200 



HEREDITY AND SEX 



does not show whether this is due to each strain having 

 degenerated in certain directions, or to some other 

 kind of a change in the heredity complex. 



The egg counts show that in the inbred stock many 

 of the eggs are not fertiHzed, or if fertihzed (32%) 

 they still fail to develop. This means a decrease 

 in fertility in the sense in which that word is here 

 used. The offspring that arise from the cross-fer- 

 tilization of these strains are more vigorous than their 

 parents, if their increased fertility be taken as the 

 measure of their vigor. The latter result is not shown 

 in the table, for here 52% and 58% are the percent- 

 ages of fertile eggs produced when the two strains are 

 crossed. 



M\sTovy oF lnt>recl Stock. 



Fl a 3 4 5 6 7 



368 209 i9f m es m - 



8 9 10 11 iZ R5 



- - - - - 156 



Cro66 of fia i^^-utxc^tc 



52% 58% 



Fig. 96. — The horizontal line Fi-Fn gives the average number of flies 

 per pair that emerged from inbred stock, decreasing from 368 to 156 per pair. 

 Below is shown the results of a cross between a race of Truncates (short 

 wings) and F13. The percentages here give the number of eggs that hatched 

 in each case. 



Darwin found that cross-fertilization was bene- 

 ficial in 57 species of plants that he studied. In the 



