
942 THE AMERICAN NATURALIST. [VOL XXXVI. 
apparently been but recently acquired, judging from the fre- 
quent lateness in appearance of spines in individuals. Thus 
there would be more vitality or spine-forming power left, which 
would be able to overcome any weakening effect of senility 
appearing in other features of the group. Hence, though 
a gerontic form of coiling is assumed by the whorls of Melon- 
gena, the power to form spines is still retained, and these 
appear after an interval in the senile portion of the whorl. 
Lllustrations of the Law of Ti achygenesis and of Parallelism 
among Gastropoda. — Hyatt’s Law of Tachygenesis, or accelera- 
tion in development, teaches us that the adult characters of 
a species are inherited at an earlier stage in the members of a 
succeeding generation. This of course implies a condensation 
of preceding characters, which in some cases may even be 
dropped entirely. The diagram on page 941 (Fig. 16) will give 
a clearer representation of the working of this law. 
Let each of the lines A to / represent the life history of an 
individual gastropod in a phylogenetic series, beginning with A. 
Let the life history be divided into stages numbered from 1 up, 
each stage being characterized by some definite feature in the 
shell. 
No. 1 is the protoconch stage, which persists throughout ; 
No. 2, the smooth, round, whorled naticoid stage; No. 3, the 
primitive ribbed and spiraled stage; No. 4, the angular stage, 
with formation of nodes at the crossing of the ribs; No. 5, 
the keeled stage ; No. 6, the second round-whorled stage; No. 7, 
the spinous stage; No. 8, the spineless smooth stage. 
It becomes apparent that as new characters appear, the older 
ones persist for a shorter and shorter time. This is neces- 
sary, for otherwise each succeeding species must be larger 
to accommodate the new characters. This is acceleration by 
condensation. A character newly acquired in one member 
of the series appears earlier and earlier in the succeeding 
members of that series, thus changing from an adult to an 
infantile character. Some characters are less resistant than 
others, and these will be more quickly condensed. Such is 
the case with those of stages 5 and 6, the keel and the second 
Tround-whorled stage. The nodes are very persistent, and the 
