68 



INORGANIC EVOLUTION. 



[CHAP. vii. 



Stars of lowest Temperature. 

 Ascending Series. Descending 



Antares, one of the brightest 

 stars in Duner's Catalogue of 

 Class IILr.* 



[Nebulse.] 



19 Piscium, one of the brightest 

 stars in Duner's Catalogue of 

 Class lllb. 



[Dark Stars.] 



In order to make quite clear that both an ascending and a 

 descending series must be taken into account, I give herewith two 

 photographs showing the phenomena observed on both sides of the 

 temperature curve in reversing layers of stars of nearly equal mean 

 temperatures, as determined by the enhanced lines. The stars in 

 question are : 



Sirius (descending), 

 a Cygni (ascending). 



Procyon (descending). "1 -p.^ O g 

 y Cygni (ascending). ] 



The main differences to which I wish to draw attention are the very 

 different intensities of the hydrogen lines in Sirius and a Cygni, and the 

 difference in the width and intensities of the proto-metallic and metallic 

 lines in Procyon and y Cygni. These differences, so significant from a 

 classification point of view, were first indicated in a communication to 

 the Eoyal Society in 1887f, and the progress of the work on these lines 

 has shown how important they are. I have based the group or generic 

 words upon the following considerations. 



As we now know beyond all question that a series of geological 

 strata from the most ancient to the most recent brings us in presence 

 of different organic forms, of which the most recent are the most com- 

 plex ; is it possible that the many sharp changes of spectra observed 

 in a series of stars from the highest temperature to the lowest, bring us 

 in presence of a series of chemical forms which become more complex 

 as the temperature is reduced 1 If so, we are in the stars studying 

 the actual facts relating to the workings of inorganic evolution on 

 parallel lines to those which have already been made available in the 

 case of organic evolution. I shall discuss this matter later. 



In the meantime, regarding the typical stars as the equivalents of 

 ihe typical strata, such as the Cambrian, Silurian, &c., it is convenient 



* Sur les etoiles a spectres de la troisn j ine elasse. 

 f Proc Roy. Soe., vol. xliii, p. 145. 



