1 84 EVOLUTION, GENETICS, AND EUGENICS 



anisms of evolution is the province of that branch of evolutionary 

 biology known as Genetics. 



Three principal methods of attack upon the problems of genetics 

 are as follows: 



a) Experimental breeding. — This method, first carried out system- 

 atically by Mendel, consists of breeding together two individuals dif- 

 fering in certain well-defined characters and of determining the ratios 

 in which the contrasting parental characters reappear in successive 

 generations of descendants. Mendel's method has been extremely 

 fruitful and in connection with the second method, that of cytology, 

 has thrown a flood of fight on the actual workings of the mechanisms 

 of evolution. 



b) Cytology. — This method involves the microscopic study of the 

 germ cells, especially during the period of reproduction. The ob- 

 served changes that go on in connection with sexual reproduction have 

 been found to constitute a mechanism adequate to explain the peculiar 

 hereditary regularities constituting Mendel's laws of heredity. Also 

 it has been found that most aberrations in heredity are associated with 

 correlated aberrations in the mechanism of reproduction. Experi- 

 mental breeding and cytology have been most intimate and successful 

 collaborators. 



c) Statistical analysis. — -While experimental breeding and cytology 

 lay stress upon the type of change or the mode of heredity in in- 

 dividuals, statistical methods deal chiefly with group changes and with 

 the characteristics of populations. Often evolutionary trends or group 

 changes, quite undetectable in separate individuals, can be detected 

 by statistical methods. Also, the statistical consequences of a cer- 

 tain type of change in a few or many individuals can be predicted for 

 any given number of generations. While the methods of statistical 

 analysis are as a rule too difficult for any but the expert, they are be- 

 coming more and more essential as a part of the technique of genetics. 



d) Observations of changes going on in nature. — Attempts are 

 made to discover evidences of changes in nature equivalent to those 

 observed in the laboratory, and there is now considerable evidence 

 favoring the conclusion that laboratory observations are reliable cri- 

 teria of what takes place in nature. 



PREREQUISITES FOR THE STUDY OF GENETICS 



In order profitably to pursue the study of genetics, one must first 

 understand the fundamentals of biology in general, for the mechanism 



