52 . GENERAL EVOLUTION. 



the fourth, when amounting to anything more important than 

 the loss of a few ornaments, was marked by a retrogression of the 

 whorl to a more tabular aspect, and by the partial degradation of 

 the septa." 



I will here quote an entirely antagonistic statement of 

 Bronn's,* as follows : ** In the development of lamellibranchiate 

 mollusks it is not possible to estimate the successional changes of 

 one genus by those of another, though nearly related ; so diverse 

 are the most significant relations in the manner of progress among 

 nearest allies. Therefore, embryologic indications are throughout 

 useless in classification, and it is necessary to keep carefully se^oa- 

 rate the statements of observations on development of a given 

 species, and not transfer such facts to the history of another spe- 

 cies for the purjDose of completing it. We can not even range 

 these histories in conformity with family groups." For us this 

 statement, though no doubt largely true, is an indication of im- 

 perfection — first, of knowledge of true affinities of recent, but 

 esi^ecially of extinct adults, and, second, of imperfection of knowl- 

 edge of development. The position api^ears to be based on nega- 

 tive evidence, while the opposing can and does stand on nothing 

 but positive. 



/?. Examples of the Inexact Parallelism. 



1. The genera of the batrachian family Scaphiopodidae form 

 a series of steps differing a little more than as repressions or joer- 

 manent primary conditions in the development of the highest, f 

 Thus two of the genera, which are North American, maintain 

 their tubae eustachii and tympanum through life, while three 

 European lose them at an early period. J; The three European 

 genera also advance beyond the larval character of the Ameri- 

 can in the ossification of the basis of the xiphisternum into a 

 broad style. Thus we have two series established, which differ 

 only in the two characters named. Each shows its develop- 

 mental steps in a similar manner, the European series extending 

 further ; thus : 



* " Classen u. Ordnungen des Thierreichs," iii, p. 445. 



f See "Journal Academy," Philadelphia, 1866, on Areifera. 



X According to Bruch and Tschudi, in Pelobates. I have found traces of the 

 eustachian diverticula, in a tailed Pelobates fuscus, whose body measured 1 in. 4 lin., 

 from Mus. Peabody Institute, Salem, Mass. 



