216 THE DOCTRINE OF DESCENT. 
from one another. It is extremely interesting to study 
the development of the Inflata of the upper white Jura, 
which follow the Ammonites liparus (whose externally 
visible convolutions display only one row of spines), and 
carefully break off convolution by convolution. Towards 
the middle there is a region in which there are always 
two rows of spines ; nearer the centre the innermost row 
disappears ; soon afterwards the outer one also; and the 
nucleus, some millimetres in diameter, now appears for 
about half a turn as a Planulatum, with distinct ribs, 
which, towards the beginning, likewise disappear. Thus 
even the Planulate ribs, which prevailed among the 
Liassic ancestors of these Inflata, and were supplanted by 
the spines as early as in the brown Jura, still distinguish 
these later and essentially modified descendants during 
a short period of their youth.” 
Wiirtenberger further shows how 
these relations can be simply ex- 
plained by the Darwinian theory 
alone; “ without it we should have 
only an extraordinary problem.” 
It was natural to test the applica- 
bility of the theory of selection also 
on the forms allied tothe Ammonites, 
suchas the Ancyloceras; namely, the 
genera in which the convolutions do 
not touch and partially conceal one 
another, as in genuine Ammonites, 
and which, as late comers and side 
shoots of the group, seemed des- 
Fic. 2. Ancstecerss ~— tined to decay. Selection and decay? 
Wiirtenberger shows how the abandonment of contact 
