qualities of the proteids or of the unknown carriers
of life. In the case of many organic compounds their morphological
characters (the physical condition, crystalline form, etc.) are at once
changed by alteration of atomic relations or by incorporation of new
radicals. (For instance ethylchloride (C2H5Cl) is a gas at 21 deg
C., ethylenechloride (C2H4Cl2) a fluid boiling at 84 deg C., beta
trichlorethane (C2H3Cl3) a fluid boiling at 113 deg C.,
perchlorethane (C2Cl6) a crystalline substance. Klebs, ("Willkurliche
Entwickelungsanderungen" page 158.) Much more important, however, would
be an answer to the question, whether an individual variation can be
converted experimentally into an inherited character--a mutation in de
Vries's sense.

In all circumstances we may recognise as a guiding principle the
assumption adopted by Lamarck, Darwin, and many others, that the
inheritance of any one character, or in more general terms, the
transformation of one species into another, is, in the last instance,
to be referred to a change in the environment. From a causal-mechanical
point of view it is not a priori conceivable that one species can
ever become changed into another so long as external conditions remain
constant. The inner structure of a species must be essentially altered
by external influences. Two methods of experimental research may be
adopted, the effect of crossing distinct species and, secondly, the
effect of definite factors of the environment.

The subject of hybridisation is dealt with in another part of this
essay. It is enough to refer here to the most important fact, that as
the result of combinations of characters of different species new and
constant forms are produced. Further, Tschermack, Bateson and others
have demonstrated the possibility that hitherto unknown inheritable
characters may be produced by hybridisation.

The other method of producing constant races by the influence of special
external conditions has often been employed. The sporeless races of
Bacteria and Yeasts (Cf. Detto, "Die Theorie der direkten Anpassung... ",
pages 98 et seq., Jena, 1904; see also Lotsy, "Vorlesungen", II. pages
636 et seq., where other similar cases are described.) are well known,
in which an internal alteration of the cells is induced by the influence
of poison or higher temperature, so that the power of producing spores
even under normal conditions appears to be lost. A similar state of
things is found in some r