346 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
adaptively desirable directions, are not only devoid of any known 
mechanism by which the direction of mutation might be brought about, 
and devoid of evidence for the existence of such mechanisms, but they 
involve a cause “which demonstrably would not work even if it were 
known to exist.” It is therefore not surprising that in spite of re- 
peated attempts, many undertaken with impure and insufficiently 
standardized genetic material, and others in which the results were 
simply faked, no evidence has been provided that the effects of use 
and disuse or adaptive response to environmental conditions are in- 
herited or induce appropriate mutations. From the evidence provided 
by genetics, natural selection is the only mechanism capable of ex- 
plaining evolution. 
NATURAL SELECTION, “IMPROBABILITY,” AND “CHANCE” 
An argument sometimes used against the efficacy of natural selection 
involves the claim that the initial stages in the evolution of complex 
structures or functions could not have been favored by natural selection 
until such structures or functions had reached a certain level of per- 
fection. Like all other arguments of the non posswmus type, this one 
melts away before the progress of knowledge. A case in point is that 
of the electric organs of fish, developed out of muscles which are 
capable of discharges strong enough to catch prey and defend the fish 
against its enemies. These organs are clearly adaptive and confer 
survival value on their possessors, but the question arises what func- 
tions they could perform in the initial stages of their evolution, when it 
must be supposed that their power was too weak to kill prey or to 
deter predators. Darwin himself was well aware of this problem, and 
he met the argument by pointing out that “it would be extremely bold 
to maintain that no serviceable transitions are possible by which these 
organs might have been gradually developed.” He has been proved to 
be right, because of the discovery by H. W. Lissmann that weak 
electric discharges given off by certain fish function in a manner 
analogous to those of radar equipment, and serve to convey informa- 
tion of the proximity of objects in the water. Electric organs can 
therefore be adaptive even when they are too weak to kill prey or deter 
predators. 
Another case may be cited because it illustrates the manner in which 
an adaptive result may be achieved without itself being a direct object 
of selection. Color vision has been evolved independently in many 
groups of animals. Among the light-sensitive elements in the eye, 
some are specially sensitive in dim illumination; others confer acute- 
ness of vision in bright light when they are individually innervated, 
with the result that light stimuli are perceived separately by very 
small areas of the retina. In each of the two functions of seeing in 
relative darkness and seeing accurately in the light, increased efficiency 
