The _variability of species_ has also been the subject of animated discussion among naturalists. The memorable contention which arose upon this subject between Cuvier and Geoffroy is not yet forgotten, a struggle considered by Goethe as more important than the gravest political events. In the present day a school to which many of the most illustrious names in England, Germany and elsewhere belong, has taken up, with certain modifications, the ideas of Lamarck and Geoffroy; it gives support to them from retaining the term _variability of species_. There is a grave confusion of words in this formula. Lamarck, Geoffroy, Darwin and his school, consider the species not only as _variable_ but as _transmutable_. The specific types are not merely _modified_, they are _replaced_ by new types. _Variation_ is in their estimation only a phase of the very different phenomena of _transmutation_. I shall discuss these theories presently. I shall now confine myself to the remark that _true variability_, admitted even by the defenders of _dogmatic invariability_, by Blainville, for example, a variability which I fully accept, has nothing in common with the _transmutability_ of Lamarck, Geoffroy and Darwin. Let us briefly determine the limits of this variability. III. When an individual trait is exaggerated and passes a limit always very loosely defined, it constitutes an exceptional character which clearly distinguishes the individual affected by it from all those most nearly resembling it. This individual constitutes a _Variety_. The same term must be applied to all those individuals, which, like certain plants reproduced by slips, grafts, or shoots, derive their origin from the first exceptional individual, without having the power of transmitting their distinctive characters by means of normal generation. I borrow from M. Chevreul a curious example of these _multiple varieties_. In 1803 or 1805, M. Descemet discovered in his garden at Saint Denis, in the midst of a bed of acacias (_Robinia pseudo-acacia_) an individual without thorns which he describes under the epithet _spectabilis_. It is to the multiplication of this individual by the art of the gardener that all the _thornless acacias_, now distributed over every part of the globe, owe their origin. Now these individuals produce seeds, but if the seeds are sown they only yield _thorny acacias_. The _acacia spectabilis_ has remained a _Variety_. The latter may then be defined as:—“An individual or a number of individuals belonging to the same sexual generation, which is distinguished from the other representatives of the same species by one or several exceptional characters.”