102 Proceedings of Boy at Society of Edinburgh. [ses&. 
England suggested about the same time a psychical aspect of the 
hereditary continuity. The two suggestions may he so far summed 
up together. Memory is a general function of organised mat 
and the reproduction of parental likeness is the result of unconscious 
recollectionhf the past. What are ordinarily called memory, habit, 
instinct, and embryonic reconstruction are all referable to the 
memory of living matter. Hering finds the basis of this uncon- 
scious memory in the persistence of the undulatory movements sup- 
posed to be characteristic of the molecules. These undulations are 
sensitive to change, and room is thus left for variability, but their 
tendency to persist in their established harmony is the basis of 
heredity. 
11 The Perigenesis of the Plastidules .” — Haeckel also emphasised 
the luminous metaphor of “ organic memory,” and sought to analyse 
it in terms of molecular motion. His theory is summed up in ;the 
characteristic phrase “perigenesis of the plastidules.” Comparing 
the course of historic development to a complex, ramified series of 
wave lines, in which a single life is represented by a single wave,, 
he imagines a similar ontogenetic wave-motion in the development 
of the individual. The invisible activity of the organic molecules 
is, he believes, a branched wave-motion, continuous with that of 
the history — such is “ the perigenesis of the plastidules.” “ The 
developing impulse which in the one case is transferred from the 
ancestral species to the whole group of species, and in the other case 
from the ancestral cell to the entire group of cells, assumes in both 
cases the same form of a branching wave-motion.” “ The true and 
ultimate causa efficiens of the biogenetic process, I propose to 
designate by a single word, Perigenesis — the periodic wave-genera- 
tion of the organic molecules or plastidules.” The tendency that 
this periodic motion has to persist, preserving as it were a charac- 
teristic rhythm, explains the relative constancy of ordinary inherit- 
ance, while at the same time the results of new experience may be 
added on to the dominant molecular movement. In very simple 
organisms, as he says, the plastidules have, so to speak, learned little 
and forgotten nothing, while in highly perfected types the plasti- 
dules have both learned and forgotten much. Haeckel thus empha- 
sises on the one hand the psychical, on the other the physical or 
molecular aspect of the real continuity. 
