144 ON VARIABILITY AND ADAPTATION 



therefore, be built up in three ways, with the linked groups 

 arranged in one relationship to each other, or in the reversed 

 relationship, or the substance may be a combination of the 

 two. These are all the forms we can picture as existing 

 according to this system. But now a fourth form of malic 

 acid has been found which is more dextrogyrous in the 

 polariscope than is ordinary dextrogyrous malic acid. And 

 the salts of this new form of malic acid show slight but distinct 

 variations, in solubility, etc., from the ordinary malates. If 

 this is so, there may yet be further molecular modifications of 

 such a relatively simple substance as malic acid, and a fortiori 

 idioplasm (with its multitudinous carbon atoms) may be capable 

 of an enormous number of modifications. 



The Side-Chain Theory of Inheritance. The mode of the 

 atomic arrangement in the idioplasmic molecule may therefore, 

 in part, explain the variation in the properties of that idioplasm 

 seen throughout the animal and vegetable kingdoms. I say 

 in part, for if we assume that the structure of the individual is 

 primarily the outcome of the structure and properties of the 

 idioplasm, then for each different form of living being, nay, for 

 each individual being, we have to assume a different molecular 

 structure of the idioplasm. Or, otherwise, we have to assume 

 that the modifications of this idioplasm are infinite in number. 

 This, it seems to me, asks too much. The matter cannot be 

 quite so simple. Each molecular modification may play some 

 part, it is true, but our conception' of the structure and modifica- 

 tion undergone by the idioplasm must be more elaborate. We 

 must, I think, formulate a theory of structure somewhat akin 

 to that laid down by Ehrlich x in his now well-known theory 

 of the nature of immunity. We can picture to ourselves the 

 primitive idioplasm as composed of a mass of material each 

 molecule of which is formed of a central ring, to which there 

 can be attached side-chains, and from which sundry side-chains 

 can be detached without the central ring being destroyed. 



This conception, which upon first encounter appears revolu- 

 tionary and opposed to our ordinary chemical ideas, is, after a 

 little deliberation, recognized as being but a statement, in 

 chemical terminology, of what has been for long years the 

 accepted physiological conception of the nature of protoplasm. 



1 Vide Plimmer, Journal of Pathology, v., 1898, p. 489. 



