30 REPORT—1905. 
star of a cluster stands out clear by itself. These and other observations 
force on us the conviction that the wispy clouds represent the earliest 
stage of development, the more condensed nebule a later stage, and the 
stars themselves the last stage. This view is in agreement with the 
nebular hypothesis of Laplace, and we may fairly conjecture that the 
chains and lines of stars represent pre-existing streaks of nebulosity. 
As a star cools it must change, and the changes which it undergoes 
constitute its life-history, hence the history of a star presents an analogy 
with the life of an individual animal. Now, the object which I have had 
in view has been to trace types or species in the physical world through 
their transformations into other types. Accordingly it falls somewhat 
outside the scope of this address to consider the constitution and history 
of an individual star, interesting although those questions are. I 
may, however, mention that the constitution of gaseous stars was first 
discussed from the theoretical side by Lane, and subsequently more com- 
pletely by Ritter. On the observational side the spectroscope has proved 
to be a powerful instrument in analysing the constitutions of the stars, 
and in assigning to them their respective stages of development. 
Tf we are correct in believing that stars are condensations of matter 
originally more widely spread, a certain space surrounding each star 
must have been cleared of nebulosity in the course of its formation, 
Much thought has been devoted to the determination of the distribution 
of the stars in space, and although the results are lacking in precision, 
yet it has been found possible to arrive at a rough determination of the 
average distance from star to star. It has been concluded, from investi- 
gations into which I cannot enter, that if we draw a sphere round the sun 
with a radius of twenty million millions of miles,' it will contain no 
other star ; if the radius were twice as great the sphere might perhaps 
contain one other star ; a sphere witha radius of sixty million millions of 
miles will contain about four stars. This serves to give some idea of the 
extraordinary sparseness of the average stellar population ; but there are 
probably in the heavens urban and rural districts, as on earth, where the 
stars may be either more or less crowded. The stars are moving rela- 
tively to one another with speeds which are enormous, as estimated by 
terrestrial standards, but the distances which separate us from them are 
so immense that it needs refined observation to detect and measure the 
movements. 
Change is obviously in progress everywhere, as well in each individual 
nebula and star as in the positions of these bodies relatively to one another. 
But we are unable even to form conjectures as to the tendency of the 
evolution which is going on. This being so, we cannot expect, by con- 
sidering the distribution of stars and nebule, to find many illustrations of 
the general laws of evolution which I have attempted to explain ; accord- 
ingly I must confine myself to the few cases where we at least fancy our- 
1 This is the distance at which the earth’s distance from the sun would appear 
to be 1”. 
