SOME ASPECTS OF ZOOLOGY 53 



vidual. But in this case, and probably in many others, the 

 grey colour depends on the co-operation of several factors, 

 which, when all are present together, produce grey. But if 

 any one or more is absent, other colours, or absence of colour, 

 as in the albino, are the result. 



The experiment just described is a good instance of the 

 phenomenon of " reversion " to an ancestral character. The 

 albino retained some and the waltzing mouse the remainder of 

 the factors necessary to produce grey. When these are united 

 a grey mouse results. Thus we have a simple explanation of 

 a phenomenon that puzzled Darwin and many other students 

 of inheritance. 



Let us turn back for a moment to the fruit-fly, Drosophila. 

 I said that in Morgan's flies a vestigial-winged form " appeared." 

 So also did the form called " ebony." How did they 

 " appear " ? The facts are that out of a collection of many 

 hundreds of ordinary flies some one, or perhaps two or three, 

 individuals suddenly made their appearance with these 

 peculiarities. Morgan tells us that in the course of five years' 

 breeding of fruit-flies he found no less than one hundred and 

 twenty-five new types, all of which bred true. Tower has 

 given an analogous record for the potato-beetle Leptino- 

 tarsa (Doryphora). In both species some of the new types 

 that appear in captivity are the same as types that may 

 occasionally be found among wild flies or beetles. Indeed, 

 Tower says that some of his new types were identical with 

 local races of potato-beetles established in the Southern United 

 States or in Mexico. There are several cases in which it has 

 been shown that new types appearing in confinement are 

 paralleled by types existing in nature, and this is important, for 

 it shows that the changes are not due to artificial conditions. 



Such new types are said to arise by " mutation " and are 

 called " mutants." They are often strikingly different from 

 their parents, and they appear suddenly with their full char- 

 acters. There is no question of a series of gradations, accumu- 

 lating through several generations, till the new type is reached. 

 As they hand on their peculiarities to their descendants in 

 conformity with Mendel's Law of Segregation, they must owe 



