4 HEREDITY [ch. 



We will now attempt, by means of a few examples, 

 to illustrate some of the questions which must be 

 answered, and some of the facts which must be 

 brought into relation, by any consistent account 

 of the process of heredity. A tall man on the 

 average has taller children than a short man, but 

 if all the sons of a number of tall men were measured, 

 it would be found that they showed every gradation 

 in height between the tallest and shortest ; some 

 would be taller than the fathers, others shorter, but 

 every gradation between them would occur. Also, 

 if a tall man marries a short wife, the sons are neither 

 all as tall as the father, nor divided sharply into a tall 

 group and a short group ; again they make a graded 

 series from short to tall. But if we cross a tall 

 variety of the sweet-pea with a dwarf variety, all 

 the offspring are as tall as the tall parent, and among 

 the offspring of these crossed tails, some are tall and 

 some short, but none are intermediate. Here then 

 we get two distinct modes of inheritance, and also 

 two kinds of variation ; in the first case the character 

 varies in such a way that all intermediates are found 

 between the extreme conditions, and in the second 

 the individuals can be classified sharply into two 

 groups. Again, we cross a white mouse or rabbit 

 with a black one, and all the offspring may have the 

 grey-brown colour of the wild animal — we have pro- 

 duced what is called reversion to the wild type, and 



