Heredity 139 
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 con- 
servative 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 morpho- 
logical 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 off- 
spring 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 modi- 
fications (degeneration, retrograde metamorphosis, etc.) come under 
the same head. 
Transformative heredity—or the transmission of acquired charac- 
ters—is one of the most important principles in evolutionary science. 
Unless we admit it most of the facts of comparative anatomy and 
physiology are inexplicable. That was the conviction of Darwin no 
less than of Lamarck, of Spencer as well as Virchow, of Huxley as well 
as Gegenbaur, indeed of the great majority of speculative biologists. 
This fundamental principle was for the first time called in question 
and assailed in 1885 by August Weismann of Freiburg, the eminent 
zoologist to whom the theory of evolution owes a great deal of 
valuable support, and who has attained distinction by his extension 
of the theory of selection. In explanation of the phenomena of 
heredity he introduced a new theory, the “theory of the continuity 
of the germ-plasm.” According to him the living substance in all 
organisms consists of two quite distinct kinds of plasm, somatic and 
germinal. The permanent germ-plasm, or the active substance of 
the two germ-cells (egg-cell and sperm-cell), passes unchanged 
