288 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



mechanism of heredity may be obtained by an analysis of its modes of 

 heredity. It is this new experimental and analytic method of study- 

 ing evolution that we have come to designate as genetics. 



Three principal methods of attack upon the problems of genetics 

 have been successful in advancing our knowledge. 



a) Experimental breeding. — This method was first systematized 

 by Mendel and consists of breeding together two individuals possessing 

 certain more or less contrasting characters and determining the ratios 

 in which the parental characters reappear in the offspring. This 

 method has been extremely fruitful and in connection with the second 

 method, that of cytology, has made clear much that was obscure to 

 Darwin and his followers. 



b) Cytology. — This second method involves the microscopic 

 study of the germ cells during the most critical periods of their cycle. 

 It seems very probable that we can now view under the microscope 

 the actual heredity machine and see how it works. 



c) The statistical method. — ^It is usually conceded that Sir 

 Francis Galton was the first to use the method of statistics in the study 

 of heredity. By means of correlation tables he was able to compare 

 large groups of parents with large groups of offspring with respect to 

 any unit character, and to state the degree of heredity in defin- 

 ite mathematical terms. The modern science of biometry is used 

 extensively at the present time for determining the degree of vari- 

 ability of characters which vary only slightly or irregularly and the 

 exact degree of correlation that exists between different hereditary 

 characters. 



All three of these methods of attacking the problems of genetics 

 have been fruitful in results and all are essential to an adequate under- 

 standing of the workings of evolution. 



The subject-matter of genetics consists of: {a) a knowledge of the 

 principles of ontogeny, the development of the individual from the 

 germ-cell stage to the adult stage; {h) a knowledge of the behavior of 

 the germ cells from one generation to the next, involving the so-called 

 "origin" of germ cells, maturation and fertilization of germ cells, and 

 the exact behavior of the chromosomes during the entire germ-cell 

 cycle; (c) a knowledge of variation, including a determination of 

 what distinct kinds of variation occur, where in ontogeny variations 

 are initiated, the causes of variation, etc. ; {d) what kinds of variations 

 are inherited and according to what laws — the whole subject of Men- 

 delian heredity; {e) the determination of sex and the relation of 'sex 



