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 nature. The cause—the primary process—escapes our notice, partly because at the time we do not know how best to state the problem, partly because we have no methods at our disposal for an experimental investigation of the case.

In all biological problems it is remarkable how entirely dependent we are upon the philosophy and the methods employed in the exact natural sciences. We transfer all that is there obtainable to the problems of biology. For some years certain ideas prevail, only to recede as soon as a new impulse or a new success in the domain of physics and chemistry directs a host of workers into new paths. We drill and work until a new gallery is driven into the rock of puzzles which is found in every cell. Very often the gallery ends blindly, but on its way has given rise to numerous interesting discoveries. Sometimes, however, the pioneer work is crowned with success. An important stage is left behind, and a new outlook gained. The final aim, however—a complete insight into the metabolism of the cell—still lies far ahead. Yet the knowledge we have acquired serves as a compass to keep us on the right road. The careful traveller will never leave anything unnoticed, for observations which often seem but trifles may point the way to entirely new problems.

In studying the functions of the cell we must never forget that there is not a single substance which is of