82 THE GERM-PLASM 



to the end of the ontogeny. At the same time, the determinants 

 in the germ-plasm increase in number, and each of them be- 

 comes differentiated in a fresh manner. As, however, every two 

 new determinants always follow the same course from the id 

 of germ-plasm to the final stage in ontogeny as was taken by 

 the single original determinant, they will pass through the same 

 determinant figures as before, and only lead to the formation of 

 new structures in the final stages, when they become separated 

 from one another. 



The ontogenetic stages of the ancestors will be repeated less 

 accurately the nearer development approaches its termination. 



The disappeat^ance of a character or of a part which has 

 become useless, may also be traced to the mechanism of the 

 idioplasm. The group of determinants which gives rise to a 

 particular character, will have to be removed entirely from the 

 germ-plasm if the corresponding part is to disappear com- 

 pletely. But this is a very complicated process, and one of long 

 duration as regards more complex organs, such as, for instance, 

 the limbs of Vertebrates. For the determinants which take part 

 in the formation of an extremity are very numerous, and of many 

 different kinds ; and moreover, they cause the mdiment of the 

 limb to appear very early in ontogeny. Hence the determinants 

 will have to suffer successively many retrogressive and simplify- 

 ing changes before a noticeable reduction of the organ occurs. 

 The functionless and rudimentary wings of the Australian Kiwi 

 {Apteryx), which are concealed by the plumage, possess all 

 the bones of the perfect wing, though these are very much re- 

 duced in size. This is to be explained by supposing that the 

 entire group of determinants for the wing still remain in the id 

 of germ-plasm, but that it has decreased in strength, — that is to 

 say, its elements no longer increase so rapidly, — and they there- 

 fore can only control smaller groups of cells. If the process of 

 degeneration continued, the organ would not only grow smaller 

 and smaller, but its component parts would also disappear at 

 different rates, and, losing their characteristic form, would 

 appear as indistinguishable rudiments. Such a degeneration 

 has occurred in certain species of whales, in which the rudi- 

 ments of the posterior extremity lie concealed beneath the skin ; 

 while in other species, the form of the separate bones has been 

 to some extent preserved, and those of the thigh and shank can 

 still be plainly distinguished. In these cases, many of the 



