234 THE ZOOLOGIST. 
over the country. In general they occur in isolated couples in 
brushwood, cornfields, and meadows, but nobody will be aware of 
their presence unless he detects one of their nests; and if he has 
the rare luck to find one, he will soon conclude that the species 
is spread over the country in single couples, living at great 
distances from one another. It is indeed an exceptional case 
when they are found forming a colony, and such a one is some- 
times restricted to a colony of little extent. When surprised by 
inundation of the meadows, they are sometimes seen flocking 
together in considerable numbers trying to save themselves by 
climbing up to the crown of grass and plants. 
The system of colonisation of this animal is, however, not 
permanent, the colony being often reduced in the following year 
to a small number of couples. No doubt that the increased 
number produced by a favourable multiplication in certain years 
contributes to the fluctuation observed in the distribution of these 
animals. 
I now purpose to enter into some details about a colony of 
the Harvest Mouse I met with in the summer of the year 1868, in 
a locality not examined before that time in its whole extent, and 
of which colony only a small number of couples remained in the 
following years. This locality is situated at the distance of about 
two miles from the town of Leyden, in the neighbourhood of the 
Castle of Endegeest, celebrated for having served as a refuge to 
the philosopher Descartes after his exile from France. There 
exists, on the right side of the road leading to the neighbouring 
village of Rynsburg,—not less celebrated for its Abbey and as the 
residence of the freethinker Spinoza,—a ditch of about a quarter 
of a mile in length and six paces in width, intersecting a field 
planted with vegetables. Its right border was for one-half of the 
length grown with high reeds, the other longitudinal half showing 
no vegetation. The ditch, however, being blind at the end, 
became partly dry, by evaporation, during the hot season. It 
was in the reeds of this ditch that a part of the colony of the 
Harvest Mouse had settled and had built their nests, also making 
use for this purpose of the herbs growing near the border side. 
After having detected the colony, I gave my orders to a man 
who for more than forty years has been in the service of the 
Museum, and who has no other charge than accompanying myself 
or our sportsmen when out in the field—a man who catches birds, 
