208 COOK 



the characters which a particular individual shall express as an 

 adult may be determined b}' external conditions constitutes 

 additional evidence that the expression of characters does not 

 depend upon the same factors as the transmission of characters. 

 Cases of politism occur also among plants. The vegetative 

 internodes, which correspond to the neuter individuals of the 

 insect colonies, may become unlike. Some of the branches 

 are then unable to replace the main stem, though in most plants 

 all the branches and internodes are potentially equal. 



Polar Inheritance in Crosses of Narrow-Bred Varieties. — 

 The general result of this class of experiments has been to prove 

 that narrow-bred varieties may assume the same or very similar 

 relations of polar inheritance as those regularly manifested by 

 the sexes and castes of dimorphic species. Indeed, the narrow 

 breeding which has restricted the descent of the parental groups 

 appears to intensify the polarity phenomena in such a way that 

 very definite mathematical relations are sometimes assumed. 



Equifolar Inheritance. — Equipolar inheritance is a method 

 of descent in which the diverse characters of the parents are 

 expressed in equal numbers of the conjugates and perjugates, 

 just as in sex-inheritance. This shows that the diversities are 

 also represented in equal numbers of the gamete parents and 

 the gamete offspring, but they are not united with the sexual 

 characters. There are many secondary sexual characters which 

 are to be distinguished from equipolar characters only by this 

 relation to sex-differentiation. 



Instances of equipolar descent like that of Professor Daven- 

 port's cross between the four-toed and five-toed chickens are of 

 special interest as connecting links between sex-inheritance and 

 other forms of polarized descent. Thus if Professor Davenport 

 were to mate five-toed cocks with four-toed hens for a series of 

 generations we may expect that the five-toed character would 

 become definitely attached to the other polarities of sex-inheri- 

 tance, just as the enlarged spur is now a part of the regular, 

 normal equipment of the male birds. 



Some of the mutations of the coffee tree are purely pistil- 

 late and some purely staminate. If a dimorphic type of coffee 

 were desirable it would probably not be difficult of development 

 by breeding from such a pair. 



