246 REPRODUCTION 



whose colors are due to pigment in solution in the cell sap. There 

 are also stocks with cream-colored flowers due to yellowish 

 plastids; and there are white-flowered stocks with colorless sap 

 and plastids. All of these are smooth. When the red and 

 purple varieties are intercrossed the offspring are always smooth, 

 but when the cream or white varieties are crossed with the reds 

 or purples the offspring are hairy, and when the cream and 

 white varieties are crossed the offspring are both purple and 

 hairy. It might be deduced from these facts that both the 

 cream and white varieties possess the factor for hairiness, and 

 that the purple and red varieties do not contain it. It also ap- 

 pears that the factor for hairiness cannot do its work without 

 the cooperation in some way of the factor for red or purple sap. 

 Furthermore, it appears that the cream and the white varieties 

 possess factors which acting together can produce purple, while 

 separated they are ineffective. Hairiness and purple color are 

 both reversions to an old parental form. Loss of purple color 

 and loss of hairs seems to have been caused by segregation of 

 characters which only by cooperation could produce them. 

 When by crossing these characters were recombined the lost 

 parental characteristics reappeared. 



Significance of Sexuality. — In whatever manner and through 

 whatever influences sexuality may have arisen, it seems clear 

 that any device so wasteful in the countless pollen grains and 

 ovules that come to nought must in the long run bring some com- 

 pensating good to the race; and evidence of this has not been 

 wanting. In his "Animals and Plants Under Domestication" 

 Darwin gives a convincing array of evidence to show that cross- 

 fertilization as compared with self-fertilization results in very 

 marked increase in vigor. There are, as is well known, many 

 exceptions to this rule; but it has, nevertheless, such extensive 

 application as to warrant the statement that sex-differentiation 

 has in some way been the cause of a marked stimulation of vari- 

 ous vital functions. 



A very striking illustration of this we find in the behavior of 

 Indian corn under self- and cross-fertilization. Shull has iso- 



