IV.l FOUNDATION OF A THEORY OF HEREDITY. 203 



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 occurrence of re-transformation. It has been recently 

 maintained by Minot ^ that all development is cyclical, but this 

 is obviously incorrect, for Nageli has already shown that direct 

 non-cyclical courses of development exist, or at all events 

 courses in v^hich the earliest condition is not repeated at the 

 close of development. The phyletic development of the w^hole 

 organic w^orld clearly illustrates 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 de- 

 velopment 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, Monotre- 

 mata, mammal-like reptiles, and the lower vertebrate forms, 

 into worms and finally into Monera. But how can a course of 

 development, which seems to be impossible in phylogeny, 

 occur as the regular method of ontogeny ? And quite apart 

 from the question of possibility, we have to ask for proofs of 

 the actual occurrence of cyclical development. Such a proof 

 would be afforded if it could be shown that the nucleoplasm of 

 those somatic cells which (e. g. in Hydroids) are transformed 

 into germ-cells passes backwards through many stages of de- 

 velopment into the nucleoplasm of the germ-cell. It is true 

 that we can only recognize differences in the structure of the 

 idioplasm by its effects upon the cell-body, but no effects are 

 produced which indicate that such backward development takes 

 place. Since the course of onward development is compelled 

 to pass through the numerous stages which are implied in seg- 

 mentation and the subsequent building-up of the embryo, etc., 

 it is quite impossible to assume that backward development 

 would take place suddenly. It would be at least necessary to 

 suppose that the cells of embryonic character, which are said 

 to be transformed into primitive germ-cells, must pass back 



1 • Science/ vol. iv. No. 90, 1884. 



