x INTRODUCTION. 



to understand how this process can cause any development or evolution of 

 species, it is necessary to know two facts : first, that there is a difference 

 between individuals, so that one is more fit than another to conquer in the 

 struggle for existence ; and, second, that these individual idiosyncrasies 

 are for the most part hereditary, and are capable of being transmitted to 

 offspring. These two facts are well known to every breeder of cattle, 

 horses, dogs, or pigeons, and are the main facts upon which the horti- 

 culturist relies for success. The artificial selection of the farm or the 

 garden has its counterpart in "natural selection." This is the broad 

 theory of evolution as propounded by Darwin and Wallace. Respecting 

 the details of its application, some difference of opinion still exists. Most 

 writers consider that the differences in individuals from which Nature 

 selects the fittest to survive by killing off those which are less fit to cope 

 with the difficulties of life are accidental differences. Others hold the 

 theory that the tendency to vary from the ancestral type is a tendency in 

 a certain direction towards a fixed goal ; it may be as mysterious and in- 

 explicable as the tendency of a stone to gravitate to the earth, or of a 

 needle to fly to a magnet, but not the less a fact, the one tendency being 

 as originally inherent in organic matter as the other in inorganic matter. 

 All that can be said is that it was originally made so. But, be this true 

 or not, the peculiarities of form and colour which we find in birds and 

 other animals do not seem to be all accounted for by the theory of the 

 survival of the fittest. There seems to be a correlation of the external 

 colour of many birds with their internal organization, which is inexplicable 

 on the commonly received view. Many internal characters are, as my 

 friend Mr. Alfred Tylor expresses it, emphasized on the plumage. It seems 

 possible also that in some cases there may be a direct influence of climate 

 upon colour, independent of the indirect influence of protective selection. 

 The selection of Nature is in different directions. The fitness for the posi- 

 tion in which a bird or other animal is placed which ensures its survival 

 may be of various kinds : muscular strength or other superior organi- 

 zation to enable it to conquer its enemies of its own or other species ; 

 special adaptations to enable it to secure a better supply of food ; special 

 coloration to enable it to escape the observation of its enemies or attract 

 the attention of the opposite sex ; or it may mimic the colour or shape of 

 some other animal known to be dangerous ; or the special fitness may be 

 in the habits of the bird, in its choice of a nesting-site, in its migra- 

 tions in fact, in every variation of structure or habits which distinguishes 

 one species from another. 



The acceptation of the hypothesis of evolution implies the recognition 

 of species in the process of formation. If this theory be correct, there 

 must be always some species which are not yet finished. In the slow 

 process of evolving two species from one there must be a period when the 



