When it came to be my turn to speak on a solemn occasion to this gathering, attended by so many to whom I owe my scientific education, I was well aware how difficult was the honourable duty I had undertaken and only reluctantly began to shoulder it. Forgive me, therefore, if I feel I must devote some words of apology to the very choice of my subject. This choice is no doubt easier for the philosopher and historian who remain in constant touch with the general public. In natural science it used often to be the custom to discuss more general topics of so-called philosophic or metaphysical interest. If today I depart from this custom, I certainly do not wish to provoke the suspicion that these more general questions seem to me insignificant or unimportant as against the countless special problems raised by contemporary science. It is only the manner in which they have been treated to date, in some cases one almost feels tempted to say the fact that they are treated at all at this time, that seems to me mistaken.