498 EVOLUTION, GENETICS, AND EUGENICS 



tion theory. We know nothing, as yet, about the mechanism of muta- 

 tion, or about the nature of the gene — aside from the fact that nearly 

 all genes hitherto studied behave like material particles existing in the 

 chromosomes. Nevertheless there is already evidence for a number of 

 empirical principles regarding the changes of the genes, some of which 

 may conveniently be listed here in the form of 14 statements. I shall 

 have opportunity merely to present these principles, without attempt- 

 ing any adequate explanations of how they have been derived from the 

 data. 



1. The first and probably most important principle is that most 

 genes — ^both mutant and "normal" — are exceedingly stable. Some 

 idea of the degree of this stability may be obtained from some quantita- 

 tive studies of mutation which Altenburg and I have made in the fruit 

 fly Drosophila. It may be calculated from these experiments that 

 a large proportion of the genes in Drosophila must have a stability 

 which — at a minimum value — is comparable with that of radium 

 atoms. Radium atoms, it may be recalled, have a so-called "mean 

 life" of about two thousand years. 



2. Certain genes are, however, vastly more mutable than others. 

 For example, a gene causing variegation in com, studied by Emerson, 

 and another in the four-o'clock, studied by Maryatt, ordinarily have a 

 mean life of only a few years; and that causing bar eye in Drosophila 

 has a mean life of only about 65 years, as is shown by the results of 

 Zeleny. (In expressing these results we are here using the physicists' 

 index of stability, which seems most appropriate for the present pur- 

 pose also.) 



3. External agents do not ordinarily increase the mutability sufii- 

 ciently (if at all) to cause an obvious "production" of mutation. 



4. The changes are not exclusively of the character of losses; this 

 is shown by the well established occurrence of reverse mutations, in 

 bar-eyed and white-eyed Drosophila, in Blakeslee's dwarf Portulaca, 

 Emerson's variegated corn, and probably in a number of other recorded 

 instances. It is known that mutations having an effect similar to that 

 of losses do occur, however, and they may be relatively frequent. 



5. The change in a given gene is not in all cases in the same direc- 

 tion, and it does not even, in all cases, involve the same characters. 

 The latter pomt is illustrated by a series of mutations which I am 

 investigating in Drosophila, which all involve one gene, but which pro- 

 duce, as the case may be, either a shortened wing, an eruption on the 

 thorax, a lethal effect, or any combination of these three. 



