444 INFERIOR OOLITE AMMONITES. 



Table IX presents the classification in another manner. It is constructed 

 in the form of a genealogy ; but in reality it only expresses the same thing as the 

 Classification (pp. 384 — 391) is designed to show, namely, the bringing together 

 of the species which may be considered to be genetically connected, because they 

 have the greatest similarity — not merely that similarity in external features, for 

 such forms are only morphic equivalents, and their relative positions may be seen 

 in Table X (p. 449), — but developmental similarity in all features, whether they be 

 striking or obscure. It is the relationship which any particular feature bears to 

 the other features present that is taken into consideration; in other words, that 

 degree of development of one or more features which may happen to be correlated 

 with a given degree of development of other features in any one species becomes a 

 determinant for similarity. Further, in regarding one particular species in 

 relation to any other particular species consideration must be given to the 

 degree of likeness in septation, or whether the differences exhibited are such as 

 would accord with the unlikeness, if any, in whorl-shape or other characters. 

 Guidance in this matter may perhaps be obtained by knowledge of what differences 

 would accompany change in whorl-shape, either from a purely mechanical point of 

 view, or as shown by the ontogeny of any kindred species. In the same manner, 

 difference in whorl-shape and difference in ornament must be considered in relation 

 to the ontogenetic changes shown in the species with which comparison is 

 instituted. 



Certain critics of molluscan genealogy appear to suppose that it is only lack 

 of imagination which prevents the tracing of a genetic series from anywhere to 

 anywhere. This statement is not worthy of serious scientific attention, but for the 

 sake of young students the following rather dogmatic remarks may be made. A 

 series of Ammonites, each one having distinctly different septa from its predecessor 

 or successor, cannot be arranged in a direct genetic line, however much their 

 external similarity of form may suggest such a blunder. Nor can Ammonites 

 with different proportions — thick-whorled and thin-whorled, latumbilicate and 

 augustumbilicate species — be arranged indiscriminately ; nor can uncarinate, 

 carinate, and carinate-bisulcate forms be arranged in a haphazard manner, — in 

 fact, species cannot be arranged phyletically without a distinct knowledge of the 

 ontogenetic sequence of the various features which they possess. Attention to 

 these matters, and a due observation of rudiments, whether the features be 

 nascent, and therefore such as may be expected to increase in the descendants, or 

 whether they be obsolescent — in other words, vestigial of what has gone before, — 

 will make the possible genealogies exceedingly restricted, and banish imagination 

 from the field altogether. One thing should be carefully remembered. Mistakes 

 will arise from not paying due attention to all the characters ; and this is the 

 hardest faculty to acquire. A superficial similarity — in Ammonites, generally 



