Genetics and Evolution 
131 
nation which will not or do not generally intermarry are also species. The 
existence and relative permanence of such groups rest upon many varied 
grounds, but very largely on the feeling of likeness between the members of 
each group and the feeling of difference between their group and other groups— 
a likeness and difference that may depend on race, religion, occupation, mode 
of life, possession of land, or similar factors. To set against these differentiating 
factors we have the widespread desire of the bulk of the members of a nation 
to be alike, to be one, at least in certain respects, and even the idea of “the 
brotherhood of man.” Mass-immigration has tended to keep the population 
of the United States (except the negroes and the Japanese on the Pacific coast) 
“ one species ” in spite of the great diversity of its origin, because the immigrants 
are, on the whole, successfully absorbed. It is probably more truly one species 
than the population of any other great country. 
The authors rightly insist that a study of these things is essential if we 
are to apply the results of genetic science to political machinery, and they 
deprecate the exclusive attention of the eugenists to single human characters 
(particularly defects) of no more than varietal value at most, even when 
hereditary, rather than to the natural group units of the human race, which 
have arisen and will continue to arise as the inevitable result of universal 
biological and psychological processes. 
The authors have scarcely taken into consideration the direct effect of 
environment on organisms which many botanists, in particular, hold to be one 
of the most important factors of evolution, though they do refer to the possi¬ 
bility that mutations might be induced by varying the composition of water 
culture solutions. This view is much more prevalent among botanists today 
than it was 30 years ago, when the Darwinian influence, and especially Weis- 
mann’s version of Darwinism, were at the height of their power, though many 
botanists have always held to it. If genes are really chemical substances, it 
is certainly possible to believe that the long continued influence of a given 
environment may favour in the protoplasmic complex the production of one 
or several genes (for instance by dissociation or combination or by dissociation 
and recombination) at the expense of others. A varying “set” of metabolism 
may be initiated by a given environment and inherited, if only through the 
cytoplasm of the egg-cell; and this means the increased production of certain 
substances and the decreased production of others. For instance the peculiar 
carbohydrate metabolism of typical succulent plants (Cactaceae) leading to the 
accumulation of pentosans and thus to the characteristic succulent phenotype 
might be a case in point. But it is true that we cannot exclude the possibility 
that the origin of this type of metabolism, which has been shown to exist in 
plants that are not succulent in the normal habitat, may depend on a segrega¬ 
tion of genes without any initial relation to environment. Until we have 
penetrated a good deal deeper into the mysteries of bio-chemistry we cannot 
speculate on such points with any prospect of success. Meanwhile we cannot 
altogether rule out the possibility of the direct effect of environment in evolu¬ 
tion. And if it has any effect it probably has a very important effect. 
A. G. T. 
