154 Life and Death, Heredity and Evolution 



most room for acquiring valuable knowledge; but it is ex- 

 tremely difficult to carry out the necessary experiments. 

 The facts for Paramecium are as follows : 



[1^ In Paramecium, biparental inheritance occurs with 

 reference to size; after conjugation the two sets of progeny 

 are more alike in size than before (See Jennings and Lash- 

 ley, 1913a). 



(2) It occurs also with relation to rate of fission. Here 

 it is often very striking. If the descendants of one member 

 of a given pair multiply rapidly, the descendants of the other 

 member are likely to multiply rapidly also ; if one set multi- 

 ply slowly, so also as a rule does the other set. One or two 

 examples (from Jennings and Lashley, 1913) of this are 

 perhaps worth while. Calling the two members of a given 

 pair a and b, the numbers of fissions for four successive 

 periods of ten days each were for several pairs, all under the 

 same conditions : 



As you see, both members of pair 41 divide rapidly; of 

 pair 47 slowly ; of 9, at an intermediate rate ; while in pair 

 108 both members divide still more rapidly than in pair 

 41. This condition of affairs is typical. 



(3) There is a similar resemblance between the offspring 

 of the two members in respect to vigor and vitality. Some- 

 times the descendants of one pair are weak and the stock 

 gradually dies out. When this happens, the family derived 



