EVIDENCE FROM EXPERIMENT 161 



easy to see why segregation must occur in each gener- 

 ation of the hybrids. 



The complexity of the calculation rises very rap- 

 idly, when, instead of experimenting with only a 

 single pair of alternative characters, as in the cases 

 previously cited, combinations of several such pairs 

 are effected. Mendel himself used as many as seven 

 pairs, combined in many different ways. Yet, if 

 regard be paid only to the behaviour of a single pair 

 of alternatives, it makes no difference how many 

 other unit-characters are associated in the combina- 

 tion, as they all act independently. When the be- 

 haviour of all the characters is analyzed, it is found 

 that the mathematical laws of probability and com- 

 bination are followed as closely as when but a single 

 pair is dealt with. Indeed, the correspondence be- 

 tween the calculated and the experimental results 

 is highly remarkable. 



Mendel's own experiments were made with plants, 

 but the same laws of heredity apply to animals. 

 Lang experimented with the garden-snail of Europe, 

 one race of which has a yellow, unstriped shell and 

 another has the shell with spiral stripes of black. 

 Crossing these, the unstriped yellow proved to be 

 dominant, all the individuals of the first hybrid 

 generation having shells of that kind. In the second 

 generation segregation began and in just the same 

 proportion as in the violet and white peas; one-fourth 

 of the shells were striped, three-fourths unstriped. 

 All the descendants of the former were constantly 



