346 cook 



There also appears to be a complete series of stages of accentua- 

 tion of this polarity of descent. The most primitive condition 

 is that of indiscriminate or unspecialized heterism, in which a 

 character shows all degrees of expression from the lowest 

 minimum to the highest maximum, with a preponderance at 

 some intermediate or optimum point. 



The physiological advantages of diversity of descent not only 

 prevent the species from concentrating or stagnating on a cen- 

 tral average or optimum point, but they often favor the develop- 

 ment of two optima. The connecting series of character-stages 

 may weaken, or it may entirely disappear, except for rare 

 abnormalities, the normal form of the species being represented 

 by the two separated extremes. The typical and most familiar 

 instances of specialized heterism is to be found, of course, in 

 the phenomena of sex. The primary sexual characters are 

 now so intricately involved with the functions of reproduction 

 that their significance as specializations of heterism is much 

 obscured, but large numbers of secondary sexual characters 

 are quite functionless for any purpose thus far detected, except 

 this of increasing the diversity of descent inside the species. 



When once a species has reached the stage of sex-differen- 

 tiation, and has thus established a polarity of descent, the ten- 

 dency seems to be for other specializations of heterism to group 

 themselves with sex. The result is to give each generation 

 the benefit of full diversity of descent, instead of losing this 

 advantage in cases where similar individuals might breed 

 together. No doubt it is easier, too, for a new character to 

 join with and accentuate an already established polarity than to 

 establish a new one for itself. Even among the plants which 

 have not attained differentiation into separate sexes there are 

 definitely alternative characters, and sometimes there are not 

 merely two alternatives, or two groups, but several, and in a 

 variety of combinations, as in the genus Lythrum. In insects 

 the phenomena of alternative descent reach their highest ac- 

 centuation and complexity, for there they are superposed upon 

 the sex-differentiation. There may be two distinct forms of 

 one of the sexes, as among the bees. In some species of 

 termites both sexes are capable of specialization in several dif- 



