INTRODUCTION. xv 



whole aspect of the study of ornithology was so changed by the fact that 

 scientific men had in the meantime begun to consider the subject from the 

 evolutionary point of view, that the earlier portion is already out of date, 

 and the latter, being almost necessarily finished on the same lines, is to a 

 considerable extent open to the same charge. The homogeneous character 

 of the work has been remarkably well preserved, even to its faults *, and 

 its reputation for accuracy creditably maintained. It is to be regretted 

 that neither editor recognized the importance of the fact of the intergrada- 

 tion of species, which is in many instances clearly demonstrated by a glance 

 at a large series of skins from different localities a fact which American 

 ornithologists have pointed out in the Nearctic Region, and which I have 

 endeavoured on many occasions to show is quite as obvious in the Palae- 

 arctic Region f. The progress of young birds to maturity, with the 



* To the present writer the attempt to avoid the charge of egotism, or at least to veil 

 the personality of the author by carefully abstaining from the use of the personal pronoun, 

 always appears to be a failure. The custom is quaint, not to say archaic ; but the author 

 ventures to think that it is more suitable to the irresponsible utterances of the anonymous 

 writer of the leading articles of a newspaper than to the expression of opinion, or the 

 narration of facts, in a scientific work. Nor can the present writer approve of the practice 

 of composing complicated Gladstonian sentences, so framed as to admit of several con- 

 structions, in order that the author may hereafter claim a recognition of his acumen in 

 having discerned the many-sidedness of truth. Still less can he pardon the custom of 

 " playing for safety " by the expression of an opinion, which is probably correct, whilst 

 carefully avoiding a statement of the grounds upon which the opinion is formed, because 

 the chance of their being wrong is so very much greater. The days of authority in science 

 as well as in religion are past. Modern students look for arguments, not opinions; what 

 they want are facts, and they will be grateful to any writer who provides them. If the 

 language be simple and the meaning clear, they will not stop to inquire whether the 

 writer be self-conscious of his egotism or not ; and if ignorance be candidly confessed, they 

 will thank the writer who thus suggests a useful field of labour, without losing time 

 in forming an opinion of his reputation for learning, which is a matter of little or no 

 consequence. 



t In the Introduction to my first volume I endeavoured to show the important part 

 which interbreeding plays in the evolution of species ; but I appear to have done it so clumsily 

 that my American critics have completely misunderstood the point (' Auk,' 1885, p. 89). 

 ('mss-breeding between different species is a very unimportant part of the subject, as it 

 seldom results in fertile offspring. Interbreeding is of the greatest importance when it 

 takes place between different races or incipient races of the same species. Where the 

 area of distribution is very wide it often results in a series of forms which intergrade with 

 each other. Where the area of distribution is small it prevents every little valley setting 

 up a race of its own, as happens with so many species in the Pacific Islands, for example, 

 where an extended area of distribution has been cut up into many small but interrupted 

 areas of distribution, so that interbreeding between the inhabitants of one and those of 

 another cannot take place. It is a remarkable fact that the importance of interbreeding is 

 as obvious in the vegetable as well as in the animal world, many plants having acquired 

 quite complicated mechanical contrivances to prevent self-fertilization and insure inter- 

 breeding. 



