THE METHOD OF BIOMETRY 457 



tions of the factors that condition the course of development, and, 

 therefore, the resemblance between generations, is reserved for 

 later paragraphs. A great mass of evidence indicates that the 

 mechanism of heredity and variation is carried in the so-called 

 germ plasm. While this is undoubtedly true in many of the cases 

 to be mentioned, the facts of asexual reproduction by budding in 

 multicellular animals, and of regeneration in forms like hydra, 

 planaria, and the annulates, make it certain that the mechanism 

 that controls the course of development is also distiibuted to the 

 somatoplasm. 



Several methods of studying the subject matter of Genetics are 

 in use. It is possible to observe and analyze resemblances and 

 variations from one generation to another in large groups of indi- 

 viduals as they are found under natural conditions. This is the 

 statistical method, or the method of Biometry. In contrast with 

 such mass analysis is the observation of inheritance and variation in 

 animals bred under experimental conditions for generation after 

 generation. This method of Experimental Breeding, under controlled 

 conditions that give the best environment for the organisms, makes 

 it possible to know in detail the character of heredity and varia- 

 tion in any particular individual, as well as to compare, specifically, 

 individuals of successive generations. In order to interpret the 

 results of experimental breeding it becomes necessary to study, by 

 the method of Cytology, the germ cells from which new individuals 

 arise. The greatest progress in the theory of Genetics has come 

 from correlation of results obtained from experimental breeding 

 and such study of germ cells. A fourth way of approaching the 

 study of the mechanism of heredity and variation is by the method 

 of Experimental Embryology, in which individuals of known ances- 

 try are subjected to conditions that are not usual for their develop- 

 ment. The comparison of results obtained from these several 

 ways of approach has yielded considerable information concerning 

 man}^ facts of heredity and variation, and has led to the formula- 

 tion of theories of the mechanism involved. Clarification and 

 extension of problems in Genetics may be expected to continue in 

 view of the great amount of interest in research in this field. 



The Method of Biometry 



Galton's Work. — The biometrical method is that of collecting 

 a great amount of observational data upon organisms under nat- 



