CHAPTER XIX. 



PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHAR- 

 ACTER; ILLUSTRATED BY THE EVOLUTION OF THE 

 SUTURE LINES OF AMMONOIDS. 



The Ammonoids Illustrate the Law of Acquirement of Differences 

 T>y Gradual Modification. The Ammonoids illustrate another 

 of the laws of evolution in a particularly satisfactory manner. 



When we examine the representatives of the same genus, 

 or family, or order, at the beginning and at the close of its 

 life-period, it is very common to find the two representatives 

 differing in one or more characters, which may be described 

 .as differing in the degree or extent of their development. 

 The number of parts has increased ; some part which is small 

 in one is large in the other; some structure which is simple 

 in the earlier is complex in the later; or parts which are in- 

 definite in form, or similar in the beginning are definite and 

 particular in form and structure at the close. 



It is rare, however, to be able to collect examples to show 

 the various stages by which the one was elaborated by degrees 

 of modification into the other. The famous case of the de- 

 velopment of the specialized horse foot out of a five-toed an- 

 cestor is familiar to all, with the beautiful theory of the way 

 by which the modification came about. This is a case of 

 relative rather than of direct evolution, since the prominence 

 of the one toe and line of connecting bones is produced by 

 the aborting and withdrawal from use, and finally from devel- 

 opment, of the normal number of parts which were present at 

 the beginning of the series. The Ammonoids, as we shall 

 see, illustrate the case of actual increase in complexity, grad- 

 ually and continuously ; the order of succession in the steps 

 of progress being clearly and regularly expressed by the actual 

 appearance of each form at the particular geological stage in 



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