n THE PROGRESS OF SCIENCE 77 



so that it is said to express a periodic law of re- 

 current similarities. Or the relation may be 

 expressed in another way. In each section of the 

 series, the atomic weight is greater than in the 

 preceding section, so that if w is the atomic weight 

 of any element in the first segment, w+x will repre- 

 sent the atomic weight of any element in the next, 

 and w + x + y the atomic weight of any element in 

 the next, and so on. Therefore the sections may 

 be represented as parallel series, the correspond- 

 ing terms of which have analogous properties ; each 

 successive series starting with a body the atomic 

 weight of which is greater than that of any in the 

 preceding series, in the following fashion : 



d D 8 



c c 7 



b B ft 



w w-\-x w+x+y 



This is a conception with which biologists are 

 very familiar, animal and plant groups constantly 

 appearing as series of parallel modifications of 

 similar and yet different primary forms. In the 

 living world, facts of this kind are now understood 

 to mean evolution from a common prototype. It 

 is difficult to imagine that in the not-living world 

 they are devoid of significance. Is it not possible, 

 nay, probable, that they may mean the evolu- 

 tion of our " elements " from a primary undifferen- 



