EVOLUTION OF THE UNIVERSE. SRD 
unable to bridge the gap between them ; it will have become impossible 
to see any one galaxy from any other, even by light which left it at the 
very beginning of time; only the mathematician will be able to deduce 
the existence of the other galaxies in recondite ways—and probably no 
one will believe him. Then at last these will have justified Herschel’s 
name for them—‘ island universes.’ 
When this time comes, each individual galaxy will make its own 
individual ripple of radiation ; these ripples will for ever increase in size 
and weaken in intensity, but cannot intermingle to form a uniform dis- 
tribution. The evolution of the universe will be nothing but a rapid 
spreading of ripples in a still more rapidly-spreading space. 
Even the most confirmed optimist could not, I fear, claim that any of 
the possible lines of development or final ends is of a particularly exhilarating 
nature; the most consoling thought I can offer is that when the worst 
comes to the worst, we shall none of us be there to see. 
Suppose some infallible oracle offered to give a ‘ Yes’ or ‘ No’ answer 
to two scientific questions for each of us. Personally, 1 think I might 
choose as my two questions : 
1. Does the main energy of stellar radiation come from the annihilation 
of matter ? 
2. Is the universe expanding at about the rate indicated by the spectra 
of the nebule ? 
Prof. E. A. Mitne, M.B.E., F.R.S. 
It has sometimes been asked whether the universe is evolving at all. 
Certainly its future evolution is a matter of speculation and its past evolu- 
tion a matter of inference. But that it is evolving at this present time is 
not a matter of speculation at all; it is a matter of observation. We 
actually see stars undergoing drastic changes—changes of organisation so 
fundamental that they can almost be described as mutations. I refer to 
those outbursts which we call Novae. 
From time to time in the heavens a faint star is seen to blaze up for 
a few hours or days, increasing in brightness perhaps some 25,000 times, 
then irregularly fading and after a few years returning to its pre-outburst 
brightness. Such outbursts are by no means uncommon. The first 
quarter of the current century has witnessed five novae visible to the 
naked eye. Many present will remember Nova Aquilae of 1918, which 
was visible even before dark; and Nova Cygni of 1920. These, from 
their brightness, were comparatively near-by stars—Novae Aquilae, the 
more distant, was only some 300 parsecs away. But many more have 
been observed in the most distant regions of space. Up to 1929 over 
eighty novae had been recorded in the great nebula of Andromeda, and 
were used by Hubble to determine its distance. Thus, Novae are distri- 
buted throughout space. They are also distributed fairly frequently in 
time. Hubble has estimated that some thirty novae occur each year in 
the Andromeda Nebula. It has also been estimated that in our own 
galactic system there is at least one nova per year. Such estimates are 
conflicting, but, as many have pointed out, novae are sufficiently frequent 
to imply that on the average every star ‘ becomes a nova’ at least once 
PP2 
