GENERAL TOPICS IN INHERITANCE. 79 



Weismann, on the other haud, is only less clear in expressing his hypoth- 

 esis. He accepts, of course, the idea of unit characters, each of which is rep- 

 resented in the germ cells by a "determinant." "We called," he says (1904, 

 I, p. 369), " determinants those parts of the germ-substance which determine 

 an ' hereditary character ' of the body ; that is, whose presence in the germ 

 determines that a particular part of the body, whether it consists of a group 

 of cells, a single cell, or a part of a cell, shall develop in a specific manner, 

 and whose variations cause the variations of these particular parts alone." 

 The " hereditary parts " may be small or " large regions, whole cell masses 

 of the body, which in all probability vary only en bloc, as, for instance, the 

 milliards of blood cells in man, the hundreds of thousands or millions of cells 

 in the liver and other glandular organs, the thousands of fibers in a muscle, 

 or of the sinews or fascia, the cells of a cartilage or a bone, and so on. In 

 all these cases a single determinant, or at least a few in the germ plasm, may 

 be enough." For Weismanu (1904, II, p. 151) the ultimate source of all 

 hereditary variations is the variation of the representatives of the unit char- 

 acters in the germ plasm. " If I mistake not," he says, " we may at least 

 say so much, that all variations are, in ultimate instance, quantitative and 

 that they depend on the increase or decrease of the vital particles, or their 

 constituents, the molecules What appears to us a qualitative varia- 

 tion is, in reality, nothing more than a greater or less different mingling of 

 the constituents which make up the higher unit ; an unequal increase or 

 decrease of these constituents, the lower units." The cell changes its consti- 

 tution when the proportion of its component parts " is disturbed, when, for 

 instance, the red pigment granules which were formerly present, but scarcely 

 visible, increase so that the cell looks red. If there had previously been no 

 red granules present, they might have arisen through the breaking up of 

 certain other particles of protoplasm, for instance, in the course of metab- 

 olism so that, among other substances, red granules of uric acid or some 

 other red stuff were pro.iuced. In this case, also, the qualitative change 

 would depend on an increase or decrease of certain simpler molecules and 

 atoms constituting the protoplasm-molecule." 



In criticism of the foregoing it may be said that a variation in the number 

 of ato:ns in a protoplasmic molecule is certainly also a qualitative change 

 a mutation. The only real difference between Weismann and de Vries 

 depends on the extent of the mutative modification, whether progressive or 

 complete from the beginning ; but this is a real difference, for the latter 

 view is required by the theory of immutable unit characters. The former 

 view is not in harmony with such a theory. Conversely, if it appears that 

 there are immutable unit characters, then the theory of evolution by saltation 

 is necessary ; if unit characters are modifiable, then species may have arisen 

 gradually. 



