220 FATHERLESS FROGS 



the qualities of the young are not a repetition of the 

 qualities of one parent, nor are they a mere mixture ot 

 the qualities of both parents (for contradictory qualities 

 cannot mix). They are a new grouping of qualities com- 

 prising some of the one parent and some of the other, 

 and hence a great opportunity for variation, for departure 

 from either parent's exact "make-up," is afforded, and for 

 the selection and survival of the new combination. It is, 

 it would seem, only in exceptional cases and for limited 

 periods that uni-sexual or fatherless reproduction can be 

 advantageous to a species of plant or animal. Such cases 

 are those in which abundant food, present for a limited 

 season, renders the most rapid multiplication of individuals 

 an advantage to the species. But after this exceptional 

 abundance has come to an end, the more usual process of 

 reproduction by fertilised eggs (also necessary and advan- 

 tageous for the preservation of the race by "natural selection 

 in the struggle for existence '' of the new varieties so 

 produced) is resumed until again the abundant food is 

 present, as in the annual history of plant lice and the 

 plants on which they feed. 



