462 BUMPUS. [VoL. XII. 
It will be observed that the rise in the curve of relative 
lengths is quite abrupt until the specimens are 252 mm. long, 
when the ascent is gradual until the specimens are 330 mm. 
long, from which point to the longest specimen in the series 
the ascent is again rapid. The curve then shows that the 
larger number of specimens are between 252 and 323 mm. in 
length. If the lengths are examined in profile, as shown by the 
red curve on the right of Plate C, it will be observed that the 
culminating point of constancy is around that specimen which 
has a length of 260 mm. 
As the sum total of opportunities for death are directly 
dependent upon the length of existence, it is not surprising 
that the upward trend of the curve is considerably more abrupt 
than the descent of senescence. I am inclined to look upon 
the absence of specimens measuring between 245 and 255 mm., 
causing the depression in the ascending curve, as accidental, 
though it is barely possible that the depression signifies some 
change in the rate of growth, consequent upon the approach of 
sexual maturity (Minot, ’91); or it may be dependent upon the 
season of capture. I am also at a loss to explain the depres- 
sion in the curve of descent in the neighborhood of specimens 
having a length of 280 mm. 
Before leaving this section it should be noted that the pro- 
portion of homceotic specimens is not the same among the 
shorter as among the longer specimens. Of the first fifty 
specimens, but ten are homeeotic, while of the second fifty 
twenty-five present cases of forward homeeosis. 
It has already been shown that the mere addition of a few 
terminal caudal vertebrze cannot alone account for the increased 
size of homaeotic specimens. The fact, then, appears, if our 
specimens are not exceptional, that this homceotic departure is 
a character of increasing frequency among such animals as 
have met with success in the struggle for existence. If this 
is the case, the present species of Necturus may be considered 
as undergoing a process of “mutation,” since the causes of 
transformation are acting with considerable “uniformity upon 
large numbers of individuals” and “in a definite manner” 
(Scott, ’94). 
