een republished, without any alteration, after a lapse of
forty years--I made a critical study of Lamarck's theory of descent
and of Darwin's theory of selection, and endeavoured to bring the
complex phenomena of heredity and adaptation under definite laws for
the first time. Heredity I divided into conservative and progressive:
adaptation into indirect (or potential) and direct (or actual). I then
found it possible to give some explanation of the correlation of the
two physiological functions in the struggle for life (selection), and
to indicate the important laws of divergence (or differentiation) and
complexity (or division of labor), which are the direct and inevitable
outcome of selection. Finally, I marked off dysteleology as the
science of the aimless (vestigial, abortive, atrophied, and useless)
organs and parts of the body. In all this I worked from a strictly
monistic standpoint, and sought to explain all biological phenomena on
the mechanical and naturalistic lines that had long been recognised in
the study of inorganic nature. Then (1866), as now, being convinced of
the unity of nature, the fundamental identity of the agencies at work
in the inorganic and the organic worlds, I discarded vitalism,
teleology, and all hypotheses of a mystic character.

It was clear from the first that it was essential, in the monistic
conception of evolution, to distinguish between the laws of
conservative and progressive heredity. Conservative heredity maintains
from generation to generation the enduring characters of the species.
Each organism transmits to its descendants a part of the morphological
and physiological qualities that it has received from its parents and
ancestors. On the other hand, progressive heredity brings new
characters to the species--characters that were not found in preceding
generations. Each organism may transmit to its offspring a part of the
morphological and physiological features that it has itself acquired,
by adaptation, in the course of its individual career, through the use
or disuse of particular organs, the influence of environment, climate,
nutrition, etc. At that time I gave the name of "progressive heredity"
to this inheritance of acquired characters, as a short and convenient
expression, but have since changed the term to "transformative
heredity" (as distinguished from conservative). This term is
preferable, as inherited regressive modifications (degeneration,
retrograde metamorphosis,