240 ORIGIN OF STRUCTURES AND FUNCTIONS 



and more efficient in carrying out their particular functions, 

 then it is reasonable to suppose that their component parts (as 

 it were the nuts and bolts of the mechanism) have been to a 

 great extent standardised, just as in modern engineering the 

 component parts have been standardised so that they can be 

 used to make all kinds of things from sewing machines to motor- 

 car engines. 



The idea of the standardisation of the amino acid composi- 

 tion of proteins during the process of evolution of higher 

 organisms finds support in the fact that among organisms 

 at a lower stage in evolutionary development — mainly bac- 

 teria and fungi — we find, in addition to the ordinary amino 

 acids which are constantly present in proteins, that there are 

 continually being discovered new, so-called ' peculiar ' or 

 ' uncommon ' amino acids such as /3-thiolvaline in Penicil- 

 lium spp.,^^ mf50-ae-diaminopimelic acid**" and other amino 

 acids. *^ There are also found in the proteins and peptides, 

 and particularly in the antibiotics, of lower organisms the 

 ' unstandardised ' D-forms of amino acids, among them d- 

 glutamic acid in the capsule of Bacillus anthracis*'^ and 

 related organisms, D-leucine in gramicidin,*^ D-phenylalanine 

 in gramicidin S*® and tyrocidine*^ {B. brevis), D-alanine in 

 Lactobacillus arabinosus,^^ etc. In higher organisms, on the 

 other hand, we invariably find only L-forms of amino acids 

 and apparent exceptions to this rule have always been found 

 to be artefacts arising by racemisation, usually during the 

 hydrolysis of the proteins.*^ 



Thus we see that during the course of evolution the transi- 

 tion from the lower forms, with their as yet imperfectly 

 organised metabolism, to higher forms in which the metabol- 

 ism has reached a higher degree of co-ordination, is marked 

 by a standardisation of the amino acid composition of pro- 

 teins due to natural selection. Thus, one of the essential 

 properties of the animal and vegetable proteins which we 

 have studied, their amino acid composition, is not entirely 

 determined by physical and chemical laws alone but carries 

 the imprint of its biological origin. 



Another deduction which can be drawn from a thorough 

 study of the amino acid composition of present-day proteins 

 does not bear out the optimistic expectations, referred to 



