i9o6. IMoFF AT.— The Struoglc for Existence. 105 
and draw attention to the fact that the disappearance of the 
quail from Ireland, through causes which have never been 
satisfactorily explained, was preceded by a great increase in 
the numbers of that bird. Thompson, when he published the 
second volume of his "Natural History of Ireland" in 1850, 
observed that the quail had of late decreased in England, but 
had somewhat increased, in his opinion, in Scotland, and was 
" decidedly on the increase" in Ireland. Thirty 3'ears later, by 
common agreement, the quail in Ireland was extinct. From 
all these cases I think we may judge that sudden increase of 
any species is very apt to culminate in a great reaction. 
Animals have, in fact, to compete with one another not only 
for the bare necessaries of life, but further to secure those 
conditions which will tend to generate healthy existence. 
In connection with this whole subject the field for observa- 
tion and for constant accumulation of data is very large. 
Observations are wanted as to the relative density of different 
species, as to the extent to which that density may vary in 
different years, and as to the general conditions which seem to 
accompan}- or cause variation in a given direction. We have 
not often opportunities for observing competition of that 
unrelenting kind which takes place when two species of almost 
exactly similar requirements and habits are brought into 
contact. That sort of competition was exemplified when the 
brown rat arrived in these islands and found them alread}^ 
inhabited b}' a black rat ; the requirements of the two were so 
much the same that the competition could only end, as it did, 
in the extermination of the weaker, which happened to be the 
black rat. But competition of a less obtrusive kind is always 
to be observed. For a number of j-ears I was aunuall}^ struck 
by the fact that the swarms of wasps which gather nectar so 
abundantly from the flowers of the ngwort were not the 
common wasp, but the tree wasp ( Vespa sylvestris), while 
around any other flower or fruit at which wasps assemble I 
might search all day and not find a tree wasp at all. I was 
led to the conclusion that the ground-building common wasp 
did not care for figwort. But in the autumn of 1900, for some 
reason, there were scarcely anj^ tree wasps, and the figwort 
(about Ballj^h^dand) was as full of wasps as ever, all belonging 
to the ground-building kinds ( Vespa vulgaris, V. germanica^ 
