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50 THE HARVEIAN ORATION, 1903

May it not also be in this same molecular structure of living matter that will be found the explanation of those phenomena of development and of organic evolution by which the fertilized ovum of two different kinds placed under identical conditions will each attain “ to such form and structure as best fit it for its place in nature — processes which cannot be measured or ob-

varieties in protoplasm, and that it is far from being of a uniform character in all cases as was formerly supposed, and this even amongst the simplest of unicellular organisms. The differences in behaviour exhibited by different species in response to various reagents clearly demonstrate this. For whilst some are so profoundly affected in their molecular constitution as to succumb on being subjected to certain poisons, others, in no wise differing so far as can be determined by the means at present at our disposal, are quite uninjured. Some kinds of bioplasm appear to have a general high resistance to all chemical agents, while others have a high or low resistance to particular agents only; thus nervous tissue for instance is readily and injuriously affected by substances (e.g. cocaine or nicotine) to which many protophyta are indifferent. Since also many toxic bodies which produce no effect upon dead albumen are yet violently poisonous to living protoplasm, it would seem probable that the latter contains in its construction certain unstable groups of molecules which undergo replacement by others from toxic agents. All this goes to show that protoplasm is extremely complex and consists of numerous kinds of compounds, many of which are very unstable. Also that not all protoplasm contains the same compounds but that these are dissimilar in different organisms. And further that not all of the compounds in any protoplasmic body are essential to life, and that we may so act on a protoplasmic body by a weak reagent and gradually change its composition, so that it will no longer be killed by a strong solution of the same reagent, thus effecting an acclimatization, or as we should say rendering the organism immune. (See Experimental Morphology by Dr. Davenport, 1897).