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two separate eggs. The former will have identical hereditary outfits,
the latter will have hereditary outfits as different as those of members
of the same family born at different times.
Yet it is true that in regard to intelligence tests, fraternal twins of
like sex, though as we would expect they show considerably less re-
semblance than identical twins, are more alike than pairs of brothers
or pairs of sisters born at different times. The additional similarity
of their environment, due to their developing pre-natally and post-
natally in more similar conditions, has assimilated them.
Writing of these results, Hogben x says that "the ambiguity of the
concept of causation" inherent in classical biometrical method has
"completely obscured the basic relativity of nature and nurture."
The difficulties inherent in multiple causation are here pithily summed
up, and attention also drawn to the practical impossibility of com-
paring results obtained on material from different environments,
and drawing genetic conclusions on their face value.
The same is true of racial differences. It seems clear that the very
idea of race as applied to man is a misnomer under present conditions,
Professor Gates has indeed recently asserted 2 that the major races
(colour varieties) of man should be regarded as true species. This
appears to me to be a grave error, arising from a failure to recognize
the biological peculiarities of the human species, as a species. These
are due to man's mobility and his tradition, and result in a unique
degree of variability combined with a failure of the usual tendencies
to speciation: the incipient species are brought together again by
migration and mingled by inter-crossing before any mutual infertility
has been established.
While, however, modern genetics has shown that the term race only
has meaning as a description of somewhat hypothetical past entities
or as a goal for even more hypothetical future ideals,3 yet it is of course
clear that different ethnic groups (to use the most general and non-
committal phrase) differ in genetic characters. Ethnic groups obvi-
ously differ in regard to the mean values, and also the range and type
of variability, of physical characters such as stature, skin-colour, head-
and nose-form, etc.: and these differences are obviously in the main
genetic. There is every reason to believe that they will also be proved
to differ genetically in intellectual and emotional characters, both
quantitatively and qualitatively* Butóand this caimot be too
strongly emphasizedówe at present have on this point no evidence
whatever which can claim to be called scientific. Different ethnic
cit,9 p. 95,                          3 Gates, 1934.
lc  and Haddon, 1935, especially chapter vu,