76 ON HEREDITY. 



the first time death appeared. For in each case the somatic cells 

 must have perished after a certain time, while the reproductive 

 cells alone retained the immortality inherited from the Protozoa. 

 We must now ask how it becomes possible that one kind of cell 

 in such a colony can produce the other kind by division ? Before 

 the differentiation of the colony each cell always produced others 

 similar to itself. How can the cells, after the nature of one part 

 of the colony is changed, have undergone such changes in their 

 nature that they can now produce more than one kind of cell ? 



Two theories can be brought forward to solve this problem. We 

 may turn to the old and long since abandoned ni-sus formativus, 

 or adapting the name to modern times, to a phyletic force of 

 development which causes the organism to change from time to 

 time. This vis a tergo or teleological force compels the organism to 

 undergo new transformations without any reference to the external 

 conditions of life. This theory throws no light upon the numerous 

 adaptations which are met with in every organism ; and it possesses 

 no value as a scientific explanation. 



Another supposition is that the primary reproductive cells are 

 i nn<uencec l by the secondary cells of the colony, which, by their 

 adaptability to the external conditions of life, have become somatic 

 cells : that the latter give off minute particles which entering into 

 the former, cause such changes in their nature that at the next 

 succeeding cell-division they are compelled to break up into dissimilar 

 parts. 



At first sight this hypothesis seems to be quite reasonable. It is 

 not only conceivable that particles might proceed from the somatic 

 to the reproductive cells, but the very nutrition of the latter at the 

 lexpense of the former is a demonstration that such a passage 

 Actually takes place. But a closer examination reveals immense 

 difficulties. In the first place, the molecules of the body devoured 

 are never simply added to those of the feeding individual without 

 undergoing any change, but as far as we know, they are really as- 

 similated J , that is, converted into the molecules of the latter. We 

 cannot therefore gain much by assuming that a number of mole- 

 cules can pass from the growing somatic cells into the growing 

 reproductive cells, and can be deposited unchanged in the latter, so 



1 Or is an exception perhaps afforded by the nutritive cells of the egg, which 

 occur in many animals ? 



