Functional and Ontogenetic Stimuli Alike 307 
add further the following more special ones which are 
quite similar in their nature to the preceding. 
The tails of larval amphibians, cut off obliquely in 
respect to their axes, become regenerated in such a way 
that the axis of the regenerated fragment is always per- 
pendicular to the plane of section and forms consequently 
a certain angle with the axis of the stump. Nevertheless, 
all these tails, because of regulative forces within the 
organism, tend gradually to straighten themselves. Now 
Barfurth has demonstrated that in frog larvae which are 
prevented almost entirely from swimming by putting 
them in shallow water divided into a number of small 
compartments by water plants, this straightening goes on 
in a much less complete manner and much more slowly 
than in those which are placed in deep water and which 
are thus able to swim freely. That is a proof that func- 
tional adaptation of the tail to swimming co-operates with 
the entire action of the internal regulative capacity, or in 
a wider sense with the ontogenetic stimuli, and adds it- 
self to them, intensifying and accelerating their action.??¢ 
An example of the ontogenetic stimulus having not 
yet replaced the functional, or better, being not yet 
endowed with a quantity of potential energy sufficient to 
overcome by itself the resistance which opposes its acti- 
vation, and which has therefore need of this functional 
stimulus in order to commence to act, is furnished us 
by the axolotls. These tailed batrachians can retain 
their external gills indefinitely and live out their lives 
and reproduce their kind with external gills, or be trans- 
formed into amblystomae, according as they are or are 
226Barfurth: Versuche zur funktionellen Anpassung. Arch. f. 
mikrosk. Anatomie, Bd. XX XVII; Drittes Heft. Bonn, Cohen 1891; 
especially P. 403—405. 
