"j^ ON HEREDITY. [II. 



The Maf^ospJiaera planula of Hackel proves that such perfectly 

 homogeneous cell-colonies exist\ even at the present day. 

 Division of labour would produce a difterentiation of the 

 single cells in such a colony : thus certain cells would be set 

 apart for obtaining food and for locomotion, while certain other 

 cells would be exclusively reproductive. In this way colonies 

 consisting of somatic and of reproductive cells must have arisen, 

 and among these for 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 nisus forma- 

 tivus, or adapting the name to modern times, to a ph^detic force 

 of development which causes the organism to change from time 

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

 ganism 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 influenced 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 



* It is doubtful whether Magosphaera should be looked upon as a ma- 

 ture form ; but nothing hinders us from believing that species have lived, 

 and are still living, in which the ciliated sphere has held together until 

 the encystmcnt, that is the reproduction, of the constituent single cells. 



