1 8 Heredity and Eugenics 



these facts in such numbers as to make any generaliza- 

 tion worthy, demands the culture through many generations, 

 under most rigid control, of the largest possible number of 

 plants and animals. This means long periods of time, great 

 patience, and many investigators. The number of investi- 

 gators is multiplying, the range of material is increasing, 

 and the period covered by some of the work has now been 

 sufficient to justify some presentation of the results. 



Probably the most conspicuous working hypothesis 

 today, in connection with the collection and interpreta- 

 tion of the data of heredity, is the one called "Mendel's 

 law." This is to be made a special theme in the course of 

 this series, but a brief statement in reference to it will 

 help the background and may prepare the way for the later 

 discussion. This Austrian monk, who worked in his 

 monastery garden during the middle of the last century, 

 left on record what is called a law of heredity. This record 

 was lost, so far as its influence was concerned, until 

 ten or fifteen years ago, when the modern movement in 

 experimental evolution began to be vigorous. Now the 

 Mendelians constitute a conspicuous biological cult, and 

 Mendelism has extended from its simple original state- 

 ment into a speculative philosophy, with conceptions of 

 unit-characters, dominance, ratios, etc., that the untrained 

 cannot follow. 



The fundamental conception is simple enough. If two 

 different species are crossed, the result is a hybrid which 

 combines certain characters of both parents. When this 

 hybrid propagates, the progeny splits up into three sets: 

 one set resembling the hybrid parent; and the two other 

 sets resembling the parent forms that entered into the 

 hybrid. Mendel's law is a statement of the definite ratio 



