BIRDS AND NATURE 
19 
fining ourselves to those living on the territory here consid- 
ered. There are very few birds that lay more than once a 
season. And there are but few species, if we confine ourselves 
to those under special consideration here, the nests of which 
are comparatively populous: some titmice have 12—18, the 
quail from 10 to 16, the partridge 14—18; while the most 
important insect-eaters have from 5—6 eggs. 
Over against the propagation we have the elements of 
extirpation, which are of three kinds. The first is part of the 
economy of Nature and is represented by the natural enemies 
of the birds, viz. small mammals of prey, birds of prey — 
among the latter nest- robbers. The second is the catastrophe 
which generally overcomes those birds of passage that are 
obliged to cross the sea to reach their winter or breeding 
quarters. The causes of the catastrophe are meteorological, 
— famine resulting from icy-cold winters, storms, thick 
fogs of long duration, which overtake the winged travellers 
en route. 
The above must be considered as restrictions imposed 
by Nature: and, though the first element may be combated 
by man's keeping down the number of the birds' natural 
enemies, there is nothing to be done against the second, for 
frost, storms and fogs are „ forces majeures". Yet we can 
relieve the famines of winter. The comparatively insignificant 
increase and the natural restrictions in the case of birds 
must be set over against what birds act as restricting agents 
on, — the insect world, the propagation of which is generally 
a geometrical progression, and the world of vegetation with 
its often peculiar development and its mass of seeds which 
help to secure the existence and increase of some particular 
species. 
2* 
