DARWIN-WALLACE CENTENARY—pE BEER 345 
involves not only transmission of genetic material in the form of 
genes, but also transmission of bodily characters, since the latter are 
carried over wholesale from “parent” to offspring. Adaptation to 
new environments can take place in bacteria. Furthermore, in bac- 
teria, and perhaps also in higher organisms, it is possible for organic 
molecules such as bacteriophage particles to enter organisms and be- 
come incorporated in the genetic mechanisms so as to behave like 
genes. These results are full of promise as a field of research into the 
nature of genes, and perhaps of mutations, but they do not in any way 
invalidate the principles of Mendelian genetics and inheritance. 
Mutations are chemical changes in the gene molecule, and since 
chemical stability is not absolute, the puzzle about mutations is not 
so much that they occur as that they occur so infrequently. This ig- 
norance of the causes which determine the directions in which muta- 
tions take place, if such causes indeed exist, is, strange to relate, no 
handicap to the understanding of the mechanism of evolution, because 
it is emphatically selection, not mutation, that determines the direc- 
tion of evolution. This all-important conclusion is based not only on 
detailed experimental studies on the effects of selection in nature, but 
also on the demonstration by Sir Ronald Fisher of a general principle. 
The effects of selection in changing the frequency of genes in a popu- 
lation have been calculated for various percentage benefits in survival 
value conferred by such genes. It has been found by calculation that 
at the observed natural average mutation rate of one in half a million, 
no mutant gene has the slightest chance of maintaining itself against 
even the faintest degree of adverse selection. Furthermore, if the di- 
rection of evolution were determined by the direction of mutation, it 
would be necessary to suppose that such mutations must be predomi- 
nantly favorable. In fact, the vast majority of mutations have been 
unfavorable, and natural selection has acted against them by convert- 
ing the resulting mutant genes into recessives, or by suppressing them 
into the condition of mere modifiers, or by exacting the more drastic 
price of abolition consequent on the rapid death of the organisms 
containing them. It is natural selection, not mutation, that has gov- 
erned the direction as well as the amount of evolution, and it has been 
estimated that if mutation were to stop now, there is already sufficient 
variation in the plant and animal kingdoms for evolution to continue 
for as long in the future as it has continued hitherto in the past. 
The bearing of this demonstration on hypotheses that attempt to 
explain evolution by postulating the existence of agencies capable of 
directing mutation is plain. It means that all such theories as invoke 
the effects of use and disuse, “inheritance of acquired characters,” 
environmental stimuli, “organic selection,” “inner feelings,” inherited 
memory,” momentum along particular directions, orthogenesis, nomo- 
genesis, and others, which assume that mutation can be made to follow 
