FOUNDATION OF A THEORY OF HEREDITY. 199 



fact. But there are not many cases in which the sexual cells are 

 separated so early as the third generation : and it is very rare for 

 them to separate at any time during- the true segmentation of the 

 egg. In Dapknidae (Moina) separation occurs in the fifth stage of 

 segmentation 1 , and although this is unusually early it does not 

 happen until the idioplasm has changed its molecular structure six 

 times. In Sagitta 2 the separation does not take place until the 

 archenteron is being formed, and this is after several hundred 

 embryonic cells have been produced, and thus after the germ- 

 plasm has changed its molecular structure ten or more times. But 

 in most cases, separation takes place at a much later stage ; thus in 

 Hydroids it does not happen until after hundreds or thousands of 

 cell-generations have been passed through ; and the same fact holds 

 in the higher plants, where the production of germ-cells frequently 

 occurs at the end of ontogeny. In such cases the probability of a 

 re-transformation of somatic idioplasm into germ-plasm becomes 

 infinitely small. 



It is true that these considerations only refer to a rapid and 

 sudden re-transformation of the idioplasm. If it could be proved 

 that development is not merely in appearance but in reality a 

 cyclical process, then nothing could be urged against the occur- 

 rence of re-transformation. It has been recently maintained by 

 Minot 3 that all development is cyclical, but this is obviously 

 incorrect, for Na'geli has already shown that direct non-cyclical 

 courses of development exist, or at all events courses in which the 

 earliest condition is not repeated at the close of development. The 

 phyletic development of the whole organic world clearly illus- 

 trates a development of the latter kind ; for although we may 

 assume that organic development is not nearly concluded, it is 

 nevertheless safe to predict that it will never revert to its original 

 starting-point, by backward development over the same course 

 as that which it has already traversed. No one can believe that 

 existing Phanerogams will ever, in the future history of the world, 

 retrace all the stages of phyletic development in precise inverse 

 order, and thus return to the form of unicellular Algae or Monera ; 

 or that existing placental mammals will develope into Marsupialia, 



1 Grobben, ' Arbeiten d. Wien. Zool. Instituts,' Bd. II. p. 203. 



2 Butschli, ' Zeitschrift f. wiss. Zbol.' Bd. XXIII. p. 409. 



3 ' Science,' vol. iv. No. 90, 1884. 



