80 N. W. PIRIR 



used commonly as it is still used by some tunicates. It is interesting that it is 

 the primitive species of tunicate that have blood corpuscles containing large 

 amounts of V and also normal, or stronger, H2SO4. An existence that depends 

 on a strongly acid solution of an clement as rare as V is obviously both hazardous 

 and precarious so that more highly evolved species, able to do the same job with 

 a commoner element, such as iron, have an advantage. Dependence on the use 

 of V in quantity seems to have been disappearing and we see now only the last 

 remnant of it. But selection does not operate in the same way against the use of 

 traces of V and, in traces, the element is present in many organisms. It is essential 

 for some moulds and algae [10] and probably for other plants as well. Much the 

 same can be said for aluminium. It is essential for the growth of some plants and 

 is widely distributed, particularly in the more primitive types such as tea [11] 

 and lycopodium. It is not so easy here as with V to see what kind of reaction may 

 be involved. In many species there is barium in the choroid and chromium in 

 the blood corpuscles but there is no evidence whether these occurrences are the 

 reUcs of ancient metabohc paths or the beginnings of new ones. 



Selenium is widely distributed in rocks so that dependence on it would rarely 

 be a serious disadvantage and yet it is seldom used and is poisonous to most 

 species. A few plants however grow better in the presence of Se and it is built 

 into their proteins in place of sulphur so that the protein is poisonous to animals. 

 The phenomenon, if not the interpretation, was known to Marco Polo and a 

 close study of it would be most illuminating. 



The relationship between S and Se leads naturally to a consideration of the 

 possibility of a similar relationship between carbon and germanium. There is 

 no evidence that Ge is essential for any present-day organism but the Hartley 

 coal seam in Northumberland is rich in it and it is present in workable amoxmts 

 in many other coal seams. This could be the result of adsorption of Ge by coal 

 or its precursors but it is Just as likely that coal-fonning organisms used Ge. 

 If they did, some may do so still, and a search might be rewarding both bioche- 

 mically and commercially. The presence in fossils of some other elements, for 

 example caesium, thallium and titanium suggests a biological role for them also, 

 but secondary specific absorption, during the process of fossihzation, confuses 

 the interpretation of all evidence of this type. Argument about these elements 

 may seem to be too nebulous to merit serious attention. There are two reasons 

 for setting it out; it is important to keep all the possibilities to some extent in 

 mind, and until the possible significance of this type of evidence is appreciated 

 not much effort is likely to be put into finding or co-ordinating the facts. 



Halogen metabolism offers some other hints about the early course of evolution. 

 Halide ions have probably been available ever since liquid water formed, there 

 may have been free chlorine, and chlorinated hydrocarbons are present in 

 bitumens both of terrestrial and meteoric [12] origin. The abihty to handle the 

 halogen-to-carbon bond may therefore have been useful to an eobiont. Sponges 

 still make extensive use of bromine compounds, so do the various species of 

 mollusc that make Tyrian purple; there is at least one group of plants, 

 Dichapetalum, that makes fluoroacetic acid and several of the moulds make a 

 range of chlorine compounds. But the vertebrates have nearly given up this type 



