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All that can be said is that it was originally made so.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 
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