ave-length. The intensity of the
radiation, that is to say, the amount of energy moving along the beam,
can only affect the number of electrons set in motion and not the speed
of any one of them. During the last few years a very extraordinary
theory has been developed on the basis of these and similar facts. I
doubt if it would be more profitable to give further instances at
present, but I have mentioned it because it seems to show looming on the
horizon of our knowledge another tendency of Nature to make use of the
atomic principle.

I will only add that the whole position of physics is indeed at this
time of extraordinary interest, and at any moment there may be some
great discovery or illuminating thought which will explain the present
startling difficulties and open up new worlds of thought.

FOR REFERENCE

Bragg, _Rays and Crystals_ (Ball & Sons).

FOOTNOTES:

[Footnote 70: Since this address was given, the results of the Eclipse
Expedition to Brazil are considered to have confirmed in a satisfactory
manner one of the most remarkable deductions made by Einstein from the
principles which he maintains. The matter has roused so much interest
that some of the leading exponents of the relativity principle have
published careful accounts intended for students not familiar with it:
it would therefore be superfluous to discuss the matter here.]

IX

PROGRESS IN BIOLOGY DURING THE LAST SIXTY YEARS

PROFESSOR LEONARD DONCASTER, F.R.S.

On November 24, 1859, _The Origin of Species_ was published, and this
date marks the beginning of an epoch in every branch of biology. Before
it, Biology had been almost entirely a descriptive science, but within a
few years after the publication of the _Origin_ its effects began to
colour all aspects of biological research. A co-ordinating and unifying
principle had been found, and the leading idea of biologists ceased to
be to describe living things as they are, and became transformed into
the attempt to discover how they are related to one another. The first
effect of this change of attitude was chiefly to turn biologists towards
the task of tracing phylogenetic or evolutionary relationships between
different groups of animals--the drawing up of probable or possible
genealogical trees and the explanation of natural classification on an
evolutionary basis. When once, however, the notion of cause and effect,
or more correctly of relationship, between the phenomena seen in