The Origin of Species 327 



tical conditions, are sufficient to account for the 

 principal phenomenon of hereditary transmission 

 without the assumption of the presence of a special 

 germ plasm. This view frankly admits that the de- 

 velopment of the organism not only may be directly 

 influenced by external factors, but that the changes 

 so induced may be inherited. 



Mendel's Law of Heredity. Some 40 years ago 

 a German monk, Gregor Mendel, published in the 

 proceedings of an obscure scientific society the re- 

 sults of a series of experiments upon the laws of 

 heredity. These were quite ignored by the scientific 

 world until attention was called to them by De 

 Vries and some other experimenters, almost simulta- 

 neously, in connection with the revived interest in 

 the study of heredity, aroused by De Vries' work 

 on mutation. The great value of Mendel's work 

 lies in its showing that there are definite laws gov- 

 erning the inheritance of certain characters. Men- 

 del worked especially upon varieties of garden peas, 

 and demonstrated that where any two contrasting 

 characters were crossed, as for instance long stems 

 and short stems, smooth and wrinkled seeds, that 

 these characters were inherited in definite propor- 

 tions, and one of the characters was " dominant " 

 while the other was " recessive." For instance, 

 when a variety with round seeds is crossed with one 

 producing wrinkled ones, when these are self-fer- 

 tilized, the progeny will all produce seeds of the 

 round type, which is thus shown to be dominant. 



