230 ESSENTIALS OF BIOLOGY 



on. If there are the 3 pairs of allelomorphic characters, a and h, 

 c and d, e and / we can have these combinations, 



ace, acf, ade, adf, bee, bcf, bdc, bdf (or 2^ in all), 



and if there were n pairs of characters we might conceivably 

 have 2^* combinations. Thus we see that, so far as we have gone 

 the notion of randomness enters into MendeUan theory, that is, 

 the above combinations, ace, acf, ade, etc., are the possible 

 assortments when w^e take one out of each pair of characters and 

 associate them at random. But the randomness may be restricted. 

 Thus some men see badly in the dark in conditions when normal 

 men see relatively well, and this is called night-blindness. When 

 a normal woman has children by a night-blind man all these 

 children (boys and girls) have normal sight, but if the girls grow 

 up and have children by normal men some of their sons may be 

 night-blind. Women are said not to be night-blind, but they 

 " carry " the character night-blindness, so that the latter is said 

 to be linked with sex (the " Principle of Linkage "). 



This very slight summary of Mendelism notes the aspects of 

 the subject that have general interest. There is, of course, very 

 much more, but much of it is perplexing, is loaded with detail 

 that is irrelevant from our point of view, is contradictory and 

 may (without loss) be neglected. The " principles " must all 

 be qualified by subsidiary principles : thus there is " imperfect 

 dominance," imperfect segregation (as in sex-linked characters), 

 multiple allelomorphism, etc. Thus maleness and femaleness 

 seem to be allelomorphic characters, but there may be herma- 

 phroditism in animals in which the sexes are usually separate 

 and there may be " intersexes," when the male shows some of 

 the morphological characters of the female, and vice versa, and 

 animals (hens) that are female when young may show male 

 characters (hens will crow) when they become old. Such re- 

 finements, and subsidiary hypotheses " accounting " for them, 

 may be neglected. Later we shall consider the theoretical interest 

 that Mendelism has for general biology. 



81. ON THE CYTOLOGICAL PHENOMENA ASSOCIATED 

 WITH MEN DELI AN HYBRIDITY 



In the first discussions of what we now call Mendelism it was 

 assumed that there were agencies in the ova and spermatozoa that 

 were the causes of the appearances of the characters and these 



