MA'lERIALS FOR STUDY 107 



In other words, there seem to be cases in which generation 

 succeeds generation without any variation. 



But there is every reason to suspect that in most cases the 

 apparent absence of variation is illusory, and due to a lack of 

 sufficiently intimate acquaintance with the individual organisms. 

 The sheep which seem " all the same " to the careless eye are 

 often known individually by the shepherd, and it is easy to 

 demonstrate that the peas in one pod are often far from being 

 alike. Similarly, the members of a group of individuals may 

 seem " all the same " even to the naturalist's eye, but minute 

 differences are soon detected by the expert who has devoted 

 years to becoming intimately acquainted with that particular 

 type. There are observable differences between sister-bees or 

 ants, between the rooks from one clutch or the pigs from one 

 litter. Even when there is only one parent — e.g. a self-fertilising 

 liver-fluke or a parthenogenetic water-flea — there may be 

 variations among the descendants. There is no doubt, however, 

 that the range of variability differs greatly in different types, 

 and it is obviously in cases where individual peculiarities are 

 frequent and well marked that we can most hopefully study 

 the relations of resemblance and difference between parents and 

 offspring, or between the members of a series of generations. 

 In horses and dogs, in sheep and cattle, in rats and mice, in 

 rabbits and guinea-pigs, in pigeons and fowls, in butterflies and 

 smaU, rapidly breeding crustaceans, in wheat and barley and 

 maize, in peas and stocks, and in man himself, there is ample 

 opportunity for studying the modes of inheritance. 



§ I. Though Prediction in Individual Cases is insecure, there are 

 some Common Modes of Inheritance 



When we are dealing with the generations of an animal or 

 plant in regard to which previous observation has shown us 

 that the members of the species are strikingly uniform in their 

 characters, we may venture with some security to predict that 



