known that the genus _Nautilus_ has lasted from Silurian times, through all the three geological periods, up to the present day: while all its Silurian allies (_Orthoceras_, _Gomphoceras_, _Goniatites_, &c.) became extinct at a comparatively early period. A keen and clever controversialist might still bring forward many objections against such an argument. I do not therefore place too much dependence upon the geological facts by themselves, as a disproof of the self-variability of Nägeli’s idioplasm; for it must be admitted that the facts are not sufficiently complete for this purpose. For instance, in the case of _Nautilus_ it might be argued that we do not know anything about the fossil Cephalopods of pre-Silurian times, and that it is therefore possible that the above-mentioned allies of _Nautilus_ may have existed previously for as long a period as that through which _Nautilus_ has lived in post-Silurian time. However this may be, it will be at least conceded that the geological facts do not lend any support to Nägeli’s theory, for we can see no trace of even an approximately regular succession of forms. Appendix II. Nägeli’s explanation of adaptation[202]. In order to explain adaptation Nägeli assumes that, under certain circumstances, external influences may cause slight permanent changes in the idioplasm. If then such influences act continually in the same direction during long periods of time, the changes in the idioplasm may increase to a perceptible amount, i. e. to a degree which makes itself felt in visible external characters[203]. But such changes alone could not be considered as adaptations, for the essential character of an adaptation is that it must be a purposeful change. Nägeli, however, brings forward the fact that external stimuli often produce their chief effects at that very part of the organism to which the stimuli themselves were applied. ‘If the results are detrimental, the organism attempts to defend itself against the stimulus: a confluence of nutrient fluid takes place towards the part upon which the stimulus has acted, and new tissues are formed which restore the integrity of the organism by replacing the lost structures as far as possible. Thus in plants the healthy tissues begin to grow actively around the seat of an injury, tending to close it up, and to afford protection by impenetrable layers of cork.’ Purposeful reactions of this kind are certainly common in the organic world, occurring in animals as well as in plants. Thus in the human body an injury causes a rapid growth of the surrounding tissues, which leads to the closing-up of the wound; while in the Salamander even the amputated leg or tail is replaced by growth. An extreme example of these purposeful reactions is afforded by the tree-frog (_Hyla_), which is of a light-green colour when seated upon a light-green leaf, but becomes dark brown when transferred to dark surroundings. Hence this animal adapts itself to the colour of its