THE CLASSIFICATION OF BIRDS 15 



that these very slight variations (fortuitous, so-called) are heredi- 

 tary, and that consequently the group of individuals within range 

 of each other, so far as interbreeding is concerned, gradually 

 develop in a direction favourable to their special mode of life by 

 the combined action of the longevity of the individuals possessing 

 any slightly beneficial modification, and of the early removal by 

 death of those possessing any slightly injurious modification. The 

 effect of these causes are supposed to be intensified by the fierce- 

 ness of the struggle for existence to which some individuals are 

 exposed, and to be almost inoperative where the struggle for exist- 

 ence is very slight; so that whilst some species may develop 

 rapidly, others may show scarcely any change for long periods. 



The development of a species is one thing, but the creation of 

 a new species is quite another. So long as the individuals of a 

 species live in the same area they habitually interbreed, and by 

 inheritance the favourable modifications become common property ; 

 but if a part of a species emigrate to a district isolated from the 

 rest, the slight differences of environment may make it to the 

 advantage of the species in the two localities to develop on slightly 

 different lines, and the accumulated slight differences may in the 

 course of a great number of generations produce so wide a diverg- 

 ence that, should circumstances occur by which the two parties 

 should again occupy the same area, the wish or the power to inter- 

 breed would be absent, and the two parties would be regarded as 

 specifically distinct. It is thus obvious that the two great factors 

 in the multiplication of species are variation and isolation. It is, 

 however, outside the scope of the study of classification to inquire 

 into the modus operandi of the differentiation of species ; it is enough 

 to recognise the fact that species have become differentiated, with- 

 out deviating into the endless maze whither an inquiry into the 

 cause of the differentiation must inevitably lead the student. 

 Suffice it to say that there seems to be a consensus of opinion 

 amongst the followers of Lamarck, Darwin, and Weismann that 

 the variations of the species are advantageous, whether those of 

 the individuals be accidental, controlled by natural selection, or 

 the effects, direct or indirect, of environment, or the result of 

 some unknown teleological law. 



The convert to the law of heredity naturally assumes that 

 every important character has been derived from inheritance, and 

 consequently expects that by grouping together species which 

 possess common characters a natural classification of Birds can 



