SYMBIOSIS AND SELECTION 27 



there be a very vigorous form, it may ultimately grow 

 and multiply to such an extent as to crowd out and finally 

 kill the other forms with which it is associated, and if the 

 nutrient medium equally favour two species, that one 

 which is in an excess at the beginning may outgrow the 

 other. The occurrence of what has been termed sym- 

 biosis is of considerable interest in the life of micro- 

 organisms, and too little attention has hitherto been paid 

 to it. This is the co-existence of two or more species 

 which together bring about certain changes. For ex- 

 ample, in the well-known ginger-beer plant, Marshal] 

 Ward l isolated several yeasts, bacteria, and moulds ; 

 of these, one of the yeasts and one of the bacteria together 

 induce the particular changes in a saccharine fluid to 

 which ginger has been added, which render the mixture 

 like ginger-beer, and these changes do not occur unless 

 both species develop together. 



Bacteria exhibit a selective action on certain substances 

 which contain isomerides or right- and left-handed modifi- 

 cations of a substance. The Bacillus ethaceticus attacks 

 mannitol but not dulcitol, two alcohols which are very 

 similar and possess the same simple chemical formula. 



By a series of brilliant researches Emil Fischer succeeded 

 in determining the constitution of the various sugars, 

 and, further, gynthesised them in the laboratory. The 

 natural sugars are all compounds with dissymmetric 

 molecules rotating a beam of polarised light, but when 

 prepared artificially they are without action on polarised 

 light, because the artificial product consists of equal 

 numbers of left-handed and right-handed molecules, and 

 the optical activity of the molecules of the one neutralises 

 the optical activity of the molecules of the other, thus 

 giving rise to a mixture which does not affect the polarised 

 beam. 



* Phil Tran*. Roy. Soc. Lond., vol. clxxxiii, 1892, p. 125. 



