102 
The Irish Naturalist 
April, 
young in the course of every four months ; and that the young 
ones are also mothers when five-and-a-half months old. A 
field-mouse is known to live for about six years, which would 
give her time, on the most moderate computation, to produce 
some sixty or seventy litters, or at least 200 young, in her life. 
Then when we consider her grand-children, great-grand- 
children, and great-great-grandchildren, by whom she would 
find herself surrounded when only three years old, we at once 
find ourselves running into hundreds of thousands, which 
would become thousands of millions before this venerable 
lady had finished her pilgrimage at the ripe age of six. Now 
we know that whether there are cats in a neighbourhood or 
not, mice do not increase to these enormous, incalculable 
figures j nor would they do so if stoats and owls and kestrels 
and all their numerous enemies — including black-headed 
gulls — were to vanish with poor pussy from the face of the 
earth. Every one of these creatures in destroying one mouse 
gives, on Darwin's own doctrine, a helping hand in life to the 
mouse that escapes it. I am not questioning for a moment 
the usefulness of these mouse-destroying creatures. They are 
at work all the summer and autumn, thinning the mice at a 
time when mice have plenty to eat, and thus they really, 
during a short season, reduce the total number. But as soon 
as a time of scarcit}- arrives, competition steps in to insure 
that the number of mice which survive it shall be the same 
number as would have survived it if there had been no pre- 
liminary thinning by cats and owls and the farmer's other good 
friends. So I don't think the red clover, w^hich will not need 
the good offices of the bumble-bees till the following June, 
will be much the worse off for the respite the mice have had 
in the autumn through some temporary scarcity of cats, owls, 
or vStoats. The whole questson is too complicated to allow 
me to speak very confidentlj^, but I think we should at least 
endeavour to discriminate clearly between the two points of 
view. 
There are difficulties, no doubt, in the way of our believing 
that we live in a fully crowded world. One is inclined to 
argue that it would be injurious, not only to the weaker 
individuals, whom the competition must kill, but to the health 
and general physical condition of all the individuals. But 
