that these three species of Blue Titmouse are modifications of a 

 common parent form by Isolation ; but as their areas of distribution 

 again became continuous, the two dominant races have intercrossed 



