27 
forms coil tighter and tighter, one whorl lying over the other, the 
inner whorl presses upon and obliges the outer whorl to form this 
depression on the inner side. When the shell gets old the whorl 
quits the spiral and grows out straight, and when that period begins 
in old age this depression, which is formed where the whorls close 
up, gradually disappears, so that in extreme old age you get a return 
to the rounded outline. 
Thus you get throughout the earlier systems in the earth’s history, 
throughout the Silurian and Devonian period, a transient condition. 
You will find that whenever this depression occurs it 1s always 
because one whorl laps over another. When, in the course of growth, 
the shell passes by the whorl, that bay or depression disappears, so 
that you get in every fossil the proof that this characteristic is a 
transient one, that when it occurred it was through the mechanical 
action of the growth of one whorl of the shell over another, as much 
so asa dent in a piece of putty when you put your fist in it ; in other 
words, it is not in the organism and in any shape which would en- 
able us to say it was inherited. The Weissman hypothesis is that 
evolution has taken place by other forces than those which modify 
the organization from the exterior. He says that no characteristic 
which is acquired in this way, mechanically, by growth or the 
action of the externals in any way on the animal, can be inherited. 
It is not inherited. It makes no impression on the organism so 
that it can be inherited. 
We can get the history of this characteristic throughout the 
earlier periods and it justifies the hypothesis. It was supposed by me 
for several years to be one of the strongest points in favor of the 
hypothesis, that an acquired characteristic made no impression 
on the germ and was, in fact; non-inherited. 
This last winter, following out an investigation begun in connec- 
tion with the geological survey of Texas in the carboniferous de- 
posits of that region, I was led to extend my investigation in regard 
to their development and general history. The result was the 
finding that in certain series of the carboniferous this characteristic 
was indubitably inherited. I found in the young of close-coiled 
carboniferous forms, shells which were unquestionably close coiled 
in their adult condition, that in the young of these there was a repeti- 
tion of the characteristics of the adults of the Silurian and Devonian. 
In these very young forms the whorls do not touch when they first 
begin to grow, but are all open, as much so as if I bent this piece 
