622 H. S. ^Jennings 



of the vessel, where the conditions were not such as to keep them 

 alive, while the free individuals remained at the top and multi- 

 plied. Thus by continued natural selection all specimens mul- 

 tiplying by incomplete fission were removed, and in a few days 

 the deep culture contained only normal, free individuals. In 

 shallow cultures, on the other hand, the united groups persisted 

 for about two months, as we have seen. 



In this case then we have a new characteristic, of known origin, 

 that is inherited by many individuals for many generations, and is 

 finally extinguished only by the action of natural selection. The 

 many other new characteristics that we have described were not 

 inherited (save as they were handed on directly to a single speci- 

 men). In the one case the new feature becomes a race charac- 

 teristic; in all the others it fails to do so. 



WHAT MUST BE THE NATURE OF A NEW CHARACTER, THAT IT 



MAY BE INHERITED .^ 



What is the peculiarity of the characteristic that was thus mul- 

 tiplied and inherited, and what light does it throw on the question 

 as to what must be the nature of an acquired characteristic in order 

 that it may be inherited .^ 



The characteristic thus inherited was a modifLcation of the pro- 

 toplasm of the cell, of such a character as to cause it to behave differ- 

 ently in reproduction. The other characteristics, not inherited, 

 were not such modifications of the protoplasm as to cause it to 

 behave differently in reproduction. 



Consideration of the facts of normal reproduction in the Pro- 

 tozoa, and of heredity in general, indicates that this difi^erence is an 

 essential one. In order that it may be inherited (by more than one 

 of the progeny), a characteristic must be the result of such a modi- 

 fication of the mother cell as will cause it to behave in a certain way 

 at reproduction. It makes no difference whether the mother cell 

 in question is a germ cell, in a Metazoan, or a differentiated Pro- 

 tozoan. 



Thus we know that in the inheritance of the setae of the Hypo- 

 tricha, for example (Fig. 21), these are not simply handed over in 



