2l6 THE DOCTRINE OF DESCENT. 



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), 

 carefully breaking off convolution by convolution. 

 Towards the middle there is a region in which there are 

 always two rows of spines; nearer the centre the inner- 

 most row disappears ; soon afterwards the outer one also ; 

 and the nucleus, some millimetres in diameter, now ap- 

 pears for about half a turn as a Planulatum, with dis- 

 tinct ribs, which, towards the beginning, likewise disap- 

 pear. 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 de- 

 scendants 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. 2o.-Ancyioceras. tin^j ^^ ^ecay. Selection and decay? 

 Wiirtenberger shows how the abandonment of contact 

 in the convolutions was to the spinous Ammonites an 



