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 



