I2 THE EVOLUTION OF LIFE 
tions are to be found in the fact that ‘“‘in the sun, 
and in most of the stars, we have light from a more | 
highly heated centre passing through an envelope 
of cooler vapours, and on this account absorption 
phenomena are produced,” thereby giving rise to 
the various groups of lines seen in the spectra. By 
the use of the spectroscope, therefore, and after 
prolonged labours devoted to the identification of 
the various sets of lines characterising the spectra 
of the different elements, we have gradually been 
taught that very many of the same elements which 
exist on this earth are to be found, in various com- 
binations, entering into the composition of the sun 
and of the stars, and, what is all-important, zz 
gradually increasing numbers as stars of lower 
temperature are brought under observation. 
In the early days of spectroscopic investigation it 
was supposed that an element could yield only one 
particular line spectrum. The first blow was given 
to this conception in 1865 by Plucker and Hittorf, 
who announced that ‘‘there is a certain number of 
elementary substances which, when differently 
treated, furnish two kinds of spectra of quite a 
different character, not having any line or band in 
common.” Much work has since been done in this 
direction by Sir Norman Lockyer, and his results 
with those of other workers have gone far to 
establish upon a firm basis the doctrine of Inorganic 
Evolution with which we are now concerned. 
It soon became recognised that the spectrum of 
an element depended, within certain limits, upon the 
temperature to which the substance was exposed ; 
