A PHYSICO-CHEMICAL THEORY 143 



successive and progressive generations ; then we must admit 

 that the idioplasm of the highest forms, judging from its powers 

 of controlling and directing the development of the highly 

 complicated organism, is something very much more complex 

 than the idioplasm of the unicellular organisms ; that in the 

 course of evolution this has undergone successive accretions of 

 properties, and this accretion of properties is the manifestation 

 and accompaniment of increasing complexity of constitution 

 of that idioplasm. 



This idioplasm must be capable of modification, either by 

 its environment or under the action of some law of progressive 

 modification. The fact that there exist to-day forms of life 

 practically identical in the main details of structure with those 

 of remote geological ages is against the latter view ; we must 

 hold that environment determines changes in the constitution 

 of the idioplasm. 



Chemical Theories of Inheritance : the Isomeric Theory. — It 

 must now be asked : Can we imagine a chemical substance so 

 constituted as to be capable of modification in its molecular 

 constitution, and so, in sundry of its properties, without under- 

 going complete change, without other properties being lost ? 



We can. If, as Professor Walker of McGill points out to me, 

 such relatively simple bodies as lactic and malic acids are capable 

 of existing in more than one form — as laevogyrous and dextrogy- 

 rous and optically inactive modifications — and this with no obvious 

 alterations in their chemical properties (though physically they 

 show different actions upon polarized light, and physiologically 

 we find that certain bacteria can act upon one and not the other 

 form), then, certainly, the much more complex proteid material, 

 which would seem surely to be the basis of the idioplasm, may 

 present a very great number of molecular arrangements, and 

 each of these may be accompanied by slight differences in certain 

 physical and physiological properties. 



Certain recent observations have rendered this additionally 

 probable. By our present method of conceiving and visualizing 

 the structure of chemical substances, we regard the carbon 

 atom in the molecule as being situated at the centre of the 

 tetrahedron and as capable of linking four other atoms or groups 

 one to each of the corners or apices of the tetrahedron. A sub- 

 stance like malic acid having a single central carbon atom may, 



