II.] ON HEREDITY. 73 



in a certain sense these organisms possess immortality : they 

 can, it is true, be destroyed, but, if protected from a violent 

 death, they would live on indefinitely, and would only from 

 time to time reduce the size of their overgrown bodies by 

 division. Each individual of any such unicellular species living 

 on the earth to-day is far older than mankind, and is almost as 

 old as life itself. 



From these unicellular organisms we can to a certain extent 

 understand why the offspring, being in fact a part of its parents, 

 must therefore resemble the latter. The question as to why 

 the part should resemble the whole leads us to a new problem, 

 that of assimilation, which also awaits solution. It is, at any 

 rate, an undoubted fact that the organism possesses the power 

 of taking up certain foreign substances, viz. food, and of con- 

 verting them into the substance of its own body. 



Among these unicellular organisms, heredity depends upon 

 the continuity of the individual during the continual increase of 

 its body by means of assimilation. 



But how is it with the multicellular organisms which do not 

 reproduce by means of simple division, and in which the whole 

 body of the parent does not pass over into the offspring ? 



In such animals sexual reproduction is the chief means of 

 multiplication. In no case has it always been completely 

 wanting, and in the majority of cases it is the only kind of 

 reproduction. 



In these animals the power of reproduction is connected with 

 certain cells which, as germ-cells, may be contrasted with those 

 which form the rest of the body; for the former have a totally 

 different role to play; they are without significance for the life 

 of the individuaP, and yet they alone possess the power of 

 preserving the species. Each of them can, under certain con- 

 ditions, develope into a complete organism of the same species 

 as the parent, with every individual peculiarity of the latter 

 reproduced more or less completely. How can such hereditary 

 transmission of the characters of the parent take place ? how 

 can a single reproductive cell reproduce the whole body in all 

 its details ? 



Such a question could be easily answered if we were only 

 concerned with the continuity of the substance of the repro- 



^ That is for the preservation of its life. 



