In this case there can be no doubt on that point: they are the cells on the posterior wall and the margin of the iris. And it is certainly not the _absence_ of the lens which gives rise to its renewal, as would necessarily be the case if it were due to the dominance of an adaptive force. If the lens, instead of being excised, be simply pressed back into the vitreous humour occupying the cavity of the eye, a new lens is developed all the same from the irritated margin of the pupil. And if by chance this margin has been irritated in two places while extraction of the lens was being performed, then two small lenses will develop (Fig. 99, _B_). Indeed, several may begin to develop at the posterior wall of the iris, although they do not attain to full development; mechanical irritation of any part of this cell-layer is responded to by the formation of lenses. This surely disposes of the 'mystical nimbus' which would dazzle us with a new force of life, always creating what is appropriate. We have before us an adaptation to the liability of newts' eyes to injury, which, like all adaptations, is only relatively perfect, since under the usual conditions of eye injury it gives rise to a usable lens, but under unusual conditions to unsuitable structures. It is exactly the same as in the case of animal instincts, which are all 'calculated' for the _ordinary_ conditions of life, but, under unusual conditions, may operate in a manner quite unsuited to the necessary end. The ant-lion has the instinct to bore backwards into the sand, and he makes the same backward-pressing movements when placed on a glass plate into which he cannot force the tip of the abdomen. The same is true of the mole-cricket, which makes its usual digging movements with the forelegs even on a plate of glass. The wall-bee roofs over her cell when she has laid an egg in it, but she does so even if the egg be taken out beforehand, or if a hole be made in the bottom of the cell, so that the honey which is to serve the larva for food when it emerges from the egg runs out (Fabre). Her instinct is calculated for filling the cell _once_ with honey, and _once_ laying an egg in it, because such disturbances as we may cause artificially do not occur or occur very rarely in natural conditions. There are countless facts of this kind, for every instinct and every adaptation can, in certain circumstances, go astray and become inappropriate. This should be considered by those who still persist in opposing the theory of selection, for herein lies one of the most convincing proofs of its correctness. Adaptations can only arise in reference to the majority of occurrences, and variations which are only useful in an individual case must, according to the principle, disappear again. Adaptation always means the establishment of what is appropriate in an average number of cases.