S4 H. C. SORBY. 



petals are coloured mainly by the more stable compounds 

 which absord only the blue end of the spectrum, whereas 

 when developed under the the influence of bright sunlight 

 other compounds are formed, which absorb light more towards 

 the red end of the spectrum, and are much more readily 

 decomposed. By exposing to the light the mixed solution of 

 the colouring matters developed in the living plant under the 

 influence of light, these less stable substances are soon de- 

 stroyed, and we obtain a mixture closely corresponding to 

 that met with in those petals which are developed in the 

 dark. All this may be explained by supposing that the 

 increased vitality due to the exposure of the plant to the 

 sunlight suffices to counterbalance a much greater amount 

 of mere chemical affinity. 



Such then is the best account I am now able to give of 

 what appears to me to be a most promising field for research. 

 Of course I cannot but feel that it is very imperfect, but yet 

 I trust that it will suffice to show the great value of this 

 method of research in the study of comparative physiology, 

 and the importance of attending to minute differences in 

 spectra, since this may often lead to the discovery of very 

 striking chemical differences which otherwise would never 

 have been suspected. It is much to be regretted that the 

 amount of material usually available will make it very diffi- 

 cult, if not impossible, to confirm such results by accurate 

 chemical analyses, bat at the same time this makes the value 

 of the spectrum method still more apparent, since it is pos- 

 sible by that means to learn a large number of most 

 important facts, which in all probability would for ever have 

 escaped detection by pure chemical analysis. Now that the 

 gradual evolution of complex chemical compounds essential 

 for the performance of physiological processes has been 

 forced on my attention, I can clearly see that this view of 

 the subject will throw much light on other cases ; and it 

 appears to me that for the future we must not confine our 

 attention to evolution of mere form, but materially extend 

 the scope of our inquiry, with the hope of being thereby 

 able to better understand the manner in which has been 

 brought about the wonderful correlation between structure 

 and function. 



P.S. — Since writing the above my attention has been 

 called to the following remarks made by Ray Lankester in 

 his paper in ' Pfliiger's Archives.' ^ He there says — " The 



' " Ueber das Vorkonimen von Haemoglobin iu den Muskeln der Mol- 

 lusken, und die Verbreitung desselben in den lebendigen Organismen,'* 

 ' Pfliiger's Arcbiv fiir Pliysiologie/ Bd. iv, s. 318. 



