1 86 THE METAPHYSICS OF EVOLUTION. 



groups, suggests that the seventy and odd substances 

 which are accounted elements, because we have not 

 hitherto been able to decompose them, are not final 

 and ultimate facts. The law which explains their 

 grouping must be regarded as anterior to them, 

 and its operation may be described as the genesis 

 of the elements. Hence it becomes possible to 

 speak of the evolution of the elements. 



But the analogy with biological evolution extends 

 much further. It is impossible not to be struck 

 with the great quantitative inequality in the occur- 

 rence of the elements. Some of them are widely 

 distributed and occur in large masses, whereas 

 others only occur rarely and in small quantities. 

 If, therefore, the elements are to be regarded as 

 the products of a process of evolution, it is evident 

 that the process has been much more favourable 

 to metals like iron than to one like platinum or 

 uranium. *' A rare element, like a rare plant or 

 animal, is one which has failed to develop in har- 

 mony with its surroundings," i.e., failed in the 

 struggle for existence. 



And it is even possible to guess at the cause. 

 One of the most striking facts about the rare metals 

 is that they occur in rare minerals composed of 

 several of these metals, and often occur in these 

 minerals alone. Thus rare minerals, like samar- 

 skite or gadolinite, may be found to contain three 

 or four of the rare metals, samarium, yttrium, 

 erbium, etc., and their close and constant associ- 

 ation evidently cannot be a matter of chance. Now 

 if a soluble salt of one of these earths, e.g., yttria, 

 be taken, and subjected to an extremely delicate 

 and laborious process of " fractionation," by which 

 the more soluble portions are separated out from 



