NATURAL SELECTION—DOBZHANSKY AND ALLEN 369 
pleasant, or at least painless. Instead, childbirth is attended with 
intense pain, and often imperils the life of the mother, of the fetus, 
or of both. Although the later stages of pregnancy and parturition 
are to some extent incapacitating in all mammalian females, they are 
much more so in the human species. This and the other flaws in our 
biological organization Mechnikov called “the disharmonies of human 
nature.” We cannot but suppose that these disharmonies have arisen 
during the natural course of human evolution. 
The situation will appear less incomprehensible if the mechanics 
of natural selection are considered. Natural selection cannot develop 
this or that organ apart from the rest of the body, nor can it foster 
this or that gene apart from the rest of the genotype. What is selected 
in the process of evolution is the genotype as a whole. It is the whole 
organism which survives or dies, and successfully reproduces or re- 
mains barren. The genotype is a mosaic of genes, but it is wrong to 
think of the organism as though it were a mechanical sum of parts, 
each determined by a single gene. In the process of individual devel- 
opment all genes act in concert. The whole genotype, not just some 
genes, decides what an individual will be like as a fetus, in childhood, 
in adolescence, in maturity, and in old age. Moreover, the develop- 
ment of different individuals takes place in different environments; 
and the genotype may be required to adapt its carrier to any one of the 
possible environments. Certain differences between individuals (such 
as differences between some blood groups) are ascribable to single 
genes, but even the expression of these differences may vary; what an 
individual is like is always due to all the genes this individual carries. 
The evolutionary success or failure of a species is determined by the 
fitness of its entire genotype, and of its entire developmental pattern, 
in those environments which the species inhabits. An observer may 
discern, however, that some particular feature or aspect of the organi- 
zation is most instrumental in bringing about success or failure. Thus 
with man: his body is remarkable neither by its strength nor by its 
endurance. The evolutionary success of our species has been due to 
brain power, not to body power. Evidently, some genotypes which 
enhance brain power have been selected in spite of their tendency to 
decrease body power. Darwinian fitness is the resultant of all the 
advantages and disadvantages which one genotype may have com- 
pared to other genotypes. In man, the ability to learn and to invent 
and use tools influenced this balance more significantly than did mus- 
cular strength or resistance to inclement weather, although these were 
not negligible. 
It is certainly reasonable to suppose that genotypes which combined 
the greatest brain power with the greatest body power would yield the 
highest fitness. Why, then, is man not always as wise as Socrates, as 
