12 Te) BIRDS OF LA PLATA 
crease in another more favoured species over the 
same region. It is not as if the regnant species had 
invaded and seized on the province of another, but 
appears rather as if they had quietly entered on the 
possession of an inheritance that was theirs by right. 
Mighty as are the results worked out by such a 
process, it is only by a somewhat strained metaphor 
that it can be called a struggle. But even when the 
war is open and declared, as between a raptorial 
species and its victims, the former is manifestly 
driven by necessity, and in this case the species 
preyed on are endowed with peculiar sagacity to 
escape its persecutions ; so that the war is not one 
of extermination, but, as in a border war, the invader 
is satisfied with carrying off the weak and unwary 
stragglers. Thus the open declared enmity is in 
reality beneficial to a species; for it is sure to cut 
off all such individuals as might cause its degenera- 
tion. But we can conceive no necessity for such a 
fatal instinct as that of the Cuckoo and Cow-bird, 
destructive to such myriads of lives in their begin- 
ning. And inasmuch as their preservation is inimical 
to the species on which they are parasitical, there 
must also here be a struggle. But what kind of 
struggle ¢ not as in other species, where one perishes 
in the combat that gives greater strength to the 
victor, but an anomalous struggle in which one of 
the combatants has made his adversary turn his 
weapons against himself, and so seems to have an 
infinite advantage. It is impossible for him to suffer 
defeat ; and yet, to follow out the metaphor, he has 
