2l6 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 afterw^ards 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, tow^ards 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 to the Ammonites, 

 such as 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- 

 FiG. 20. Ancyloceras. tiucd to dccay. Sclcctlon and dccay .<* 

 Wiirtenberger shows how the abandonment of contact 



