16 
GENERALIA 
If we apply this to birds alone, we can formulate the 
following thesis: man must protect those birds which are of 
value to his own interests and put a check on those which 
are inimical to the same. By protection we mean of course 
that he must allow useful birds every chance of subsistence 
i. e. means of living and opportunities of increase. 
If we turn our attention now to the differences in the 
conditions of life of animals and vegetation due to the geo- 
graphical position of the particular region and to climatic 
divergencies, we shall find beyond doubt that the life of 
birds and the effects produced by the same cannot be treated 
from one and the same point of view. 
The birds, — if we include the phenomenon of the grand 
migration and consider the northern part of the Eastern Hemi- 
sphere i. e. that part which falls between the Equator and 
the North Pole, — occupy the territory just defined; not to 
speak of those birds which entend their migrations beyond 
the Equator, i. e. to the South Pole. 
Broadly speaking, the higher we penetrate into the polar 
regions, the fewer species of birds we meet; all the larger, 
however, the quantities of each species, a fact which corre- 
sponds with the simplicity and mightiness of the phenomena 
of polar Nature: the nearer we approach to the Equator, the 
more species of birds we find; but the appearance . of any 
particular species in masses becomes rarer, a fact which, in 
its turn, corresponds to the complexity of natural phenomena, 
among others to the vast variety (multifariousness) of lower- 
grade animals and of the whole vegetable world. 
Any territory lying between the Pole and the Equator 
contains modifications corresponding to its situation; and the 
conception of the significance of birds varies with these 
modifications, for there are birds which in certain territorial 
conditions are useful, in others again are noxious. To take a 
