846 A. Hyati—Biologicai Relations of the Jurassic Ammonites. 
into being is practically unlimited except by the possibilities of 
the typical discoidal form of the embryo and young, 
quent development in each series becomes more and more limited 
according to the size of the group. The same law of inherit- 
ance which renders the embryonic form of the third stage fixed 
or invariable in each individual of the trae Ammonites of the 
Jura, subsequently accomplishes the same purpose, to a less 
degree and with greater fluctuation, for the later developed 
forms and characteristics of each separate series or group, oblig- 
ing them to evolve, if they progress at all, a certain succession 
of forms which have been described above. : 
It will be noticed that I use the word progress in a special 
sense as applicable to a certain class of paralled forms and not 
to those with which we shall presently deal, the old-age forms, 
which though equally perfect in the phenomena of parallelism, 
cannot be attributable to growth. The former are the mecban- 
ical results of the growth or increase in size of the shell of the 
common embryonic form of the third and succeeding stages of 
the young, while the latter result from the natural but inevit- 
able loss of growth-force in the adult shell and its parts. 
This growth seems to me to be due to the favorable nature 
of the physical surroundings, primarily producing characteris- 
tic changes which become perpetuated and increased by inher- 
itance within the group. We can recognize this in the con- 
are due toa healthy adult condition or to old age; whether 
they precede or succeed the supposed period of reproduc- 
series of senile parallel forms could have no existence. 
_ This constant tendency to reproduce the ancestral character- 
istics at earlier and earlier stages accounts for the reduction of 
the principal characteristics of the Nautiloids and Goniatites to 
