I.— PHYSIOLOGY. 200 



of it, certainly, it is hazardous to try to trace them. Tho attempt so 

 commonly made to trace one between the growth of living matter and the 

 growth of crystals in a saturated solution, it is safe to say, is in so many 

 respects on the wrong lines that it is merely misleading. Crystals are 

 not alive. The molecules that constitute the crystal are set in solid forma- 

 tion ; so long as the crystal exists they are stable and unchangeable. 

 These molecules collect on the growing crystal, but they exist ready-made 

 in the surrounding solution ; they do not come into being by the influence 

 of the crystal ; tliey are themselves so constituted as to take up a set 

 position in relation to each other and to those already ranged side by side 

 in the crystal, as soldiers on the drilling-ground at the word ' fall in ' ; they 

 are available because the solution is kept saturated by the dissolving of 

 smaller but similar crystals that for physical reasons are more soluble in 

 the solution than the larger ones. In contradistinction to this, the mole- 

 cules that enter into the composition of living matter exhibit the phe- 

 nomena of life only when permeated with water molecules exercising the 

 kinetic activity of the liquid state ; they are unstable and perishable ; 

 the added molecules, some of which even during growth and all of them 

 at other times, serve but to replace those that perish, do not exist ready- 

 made ; they come into being only in conformity to the pattern and under 

 the influence of those already in existence, a pattern that these alone can 

 use ; and they are formed out of material that is chemically different from 

 them. 



Let us for a moment consider what this spontaneous regeneration 

 implies. Of the various chemical components of protoplasm, proteins 

 are generally considered the most important, often the only important, 

 ones. The elucidation of the chemical principles xipon which the structure 

 of proteins rests, which took place about the beginning of this century, was, 

 like the neurone hypothesis of the structure of the nervous system, an 

 advance the magnitude of which only those perhaps can appreciate who 

 began the study of physiology well back in an earlier one. For a time it 

 seemed in each case that the problem was solved and all that was to follow 

 was simple. Those were great days. The best-known varieties of proteins, 

 when detached and uprooted from the place where they grew, consist of 

 chains of about a hundred, sometimes nearly two hundred, links. Each 

 link is an amino acid coupled by its acid group to the amino group of one 

 neighbour and by its amino group to the acid group of its other neighbour, 

 a molecule of water being lost at each linkage. There are not more than 

 about twenty different amino acids, so that some of them must occur 

 several times in the chain ; in some kinds of protein one amino acid may 

 occu]:)y thirty or forty of the hundred places in the chain. In any such 

 isolated protein it is probable that the order as well as the proportion in 

 which each amino acid occurs in the molecule is fixed, and it is this specific 

 order and proportion that accounts for the specific character and properties 

 of the protein. What could be simpler ? And only yesterday all was so 

 obscure. 



It is not recorded that in the rush of this advance anyone stopped to 



reflect what number of formations such a protein might still possibly 



have. Supposing it were a chain of only fifty links, a very simple case ; 



if all the links were different the number of possible permutations is 



1926 P 



