[II. ON- HEREDITY. 75 



which such separation does not take place until after the animal 

 is completely formed, and others, as I believe that I have 

 shown ^, in which it first arises one or more generations later, 

 viz. in the buds produced by the parent. Here also there is no 

 ground for the belief that the hereditary tendencies of the re- 

 productive molecules are in any way changed by the length of 

 time which elapses before their separation from the somatic 

 molecules. And this theoretical deduction is confirmed by 

 observation, for from the ^g% of a Medusa, produced by the 

 budding of a Polype, a Polype, in the first instance, and not 

 a Medusa arises. Here the molecules of the reproductive sub- 

 stance first formed part of the Polype, and later, part of the 

 Medusa bud, and, although they separated from the somatic 

 cells in the bud, they nevertheless always retain the tendency 

 to develope into a Polype. 



We thus find that the reproduction of multicellular organisms 

 is essentially similar to the corresponding process in uni- 

 cellular forms ; for it consists in the continual division of the 

 reproductive cell ; the only difference being that in the former 

 case the reproductive cell does not form the whole individual, 

 for the latter is composed of the millions of somatic cells by 

 which the reproductive cell is surrounded. The question, 

 ' How can a single reproductive cell contain the germ of a 

 complete and highly complex individual ? ' must therefore be 

 re-stated more precisely in the following form, ' How can the 

 substance of the reproductive cells potentially contain the 

 somatic substance with all its characteristic properties .? ' 



The problem which this question suggests, becomes clearer 

 when we employ it for the explanation of a definite instance, 

 such as the origin of multicellular from unicellular animals. 

 There can be no doubt that the former have originated from 

 the latter, and that the physiological principle upon which such 

 an origin depended, is the principle of division of labour. In 

 the course of the phyletic development of the organized world, 

 it must have happened that certain unicellular individuals did 

 not separate from one another immediately after division, but 

 lived together, at first as equivalent elements, each of which 

 retained all the animal functions, including that of reproduction. 



"^ Compare Weismann, ' Die Entstehung der Sexualzcllcn bci den 

 Hydromedusen,' Jena, 1883. 



