HEREDITY 225 



characters. (3) It includes polymorphism, so that all the bee 

 castes are still regarded as bees. (4) It takes fluctuating variability 

 into account by assigning a certain range to each measurable 

 character. 



We see also that all local races of the cod are still the individuals 

 of the specific category Gadus callarias. Greenland cod have 51 

 to 55 vertebras and Irish Sea cod have 50 to 54 : they belong 

 therefore to diff"erent categories, or local races of cod. But, from 

 its definition, the species Gadus callarias has 50 to 55 vertebras 

 so that although Newfoundland and Irish Sea cod-races are 

 different from each other they are still Gadus callarias — because 

 of the logical schemes of our classifications. When we make the 

 statement : 



Characters of parents = Characters of progeny, 



which is what we mean by " heredity," we are not making an 

 equation but what the mathematicians would call an identity. 



What we study in heredity are the ways in which the progeny 

 differ from the parents (if they differ), what regularities can be 

 found when we study these differences and how we can control 

 breeding so as to minimize or maximize these differences, or 

 bring them under control. These problems are different ones 

 according to whether the modes of reproduction of the organisms 

 concerned are vegetative, or asexual, or parthenogenetic, or sexual. 

 In vegetative reproduction the new organisms are, say, plants 

 that are multiplied by grafts, slips, cuttings, etc., and not by seeds. 

 In many cases Animal organisms (such as some protozoa) multiply 

 by simple fission, without conjugation. In parthenogenesis the 

 animal reproduces by means of ova which are not fertilized (since 

 there are no males). When there is not amphimixis the problems 

 of heredity (apart, of course, from the problem of development) 

 are relatively simple. Any species that does not reproduce 

 sexually may be regarded as being constituted by a number of 

 " pure races " and it may be possible to isolate such categories. 

 W^ithout going into detail upon this part of our subject it 

 may be sufficient, in the meantime, to regard pure races in 

 asexually reproducing organisms as being the descendants of 

 one original ancestral organism. When we study heredity in 

 sexually reproducing organisms the problem becomes much 

 more difficult. 



