304 MONTGOMERY. [Vou. XII. 
of change in the environment, and upon the degree of dif- 
ferentiation of the organism ; but that a certain maximum 
amount of variation cannot be exceeded by the organism, and 
this amount seems to differ for different organisms. Variations 
must continue to be produced until the correlation of the organs 
is fully restored, when the restraint exerted by this acquired cor- 
relation upon the physiological processes of the several organs, 
would prohibit the production of further variations. Therefore, 
if the variations continue to be produced through a long period 
of time, we must conclude that the correlation of the organs 
has been greatly disturbed, which is equivalent to saying that 
a comparatively great change has occurred in the environment. 
If, however, but one considerable change has occurred in the 
environment and the latter remains thereafter unchanged, then 
the longer the period of time is which has elapsed since this 
change, the nearer at hand will be the restoration of the cor- 
relation of the organs, and consequently the less will be the 
amount of variation. This will serve further to elucidate the 
deduction made in a former paper! of mine (p. 483): that 
“the amount of variability above or below a given mean will 
stand in inverse ratio to the length of time in which the devel- 
opment (progressive or regressive) has acted upon the given 
organ.” If the change of environment be comparatively slight, 
the restoration of the correlation of the organs might be fulfilled 
in a single generation ; but if the change had been more marked, 
this correlation might not be restored until after the lapse of 
a large number of generations, during which time the produc- 
tion of variations would continue, though their amount would 
decrease as the time became longer. 
Here, then, the phenomena of variation may furnish us with 
a criterion for deducing, to some extent at least, the previous 
conditions of existence, if not also the phylogenetic stages of 
some organisms. For we may briefly consider, e.g. the fresh- 
water Nemerteans, which are undoubtedly of marine origin. 
In studying the comparative anatomy of this group of worms, 
I was struck by the fact that while the nearest marine allies of 
1 “The Derivation of the Freshwater and Land Nemerteans, etc.,”” JOURNAL 
OF MorpPHoLoecy, XI, 2, 1895. 
