Selection and Heredity 



355 



pigment; some make for continuity and some for the break- 

 ing up of the pattern, and so on. After segregation, the 

 progeny of hybrids include many types. In an experimental 

 animal selected from such a group, the particular appearance 

 is due to the presence of specific factors related to the pig- 

 ment and pattern. But there are present also many factors 

 lying latent, and still others that merely modify the outward 

 manifestation of the inherent capacity. Selection is car- 

 ried on with the attention fixed upon the outward pattern. 



Fig. 94. Variation of Pigmentation in Hooded Rats 



This is a scale for measuring coat-patterns in experiments designed to produce ex- 

 treme types by artificial selection, beginning with the type O and working in opposite 

 directions. After Castle. 



The experimenter or breeder must inevitably carry through 

 the successive generations heritable factors of which he 

 does not know but which nevertheless influence the final 

 results. When the extreme types have appeared they are 

 composites of numerous hereditary components. Succes- 

 sive breeding over many generations is necessary to dis- 

 entangle the complex. This is the argument which the 

 geneticists have opposed to the assumption that such results 

 as Castle obtained represent a simple cumulative effect of 

 selection. 



The students of genetics have already shown (i) that 

 a change in a gene involves as a rule alterations in several 

 organs and not merely in the one upon which the experi- 



