By applying rays of strong light, electric shock, or centrifuge, the Gräfin von Linden was able to change the colours of the butterflies to which the caterpillars gave rise. Pictet experimented on twenty-one species of butterflies, or rather on their caterpillars, and found that in nearly all cases when the caterpillars ate unusual food, they developed into butterflies with abnormal colouring. Schmankewitsch made the discovery that, in the case of the crustacean _Artemia_, he could produce either of two species according to the amount of salt in the water in which these creatures were placed. He declared that the anatomical differences between the species _Artemia salina_ and _Artemia milhausenii_ depended solely on the percentage of the salt in the surrounding water. He further stated that by adding still more salt he could change the _Artemia_ into a new genus—_Branchipus_. More recent observers have cast doubt upon these results of Schmankewitsch. They, however, admit that the degree of salinity of the water has some effect on the form of the _Artemia_, although they suggest that factors other than concentration affect the result. In any case, it is now well-known that changes in the environment effect changes in the colouring of many crustacea. Pictet has shown that the alternating wet and dry seasons in some tropical countries are the cause of, or stimulus that induces, seasonal dimorphism in some butterflies. He was able to effect changes in the colouring of certain species by means of humidity. The most important cases, from our point of view, are those in which the application of heat or cold to a pupa has affected the colour, shape, etc., of the emerging butterfly. Here we have but one factor, that of temperature. All the material for the formation of the butterfly is already stored up in the pupa. The unit characters, or their precursors, are all there, and they take one form or another according to the stimulus applied. Biological Isomerism Phenomena of this kind can, we think, be accounted for only on the assumption that the unit characters affected are each developed from a definite portion of the fertilised egg, that each of these portions, these precursors of the unit characters, is, like a chemical molecule, made up of a number of particles, and that upon the arrangement of these particles in its precursor in the egg depends the form that the unit character derived from it will take. One arrangement of these particles gives rise to one form of unit character, while another arrangement will give rise to a totally different form of unit character. Thus, some organisms seem to display a biological isomerism akin to chemical isomerism, save that the particles which in organisms take the place of chemical atoms are infinitely more complex. In other words, the precursors in the fertilised egg of each of these unit characters behave in some respects like chemical molecules.