Nooti] ORGANIC VARIATION. 259 
After these preliminary explanations, we may next consider 
the phenomena of continuing development. 
II. ContTINUING ORGANIC DEVELOPMENT IS ALWAYS ACCOM- 
PANIED BY VARIABILITY. 
Although this postulate may seem at first sight to be a mere 
enunciation of a well-known biological axiom, and a sine gua 
non of the theory of development, it is nevertheless of great 
importance for arriving at a true conception of the nature of 
variation; and although evolutionists in general will grant 
with Darwin that for the action of Natural Selection the 
occurrence of variations is necessary, yet to my knowledge no 
one has particularly accentuated the fact of this actual con- 
comitance of variations with continuing development. Indeed, 
most biologists have accepted this fact, without a critical 
inquiry into its fundamental importance. In my last paper! 
I laid particular stress upon this point, by saying (p. 483): 
“ Now I consider this variability in the number of the eyes 
of the freshwater forms to be explained by the general 
law, that all organs (and propter hoc all organisms) which are 
undergoing progressive or regressive development tend to be 
variable.’ In the present paper I hope to substantiate the 
validity of this “general law’ by data from another source. 
A. Certain Criteria of Continuing Development. 
In order to prove the assumption that continuing progressive 
and regressive development is always accompanied by varia- 
bility, it is necessary to produce a series of facts, showing that 
organs (or organisms) which are undoubtedly in a state of con- 
tinuing development always evince variability. But, although 
examples of variation may be found in abundance, it is obviously 
difficult to prove conclusively that a given organ (or organism) 
is at a given time influenced by a continuing process of de- 
velopment. Accordingly, for each example to be cited, we 
1 “The Derivation of the Freshwater and Land Nemerteans, and Allied Ques- 
tions.” JOURNAL OF MorPHOLOGY, XI, 2, 1895. 
