substance from which they spring is thus clearly stated by Mr. Herbert Spencer, in his 'Principles of Biology:' 'The doctrine that all organisms are built up of cells, or that cells are the elements out of which every tissue is developed, is but approximately true. There are living forms of which cellular structure cannot be asserted; and in living forms that are, for the most part, cellular, there are, nevertheless, certain portions which are not produced by the metamorphosis of cells. Supposing that they were the only material available for building, the proposition that all houses are built of bricks would have about the same relation to the truth as does the proposition that all organisms are composed of cells. This generalization respecting houses would be open to two criticisms: first, that certain houses, of a primitive kind, are formed, not out of bricks, but out of unmoulded clay; and second, that, though other houses consist mainly of bricks, yet their chimneypots drain-pipes, and ridge-tiles do not result from combinations or metamorphosis of bricks, but are made directly of the original clay; and of like natures are the criticisms which must be passed on the generalization that cells are the morphological (structural) units of organisms. To continue the simile, the truth turns out to be that the primitive clay or protoplasm out of which organisms are built may be moulded directly, or with various degrees of indirectness, into organic structures.'
"Protoplasm consists of the four chemical elements, carbon, oxygen, hydrogen, and nitrogen, which also compose the bulk of the entire organic world. These elements are united in very complex union, the nature of which has never been determined with exactness. It is albumenoid in aspect, that is, like white of egg. A few years ago, the term protein was applied to a combination of these four elements, which was supposed to be the common basis of all albumenoid substances; but no such principle has ever been separated or proved to exist. The term, however, is still retained, though with what vagueness may be inferred from the statement of Professor Frankland, that so-called protein has probably more than a thousand isometric forms.
"Professor Huxley aims to show that, as between protoplasm and all the developed forms of life, there is an acknowledged unity of composition, so there is also a unity of power and form.
"First, as regards unity of powers, by what property is it
