SOME ASPECTS OF ZOOLOGY 51 



tested, the factors on which these different characters depend 

 act independently of one another. 



Some individuals of Drosophila having long wings are ebony 

 black. Flies with vestigial wings are grey. 



If a cross is made between a grey- vestigial <$ and an ebony- 

 long ? there are two pairs of factors : "grey" with its alterna- 

 tive " ebony " ; " long " with its alternative " vestigial." 



In Fl all the offspring are grey-long. 



But these, mated together, give in F2 offspring in the 

 proportion 9 grey-long ; 3 ebony-long ; 3 grey- vestigial ; 

 1 ebony-vestigial. 



Time forbids my working out for you exactly how this 

 happens, but I may say shortly that the result is precisely 

 what is predicted by the mathematical computation of the 

 combination of two pairs of alternative units acting inde- 

 pendently. 



And if three or four or more pairs of alternative units are 

 brought into the account, it can similarly be shown that 

 experimental results accord with mathematical calculation. 



It is clear, then, that we are not indulging in mere specula- 

 tion and hypothesis when we say that the germ-plasm contains 

 factors which produce definite effects in the individuals 

 developed from the germ, and that these effects follow mathe- 

 matical laws. The examples given are simple. In many 

 experiments much more complicated results are obtained, 

 which often seem to stand in contradiction to the simple 

 sequences just described. But careful analysis brings them all 

 under the same law of unit characters in the germ-plasm. 

 Often factors that appeared simple at first sight are found to 

 be compound. And there are factors which by themselves 

 produce no effect, but when brought into combination with 

 other factors, or groups of factors, become active and produce 

 startling results. Such factors are called " modifiers." And, 

 most interesting of all, some factors seem to stick together 

 and form " linkages," which are seldom, but occasionally, 

 broken. 



To all these statements two very legitimate criticisms may 

 be offered : 



