372 EXPERIMENTAL STUDY OF INHERITANCE 



applies only to the former, though Galton sought to make it 

 include both. 



More generally it may be said that there should not be any 

 opposition between Mendelian and biometric formulae, for that 

 is a confusion of thought. Biometric formulae are applicable to 

 averages of successive generations breeding freely ; Mendelian 

 formulae are applicable to particular sets of cases where parents 

 with contrasted dominant and recessive characters are crossed 

 and their hybrid offspring are inbred. We may refer to the 

 admirable essay by Darbishire (1906). 



Summary of Mendelism. The Mendelian theory implies three 

 main ideas. 



(1) The inheritance consists, in part at least, of " unit-3haracters " 

 which are typically continued as a whole or not at all, which behave 

 as if they were discrete units which can be shuffled about and dis- 

 tributed to the offspring to some degree independently of each other. 

 These '' unit-characters " are believed to be represented in the 

 germinal material, and probably in the chromosomes, by differential 

 features of some sort the so-called factors, determiners, or genes. 

 It may be, however, that several factors may be involved in one 

 character, or that one factor may influence more than one character. 



(2) When two parents differ in respect to two contrasted unit- 

 characters these do not blend in the offspring, but one of them 

 appears, more or less in its entirety, and is called dominant, while 

 its analogue, that drops more or less out of sight for the time being 

 in the offspring, is called recessive. Or the presence of a character 

 may be dominant to its absence, and conversely. It must be care- 

 fully noticed, however, that there are numerous instances of what 

 is called incomplete dominance, as when the crossing of a black 

 and a white Andalusian fowl yields blue Andalusians. Moreover, 

 different pairs of factors may interact, and there are many com- 

 plications now known which explain how certain distributions of 

 qualities which seem non-Mendelian at first sight, may yet come 

 under that interpretation. 



(3) The third idea is that of segregation, that in the history of 

 the germ-cells of Mendelian crosses (the first filial generation) , there 

 is a segregation of the factors of, say, two contrasted unit-characters, 

 or of the factors of a unit-character possessed by only one of the 

 parents, the segregation being such that each germ-cell, whether 

 ovum or spermatozoon, is " pure " as regards the character in 

 question, either having or not having the corresponding factor. 



