We may by way of a digression stop for a moment to consider the most astonishing and uncanny case of adaptation; namely, the formation of the transparent refractive media, especially the lens in front of the retina. It is due to these media that the rays which are sent out by a luminous point can be united to an image point on the retina. One part of this process is understood; namely, the formation of a lens. Wherever the optic cup of the embryo is transplanted under the epithelium the latter will be transformed into a transparent lens. When the upper edge of the iris is injured in the salamander so that the cells can multiply, the mass of newly formed cells also becomes transparent and a lens is formed. This indicates the existence of a substance in the optic cup which makes the epithelial cells transparent; and which also limits the size of the lens which is formed. The lens is not always a perfect optical instrument, on the contrary, it is as a rule somewhat defective. Of course, a great many details concerning the process of lens regeneration have still to be worked out. 3. It is well known that most marine animals die if put into fresh water and _vice versa_; and in salt lakes or ponds with a concentration of salt so high that most marine animals would succumb if suddenly transferred to such a solution we have a limited fauna and flora. The common idea is that marine animals become adapted to fresh water or _vice versa_; or to the conditions in salt lakes; especially if the changes take place gradually. Yet it can be shown that the existence of these different faunas can be explained without the assumption of an adaptive effect of the environment. The writer has worked with a marine fish _Fundulus_ whose eggs develop naturally in sea water which, however, will develop just as well in distilled water; and the young fish hatching in distilled water live and grow in this medium. Most of the adult fish die after several days, when put suddenly into distilled water, but they can live in fresh water which contains only a trace of salt. They can also live in very concentrated sea water, _e. g._, twice the normal concentration. Suppose that a bay of the ocean containing such fish should suddenly become landlocked and the concentration of the sea water be thus raised to twice its natural amount; the majority of forms would die and only _Fundulus_ and possibly a few other species with the same degree of resistance would survive. An investigator examining the salinity of the water and not knowing the natural resistance of _Fundulus_ to changes in concentration would be inclined to assume that he had before him an instance of a gradual adaptation of the fish to a higher concentration of the sea water; whereas the fish was already immune to this high concentration before coming in contact with it.