to which they are subjected, and exert scarcely any, or only a very small form-determining influence upon each other, as has been amply proved in the preceding essay. In any case the power of these larvæ (the _Cecidomyiæ_) to propagate themselves asexually was first acquired as a secondary character, as appears from the fact that there exist numerous species of the same genus which do not “nurse.” In the form which they now possess they could never have played the part of the final stage of the ontogeny, nor could they formerly have possessed the power of sexual reproduction.[246] In brief, we are here concerned with true larval reproduction, whilst in Triton we have reversion to an older phyletic stage.[247] I cannot agree with my friend Professor Haeckel when he occasionally designates the reversion of the Tritons as an “adaptation” to a purely aqueous existence.[248] We could here only speak of “adaptation” if we took the word in a quite different sense to that in which it was first introduced into science by Darwin and Wallace. These naturalists thereby designate a gradual bodily transformation appearing in the course of generations in correspondence with the new requirements of altered conditions of life or, in other words, the action of natural selection, and not the result of a suddenly and direct acting transforming cause exerted but once on a generation. Just because the word “adaptation” can be used in ordinary language in many senses, it is desirable that it should have only _one_ precise signification, and above all that we should not speak of adaptation where scarcely any morphological change occurs, but only a kind of functional change in the sense used by Dohrn.[249] This is the case for example, when Forel[250] shows that fresh water Pulmonifera, the organization of which is attributed to the direct respiration of air, can nevertheless become settled in the greatest depths of mountain lakes through their lungs being again employed as gills. That not the least change in the lungs hereby takes place is shown by the observations of Von Siebold,[251] who saw the shallow water Pulmonifera using their lungs alternately for direct aërial and aquatic respiration, according to the amount of air contained in the water. If with Von Siebold we merely apply the word “adaptation” to such cases, this expression would lose the special sense which it originally conveyed, and the word would have to be abandoned as a _terminus technicus_; still, such cases may perhaps be spoken of as _physiological_ adaptation.