pointed out, reagents acting on the characteristic, typical structure of the components of definite kinds of cells. We may illustrate this idea by an example. A great sensation was caused at one time bv the observation, that there were unicellular organisms which apparently showed signs of intelligence. It could be seen under the microscope how the uni-cellular organism, called Vampyrella spirogyræ, hurried from one alea thread to another, until it stopped at a particular kind of alga in order to use it as food. However many kinds of alga were offered to it, it would always pick out the same kind. This phenomenon, which seems so amazing at first sight, may doubtless be explained in the following way: Every living being has ferments at its disposal which, as Emil Fischer pointed out, may be compared with keys, and the substrate, against which they are directed, with locks. Just as a particular key generally unlocks and locks only a particular lock, so can particular ferments only decompose or reconstruct substrates of a particular constitution.
The Vampyrella spirogyræ, then, hurries from alga to alga, bearing with it ferments, by means of which it intends to convert nutriment into a suitable form. It is always trying to effect an entry by means of its "keys," and it only succeeds in certain cases, namely, when the key fits the lock, which is to say, when the cell-wall of the particular alga is of
