INTRODUCTION 3 



2. THE TRIANGLE OF LIFE 



Accordingly within a generation the center of bio- 

 logical interest gradually swung from the origin of 

 species to the origin of the individual. The nineteenth 

 century was Darwin's century. His monumental work 

 "On the Origin of Species by Means of Natural 

 Selection," which appeared in 1859, not only dominated 

 the biological sciences but also influenced profoundly 

 many other realms of thought, particularly those of 

 philosophy and theology. 



Now, in the first decades of the twentieth century, 

 a particular emphasis is being laid upon the study of 

 heredity. The interpretation of investigations along 

 this line of research has been made possible through 

 the cumulative discoveries of many things that were 

 not known in Darwin's day. Trained students, pa- 

 tiently and persistently bending over improved micro- 

 scopes, have untangled the mysteries of the cell, while an 

 increasing host of investigators, inspired by the 

 Austrian monk Mendel, have industriously devoted their 

 energies to breeding animals and plants with an insight 

 denied to breeders of preceding centuries. 



The study of the origin of the individual, which 

 has grown out of the more general consideration of 

 the origin of species, forms the subject-matter of 

 heredity, or, to use the more definitive word of Bate- 

 son, of genetics. 



(It is not with the individual as a whole that 

 genetics is chiefly concerned, but rather with char- 

 acteristics of the individual. 



