to represent the molecule of every compound, or, at any rate, of every inorganic compound, by what is known as a graphic or structural formula. Thus, ethylic alcohol is represented by the formula:— H H | | H––C––C––O––H = CࠢHࠦO, | | H H and methylic ether by the structural formula:— H H | | H––C––O––C––H = CࠢHࠦO. | | H H The formulæ indicate a very different arrangement of the nine atoms which compose the molecule in each case. And to this different arrangement the differing properties of the two compounds are supposed to be due. A rough illustration of the phenomenon of isomerism is furnished by written language. Thus, three different words can be made from the letters t, a, and r, _e.g._ tar, art, and rat. They also form tra, which does not happen to be an English word, although it might have been one. Experiments of Gräfin von Linden Among organisms we sometimes observe a phenomenon which looks very like isomerism. The classical example of this is furnished by the butterflies _Vanessa prorsa_ and _Vanessa levana_. At one time these were supposed to belong to different species, since they differ so greatly in appearance. _Vanessa levana_ is red, with black and blue spots. _Vanessa prorsa_ is deep black, with a broad yellowish-white band across both wings. It is now known that the _levana_ is the spring form and the _prorsa_ the summer and autumn form of the same species. The pupæ of _levana_ produce the _prorsa_ form, but Weismann found that after being placed in a refrigerator they emerged, not as _prorsa_, but partly as _levana_ and partly as another form intermediate in many respects between _levana_ and _prorsa_. Weismann also succeeded, by exposing the winter pupa to a high temperature, in making it give rise to the _prorsa_ form, and not to the _levana_ form, as it would ordinarily do. Similar results have been obtained with the seasonally dimorphic _Pieris napi_. Standfuss, the Gräfin von Linden, and others have obtained like results in the case of other seasonally dimorphic butterflies. In some instances it has been proved that the change in the pigment is a purely chemical one; a similar transformation can be effected in the extracted pigment. But, we must bear in mind that the changes which are induced in this way are not confined to colour; they occur in the marking and shape of the wing. Even more remarkable is the fact that in some sexually dimorphic species a change of temperature alters the female, so as to cause her to have the outward appearance of the male. For example, it has been found that warmth changes the colours of the female _Rhodocera rhamni_ and _Parnassius apollo_ into the colours of the male.