XX INTRODUCTION. 
it is by these slight variations, which, when beneficial, are preserved by 
natural selection, that birds adapt themselves to any changed conditions 
of life. 
With birds, as with man, “ when once a particular mode of building has 
been adopted and has been confirmed by habit and by hereditary custom 
it will be long retained, even when its utility has been lost through changed 
conditions ” (Wallace). Although many habits have long since ceased to 
be of any service, they are retained. Witness the fact of the hole-building 
Ducks covering their eggs like their congeners nesting in the open; the 
Jackdaws often elaborate a nest in a position where one even of the 
slightest description is of small necessity ; and our domestic Swan adds 
to its nest (undoubtedly a habit originally acquired for its protection from 
floods) when that nest is far removed from the waters. Neither birds nor 
men can change old habits suddenly. Witness how we still retain the 
side-straps and the arms in our first-class railway carriages (a custom 
handed down from the old coaching days), or the buttons on the backs 
of our coats (which were formerly used to fasten up the long tails), and 
many other cases which are now quite as useless as the instances noticed 
among birds. Another instance, the Apteryx of New Zealand (Apteryx 
australis) when it sleeps goes through the formality of placing as much of 
its head as possible under its rudimentary wings*. With regard to birds, 
however, these superfluous actions are in no way injurious to the species 
performing them—were they so, natural selection would assert its influence 
and would eliminate those individuals who did not conform to their 
changed conditions of life. 
It is thought a remarkable fact by some naturalists that species of very 
wide range should build typical nests throughout their distribution. But 
surely there is nothing extraordinary in this if the area of distribution is 
continuous! Cetti’s Warbler (Cettia cetti) is a good instance. This bird 
breeds from Spain and Algeria to Turkestan, and examples of its nest 
almost from these two extremes do not differ in the least in their con- 
struction ; but I do not see any thing remarkable in this, even though this 
bird is not migratory, for it breeds “ along the whole line,” and there is 
nothing to prevent one style of architecture being common to the speciest. 
The Woodchat Shrike (Lanius rufus) is another good instance. 
One of the great points brought forward in favour of instinct is the 
uniformity of the nests of the birds of each species, even though they be 
* Trans. New Zealand Institute, ii. p. 75 (1869). 
+ It would be very interesting to know if those non-migratory species that are separated 
by discontinuous areas of geographical distribution build typical nests throughout. This 
is a subject of which we possess no information, and is well worthy the attention of 
those observers suitably situated for studying this interesting question, 
