As soon as one tube is filled, I replace it by another.
Moreover, I have scattered in front of either hive a few handfuls of empty Snail-shells, specially chosen for the object which I have in view.

Reasons which I will explain later led me to prefer the shells of Helix caespitum.

Each of the shells, as and when stocked, received the date of the laying and the alphabetical sign corresponding with the Osmia to whom it belonged.

In this way, I spent five or six weeks in continual observation.

To succeed in an enquiry, the first and foremost condition is patience.