1898] SPECIES, SEX, AND THE INDIVIDUAL . 189 
be combined into the adaptations we see, do occur apart from the 
special habits or conditions to which the adaptations are related. 
The variations that occur constantly in the form of individual 
differences, have been minutely investigated in the past few years by 
statistical methods, with the aid of the higher mathematics, The 
greater the difference, the more rarely it occurs; and occasionally 
striking abnormalities are observed, the character of which points to 
definite principles of symmetry and repetition in development. But 
it is not proved that, without change of conditions, variations occur 
which could by selection give rise to such special adaptations as 
abound in the animal kingdom, For example, the power of partial 
or complete flight by means of a membranous fold of skin, has been 
evolved in many independent cases in the vertebrate sub-kingdom, 
in the extinct pterodactyl reptiles, in bats, in flying foxes, flying 
squirrels, etc. But the variations in the condition of the skin and 
limbs in animals that do not fly or take long leaps through the air, 
are not such as to justify the belief that by the mere selection of 
the maxima among such variations, a membranous organ of flight 
could be evolved. To take another instance, there is a fish which 
has its eyes in a very remarkable condition. Spectacles for our own 
eyes, for human eyes, are sometimes made in which the upper half 
has a curvature different from that of the lower. The fish to which 
I refer, Anableps, does not wear spectacles, but actually has its eyes 
made in two parts, in the upper part of which the lens has a 
different curvature from that of the lower. The pupil is also 
divided into two by prolongations from the iris. This fish is in the 
habit of swimming at the surface with its eyes half out of the water, 
and the upper half of the eye is adapted for vision in air, the lower 
half for vision under water. Now, however various the individual 
variations in fishes’ eyes, there is no evidence that variations which 
could by selection give rise to this curious condition, occur in other - 
species of fish. It seems to me that we have no reason to suppose 
that the required variations ever occurred, until the ancestors of 
Anableps took to swimming with their eyes half out of the water. 
A similar argument applies to many other cases of special adapta- 
tion, and the logical conclusion is that the habits and conditions 
determined the modification. 
On the other hand, it may be asked, what positive evidence have 
we that special habits or conditions do determine special modifica- 
tions. The reply is, that we have abundant and admitted evidence 
as to the effect on the individual; and as hereditary modifications 
are, in many cases, of the same kind as these, the presumption is, 
that the effect on the individual has become hereditary. The 
question, however, of the origin and causes of adaptations cannot be 
considered apart from the phenomena of development and individual 
