28 
of paper this way and simply curled it in that shape. (Illustrating. ) 
The young of these carboniferous forms are formed like that; the 
whorls do not touch. When you take a young cone like that and 
examine this portion of it you find this depression, which was 
purely mechanical in origin throughout the Silurian and Devonian, 
and dependent upon close coiling is here inherited before the 
whorls touch. 
That, then, seems to be as far as possible, without demonstration 
by experiment, a clear case of the acquisition of a characteristic in 
the earlier periods of the evolution of a group, through the purely 
mechanical effect produced by the mode of growth of the shell, and 
then the inheriting of the same in the young of carboniferous forms 
before any of those mechanical causes which originated this charac- 
teristic could have their influence on the growth ofthe shell. While 
it was still young, still uncoiled, still like its ancestors in every 
- way, it inherits this acquired character, which never appeared in 
them until later in life, and was retained in them only so long as 
the originating mechanical causes continued to bear on the shell 
during its growth. 
Then to complete the history after the carboniferous, I have inves- 
tigated the different forms to see if it were continuous. We find it 
is present in the same type throughout the jura, cretaceous and 
trias, and finally, examining the last existing forms, of which there 
are only four species, of nautiloids now living, the same character- 
istic is well developed in the young. 
Then following up another line, taking the Ammonoids, which is 
the more complicated type, and which terminates in the cretaceous, 
we can pass through the entire group, and we find this character- 
istic increasing and becoming more and more important. Finally, 
we strike in the jura certain degraded forms, and ultimately in the 
cretaceous forms which are the reverse of those with which they began. 
Just asin old age we are ina measure the reverse of our adult period, 
just as in that condition we put on certain infantile characteristics, 
which are produced by the wear and tear of life, these types through 
their evolutionary history go back on their history, and part with 
characteristics that distinguish their higher development and become 
simpler. Instead of being coiled up they become uncoiled, having 
young which are coiled up and adults which are uncoiled, and in 
following this characteristic through that long reverse series of 
forms it is found to disappear precisely in accordance with certain 
