formation of
varieties and species; they always appear as the results of more or less
secondary reactions which make their appearance with stimuli exerted by
external causes.

External stimuli exerted on the organism are reproduced in the
idioplasm. Since the stimulus is discontinued with each change of the
ontogeny and only the idioplasm persists, permanent variations are
produced only in the idioplasm by those conditions that produce visible
transformations in the mature organism.

The phylogenetic action of external stimuli gives the definite character
of adaptation to the idioplasm as it becomes more complex from inner
causes and probably these external stimuli have the power to alter this
impress only as new idioplasm is automatically formed.

If an external cause acts continuously upon a phylogenetic line, the
corresponding variation of the idioplasm reaches, after a time, a
maximum, and thus comes to an end, either because the nature of the
substance permits no new rearrangement or because the stimulus is no
longer active. The cessation of the stimulus results from a micellar
rearrangement which indicates the character of the adaptation. If the
action of the stimulus lasts for only a short time, the incipient
rearrangement of the idioplasm stops, or proceeds independently on
account of the impulse received, and the determinant becomes capable of
development, even after the impulse has long ceased to act.

Since various intervening transpositions follow upon a stimulus in the
organism, the final result which appears as a reaction may turn out
variously. The same external causes may, according to the nature of the
organism and other circumstances, have very unlike variations as a
result. But the internal rearrangement produces in a definite case very
definite variations.

On account of the various intermediate steps it is often difficult to
discover the external cause of a given adaptive variation. In many cases
we recognize it without difficulty in a definite mechanical process or
in warmth, light or evaporation. For the most part the stimulus awakens
in the organism merely a want, which the reaction of the organism
endeavors to supply. Hence it appears that want or lack alone is able to
bring about such reactions. Moreover, in the sphere of sex, electric(?)
attractions and repulsions co-operate between the idioplasmic
determinants to produce phylogenetic variations.

The adaptations of the fully developed