But the second part of Nägeli’s theory of the idioplasm is also untenable. It is impossible that this substance can have the same constitution everywhere in the organism and during every stage of its ontogeny. If this were so, how could the idioplasm effect the great differences which obtain in the formation of the various parts of the organism? In some passages of his work Nägeli seems to express the same opinion; e. g. on page 31 he says, ‘It would be practicable to regard—although only in a metaphorical sense—the idioplasms of the different cells of an individual as themselves different, inasmuch as they possess specific powers of production: we should thus include among these idioplasms all the conditions of the organism which bring about the display of specific activity on the part of cells.’ It can be clearly seen from the passages immediately preceding and succeeding the above-quoted sentence, that Nägeli, in speaking of these changes in the idioplasm, does not refer to material, but only to dynamical changes. On page 53 he lays special stress upon the statement that ‘the idioplasm during its growth retains its specific constitution everywhere throughout the organism,’ and it is only ‘within these fixed structural limits that it changes its conditions of tension and movement, and thus alters the forms of growth and activity which are possible at each time and place.’ Against such an interpretation weighty objections can be raised. At present I will only mention that the meaning of the phrase ‘conditions of tension and movement’ ought to be made clear, and that we ought to be informed how it is that mere differences in tension can produce as many different effects as could have been produced by differences of constitution. If any one were to assert that in _Daphnidae_, or in any other forms which produce two kinds of eggs, the power of developing only after a period of rest, possessed by the winter-eggs, is based upon the fact that their idioplasm is identical with that of the summer-eggs, but is in another condition of tension, I should think such a hypothesis would be well worth consideration, for the animals which arise from the winter-eggs are identical with those produced in summer: the idioplasm which caused their formation must therefore be identical in its constitution; and can only differ in the two cases, as water differs from ice. But the case is quite otherwise in the stages of ontogeny. How many different conditions of tension ought to be possessed by one and the same idioplasm in order to correspond to the thousand different structures and differentiations of cells in one of the higher organisms? In fact it would be hardly possible to form even an approximate conception of an explanation based upon mere ‘conditions of tensions and movement.’ But, furthermore, difference in effect should correspond, at any rate to some extent, with difference in cause: thus the idioplasm of a