4 
From the evidence at hand it would seem so. And if it has 
occurred in such species as Juglans nigra and a host of others 
with a wide temperature-toleration range, why is not the same 
process operating at present? The writer believes it is. 
From the vast quantity of data derived from genetic experi- 
ments during the last twenty-five years on hundreds of species 
of animals and plants, and in particular on the vinegar fly, 
Drosophila melanogaster, Muller (1923) and other investigators 
have concluded that mutations take place in all directions. By 
this I mean all types of new characters arise—not just characters 
of certain kinds, or necessarily in a series in a certain direction, 
as on the orthogenetic concept. Mutations affect shape, size, 
color, scent of flowers; height, number of leaves, phyllotaxy, 
number of flower parts, stem color and shape, resistance to various 
diseases; color, shape, size, flavor, etc., of fruit; shape of wings 
and number of legs in animals, as well as length, texture and 
color of hair. In fact, characters of every description disappear 
or become modified in various degrees or become changed beyond 
recognition through mutation. So far as can now be seen from 
this immense amount of data, the innumerable structures and 
functions of all organisms are affected by mutation. In the 
vinegar fly, Drosophila melanogaster, Plough and Strauss (1923) 
found a wild strain which differed in its temperature-tolerations 
from all the other wild strains of this species that have been 
investigated experimentally. The ordinary strains will breed 
indefinitely on banana agar at 31° C., but this strain will not toler- 
ate a temperature above 29° C. Two other species of Drosophila 
have this same temperature limitation, and hence are not found 
in the tropics. 
If the assumption is granted that inherited variations in ability 
to survive various degrees of cold result only from mutation, 
and that mutations have occurred in the past and do occur now 
without reference to necessity or purpose, it seems reasonable 
to assume as a practical working hypothesis that most species 
may give rise to cold-resisting types. But if the cold-resisting 
mutant individual arises in a species confined to the tropics, to 
take an extreme case, where competition is strong, no advantage 
would accrue to it or to its progeny in the struggle for existence, 
so there is no reason to suppose that it would produce large 
