284 NATURAL SCIENCE. Oct.. 



addition of one or more cell-generations to the end of the ontogeny. 

 At the same time, the determinants in the germ-plasm increase in 

 number, and each of them becomes 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 will be repeated less accurately 

 the nearer development approaches its termination." 



Now, for my part, I cannot see that this explanation in any way 

 covers the phenomena. According to the theory, the specialisation of 

 every cell and every organ at every stage depends on determinants. 

 The problem to be explained is, therefore, why the determinants 

 for the organs of the early stages of ontogeny are not modified. 

 Weismann supposes that in the differentiated cells of any stage 

 determinants are used up, while in the undifferentiated multiplying 

 cells unconsumed determinants are held in reserve to supply later 

 generations of differentiated cells. The determinant figure is merely 

 an attempt to show why the consumption of the determinants follows 

 a definite course. It does not afford any reason why the determinants 

 which are reserved to the last should be modified, while those which 

 pass into differentiated cells belonging to early stages remain 

 unaltered. 



As to the cause of changes in the determinant, Weismann now 

 attributes them to the direct effect of external influences, which 

 influence the nutrition of the biophors and determinants during their 

 continual reproduction through countless generations ; but in his 

 chapter on variation he mentions no reason why such changes should 

 occur rather in the determinants belonging to the final stages of 

 ontogeny, than in those belonging to the earlier stages. 



It may be truly said, therefore, that we have no theory of variation 

 which necessarily involves the recapitulation of phylogeny, except the 

 theory of the inheritance of acquired characters. It is important to 

 notice that recapitulations occur chiefly in connection with adaptive 

 modifications. The young flat-fish swims -with its principal plane in 

 a vertical position, and its body is symmetrical about that plane ; the 

 adult flat-fish, on the contrary, is adapted by its asymmetry to living 

 in a position in which the principal plane of the body is horizontal. 

 The tadpole is essentially a fish adapted to live in water, while the 

 frog is adapted to breathe air and lead a terrestrial existence. These 

 obvious and striking facts point, at all events apparently, to the 

 conclusion that the change of structure was produced by the change 

 of conditions. 



Another consideration which points in the same direction is that 

 the actual ontogeny of these changes of structure in the individual 

 has been shown in some instances to be largely dependant on 



