436 



know of them ; I do not feel that I am entangled in the alleged 

 difficulty. My reply might end here ; but as the hypothesis in ques- 

 tion is one not easily conceived, and very apt to be misunderstood, 

 I will attempt a further elucidation of it. 



Much evidence now conspires to show that molecules of the sub- 

 stances we call elementary are in reality compound ; and that, by 

 the combination of these with one another, and re-combinations of 

 the products, there are formed systems of systems of molecules, un- 

 imaginable in their complexity. Step by step as the aggregate 

 molecules so resulting, grow larger and increase in heterogeneity, 

 they become more unstable, more readily transformable by small 

 forces, more capable of assuming various characters. Those com- 

 posing organic matter transcend all others in size and intricacy of 

 structure ; and in them these resulting traits reach their extreme. 

 As implied by its name protein, the essential substance of which 

 organisms are built, is remarkable alike for the variety of its meta- 

 morphoses and the facility with which it undergoes them : it changes 

 from one to another of its thousand isomeric forms on the slightest 

 change of conditions. Now there are facts warranting the belief 

 that though these multitudinous isomeric forms of protein will not 

 unite directly with one another, yet they admit of being linked to- 

 gether by Other elements with which they combine. And it is 

 very significant that there are habitually present two other elements, 

 sulphur and phosphorus, which have quite special powers of holding 

 together many equivalents the one being petitatomic and the other 

 hexatomic. So that it is a legituiate supposition (justified by analo- 

 gies) that an atom of sulphur may be a bond of union among half- 

 a-dozen different isomeric forms of protein; and similarly with phos- 

 phorus. A moment's thought will show that, setting out with the 

 thousand isomeric forms of protein, this makes possible a number of 

 these combinations almost passing the power of figures to express. 

 Molecules so produced, perhaps exceeding in size and complexity 

 those of protein as those of protein exceed those of inorganic matter, 

 may, I conceive, be the special units belonging to special kinds of 

 organisms. By their constitution they must have a plasticity, or 

 sensitiveness to modifying forces, far beyond that of protein ; and 

 bearing in mind not only that their varieties are practically infinite 

 in number, but that closely allied forms of them, chemically in- 

 different to one another as they must be, may coexist in the same 

 aggregate, we shall see that they are fitted for entering into un- 

 limited varieties of organic structures. 



The existence of such physiological units, peculiar to each species 

 of organism, is not unaccounted for. They are evolved simul- 

 taneously with the evolution of the organisms they compose they 

 differentiate as fast as these organisms differentiate; and are made 

 multitudinous in kind by the same actions which make the organism 

 they compose multitudinous in kind. This conception is clearly 



