CHAPTER I 



INTRODUCTORY. MENDEL'S DISCOVERY. 



Introductory — Some pre-Mendelian Writings — MendeVs 

 Discovery — Dominant and Recessive — Segregation. 

 Allelomorphis7n — Homozygote and Heterozygote. 

 Purity of Type. 



Among the biological sciences the study of heredity- 

 occupies a central position. Whether we be zoologists, 

 botanists, or physiologists, the facts of heredity concern 

 us. Upon this physiological function all the rest in some 

 degree depend. Every advance in knowledge of that 

 central function must affect the course of thought along 

 each several line of biological inquiry. 



Moreover though, as naturalists, we are not directly 

 concerned with the applications of science, we must perceive 

 that in no region of knowledge is research more likely to 

 increase man's power over nature. The science of sociology, 

 and in many of its developments the science of medicine 

 also, must of necessity form working hypotheses respecting 

 the course of heredity, and we cannot doubt that a percep- 

 tion of the truth in regard to the function of transmission 

 will greatly contribute to the progress of these sciences. 

 Lastly, to the industrial arts of the breeder of plants or 

 animals, the knowledge we are attempting to provide is of 

 such direct importance that upon this consideration no 

 special emphasis is required. In studying heredity, there- 

 fore, we are examining a vital problem of no mean 

 consequence, and those who engage in that work are 

 happy in the thought that they are assisting one of the 

 main advances in natural knowledge. 



But though we may approach this study o( genetics — 

 to use the modern designation — from so many different 

 sides, it is especially in their bearing on the problem of 



B. H. I 



