THE GERM-PLASM THEORY 397 



correct to a certain degree, but may also be erroneous ; no formal 

 proof of it can be obtained at present; and I am content if it be 

 simply admitted to be possible. On the other hand, the existence of 

 determinants seems to me to be, in the sense indicated, indubitable 

 and demonstrable. 



Let us return for a moment to the claws and adhesive lobes which 

 are developed on the foot of the fly. It may perhaps be thought 

 that it is possible to do without the assumption of determinants for 

 these parts, by assuming that although ' external ' influences in the 

 ordinary sense could not possibly have determined that certain cells of 

 the apex of the leg should form claws and others adhesive lobes, the 

 result might be due to the differences of intercellular pressure within the 

 apical knob ; these may have been stronger in one direction, weaker in 

 another, thus prompting the cells to grow here into claws and there 

 into adhesive lobes. If we had merely to explain from the constitution 

 of the germ-plasm the ontogeny or development of these parts in an 

 individual fly there might perhaps be no radical objection to this view, 

 though it would hardly be possible to explain the assumed difierences 

 in pressure otherwise than as due to a diff'erent intensity of growth in 

 the cells in the various regions of the limb-apex, which again would 

 have to be referred to di0"erences in the germ-plasm. But when we 

 reflect that these parts vary hereditarily and independently of other 

 parts, and owe their present form to their power of doing so, and that 

 they are diflerently formed in every genus and species, we see at once 

 that they must be represented in the germ-plasm by particular vital 

 particles, which are the roots of their transmissible variability, that 

 is, which must have previously undergone a corresponding variation 

 if the relevant parts themselves are to vary. Without previous vari- 

 ation of the determinants of the germ no transmissible independent 

 deviation on the part of the claws or adhesive lobes of the animal 

 is conceivable. 



All the opponents of my theory have overlooked this fact ; both 

 Oscar Hertwig and Kassowitz have forgotten that a theory of de- 

 velopment is not a theory of heredity; they only aim at the former, 

 and they therefore dispute the logical necessity for an assumption 

 of determinants. 



But as this is the very foundation of the theory, let me further 

 submit the following considerations in its favour. 



In insects which undergo metamorphosis, not only the external 

 but the internal parts of the caterpillar or larva go through a more 

 or less complete transformation. In the flies (Muscid^), for mstance, 

 the whole intestinal tract of the larva is reconstructed m the pupa; 



