230 GENETICS 



the constitution AaBbCc, etc., others show such constitu- 

 tions as AaBBCc, AabbCc, and so on. These when inter- 

 bred must give individuals of many different constitutions. 



(9) The rare extreme individuals — for example, those 

 that are very large or very small — when bred separately 

 commonly "breed true," producing offspring like them- 

 selves. This is because such extreme individuals are (a) all 

 of the same constitution, and (b) homozygotic. The ex- 

 treme individuals at one end of the scale have the constitu- 

 tion AABBCC, etc.; when two such homozygotes are bred 

 together, they can produce only homozygotes like them- 

 selves. The extreme individuals at the other end of the 

 scale have the constitution aabbcc, etc.; these also are homo- 

 zygotes and can therefore produce only offspring having 

 the same constitution as themselves. 



(10) Results of Selection: The peculiar relations set 

 forth in paragraphs (8) and (9) above come out strongly 

 in experimental work on the results of selection, In relation 

 to quantitative differences — sizes, weights, yield of crops 

 and the like. Attempts were early made to Improve the 

 breeds of cultivated plants and animals by selecting and 

 breeding together those of the better grades. The results 

 were found to be as follows: 



(a) If parents above the average are selected, these 

 give offspring above the average, but varying. 



(b) If from these offspring those of higher grade are 

 selected, again these give offspring of higher grade, but 

 varying. Thus by repeated selection of the higher grade 

 individuals in successive generations, progressive Improve- 

 ment is produced, "selection Is effective." Such selection In 

 the opposite direction is equally effective. 



(c) But after this has continued for a number of gen- 

 erations, Improvement ceases; no further progress Is made. 

 Selection ceases to be effective. The individuals of the high- 

 est grade, when bred by themselves, produce offspring like 



