188 



HEREDITY AND SEX 



Showing the number of male- and female-producers in the progeny 

 of five sister individuals of Hydatina senta, one line being reared in 

 spring water, the others in various concentrations of the filtrate 

 from old food cultures. 



The extent of dilution of the medium is seen to be 

 directly in proportion to the number of sexual forms 

 that appear. If the solution be dried and the dry 

 substance added to ordinary water, the same end is 

 attained. 



It has not been possible to reverse the process and 

 produce more sexual forms than are produced under 

 ordinary conditions. This seems to mean that a change 

 may be effected in one direction and not in the other. 

 We cannot make a locomotive go faster than its mech- 

 anism permits, with the most favorable conditions of 

 fuel, oil, roadbed, and engineer; but if we put in stones 

 in place of coal, we can bring it to a standstill. 



ARTIFICIAL PARTHENOGENESIS 



We have now considered some of the most striking 

 examples of natural parthenogenesis in the animal 

 kingdom. The facts show that fertilization of the egg 

 is not in itself essential for development. The in- 



