of acquired characters. But, on
the other hand, we cannot understand how the evolution of the brain
and its functions takes place, without admitting that in one way or
another the characters acquired by habits repeated during many
generations gradually accumulate in the form of hereditary
dispositions in the germinal protoplasm. It is certain that our brain
has progressed since the time when our ancestors were similar to the
gorilla, or even the cave man at the beginning of the quaternary age.
How can this cerebral progression be explained only by selection which
can only eliminate, and by crossings which by themselves can hardly
raise the average? It is here that the intervention of an unknown
power is necessary, something unexplained, the action of which has
been lately recorded in the phenomena of mutations of _de Vries_.

_De Vries_ proves that certain variations appear suddenly and without
any known cause, and have a much greater tendency to be preserved than
the variations obtained by crossing and selection. In my opinion the
phenomena of the mneme revealed by _Hering_ and _Semon_ explain the
apparent contradictions which have hitherto impaired the theories of
heredity. Mnemic engraphy explains, by its infinitesimal and repeated
action through numerous generations, how the external world may little
by little transmit to the germinal cells the characters which it
impresses on organisms. The eight hundred generations during which the
prepuce of the Jews has been cut off have not yet sufficed for the
ecphoria of the corresponding negative mnemic engraphia; while
conjugation and selection modify rapidly and strongly in a few
generations; a fact which is more striking and allows of direct
experiment. Moreover, a positive engraphia must necessarily act more
powerfully, and it seems to me that mutations must be the ecphoria of
accumulated former latent engraphias.

_Merrifield_ and _Standfuss_, by exposing caterpillars and chrysalids
for varying periods to considerable degrees of cold and heat, have
determined permanent changes in the specific characters of the
butterflies which have emerged from them.

_Standfuss_ and _Fischer_ have also shown that, after several
generations, by continuing the action of cold on the caterpillars, the
variations thus produced can be preserved even after the cold has
ceased to act. No doubt the cold acts on the germinal cells as on the
rest of the body, but the heredity of an acquired c