132 A TEXT-BOOK OF BOTANY. 



tage in a life and death struggle. Necessarily when natural selec- 

 tion first begins to operate on two individuals the differences must 

 be only slight and hardly sufficient to give one of them such a vital 

 advantage over the other. 



MUTATION. This explanation was offered in 1901 by Hugo 

 de Vries of Holland. The word mutation means a change. In 

 this sense it means a sudden change and has to do with the fact 

 that among the offspring of a certain individual may be found 

 one or more individuals markedly differing from the parent, so 

 much so as to be regarded in a few instances as a distinct species. 

 Moreover, these mutants, as they are called, continue to breed 

 true, thereby giving rise to what might very well be called a new 

 species. In the study of mutation many experiments have been 

 conducted by scientists and breeders. 



MENDEL'S LAW. In intimate relationship with the subject of 

 evolution is the question of heredity. In the middle of the last 

 century there lived an Austrian monk, Mendel by name, who ex- 

 perimented with the cultivation of peas and other plants in the 

 monastery garden. In his studies he discovered a certain law 

 underlying the transmission of characters in reproduction. This 

 law, which for many years lay hidden from the scientific world, 

 was recently brought to light and now forms the basis of most 

 of the recent breeding experiments and is of profound value in 

 the study of heredity. In the simplest case it is as follows: If 

 two different species, A and B, are crossed, the result is a hybrid 

 (AB) which combines certain characters of both parents. When 

 this hybrid propagates, the progeny splits up into three sets : one 

 resembling the hybrid parent (AB) ; and the other two sets re- 

 sembling the parent forms (A and B) that entered into the hybrid. 

 Mendel's law is a statement of the mathematical ratio expressed by 

 these three groups of forms derived from a " splitting " hybrid. 

 This means that in a series of generations initiated by a hybrid, ap- 

 proximately one-half of the individuals of each generation will 

 represent the hybrid mixture, one-fourth of the individuals will 

 represent one of the pure forms that entered into the hybrid, and 

 the remaining fourth will represent the other pure form. Of 

 course, the I : 2 : i ratio holds only when the one unit-character is 

 involved, and does not apply to the hybrids as a whole, as differ- 

 ent characteristics are generally inherited independently of others. 



