Studies on variation and selection. 



173 



ones, could Prof. Castle, by measuring, adding and dividing the 

 extent of his animals dorsal-stripe, have got the figures actually 

 obtained. 



We know, that gradually his individual rats became darker in 

 one series and lighter in the other, because be tells us so; from the 

 average grades tabulated, such a fact could never be got, they could 

 as well be the expression of a gradual preponderance of dark over 

 light rats in one, of light over dark rats in the other series. 



V. Series of genetic factors which influence the development 

 in the same direction. 



The days have passed, when people speculated upon the ways 

 in which the "determinants" "called into being" the characters. Most 

 geneticians are now fairly ^ell in accord that each character (with 

 the organism showing it, which must not be overlooked) results from 

 the cooperation of a large number of factors, genetic, inherited ones, 

 and environmental factors. This is the chief reason why it is wrong 

 to call the different genetic factors by the first letter of the name of 

 some quality they, in some combinations, help to bring about. Thus, 

 for example, was a certain gene studied in rodents called b, or s, z 

 or N by some, according to the nationality of the authors, because 

 of the fact, that animals, having this gene, in combination with a 

 few hundred others, and in the absence of a number of still other 

 factors, are black. It has been said that such names only serve as 

 an aid to the memory, but the danger remains, that some people, by 

 the use of such letters recalling characters, are strenghthened in the 

 erroneous belief that these factors "call into being" these qualities. 



We know at least seven different genetic factors in the develop- 

 ment of the rodents, which influence the coat-colour, and we know 

 exactly what combinations of these factors may produce certain colours 

 under definite circumstances. 



Thus, if a mouse or cavy shall be black, it must contain with 

 other things at least the genes a, b, c, d, e, and f, and it may not 

 contain a gene called G, lest it become agouti. 



Again, we know a series of genetic factors, which all influence 

 the colour of the eye in mice. Some of these tend, in certain com- 

 binations, to make the colour blacker, others to make it redder. We 

 happen to know these factors very well, and what other influence 

 they have on tire development of a mouse. ^ 



