362 • SCOTT. [Vol. V. 



to each other than they are to those of any other genus, recent 

 or extinct. But as genera are at present employed and in the 

 existing state of knowledge, such an exact expression of relation- 

 ship is impracticable, as that would necessitate a minute knowl- 

 edge of the phylogeny of each species, such as we are very far 

 from possessing. In the current usage, a genus is a group of 

 nearly allied species agreeing among themselves and differing 

 from all others in the possession of some common character. 

 But if the various species of the ancestral genus may acquire 

 the new character independently of each other (parallelism), or 

 if the species of widely different genera may gradually assume 

 a common likeness (convergence), then it is plain that such 

 a genus is an artificial assemblage of forms of polyphyletic 

 origin. That such parallelism of development does occur, we 

 shall see in the next section, and there are good reasons for 

 believing that convergence is not so rare a phenomenon as it is 

 generally assumed to be ; from which it follows with great prob- 

 ability that many generic groups are not real expressions of 

 relationship, but artificial assemblages of similar forms. 



While it is easy to make these distinctions theoretically, in 

 practice it is a matter of extreme difficulty, even under the most 

 favorable circumstances, at least so far as cases of parallel de- 

 velopment are concerned. Many of the known cases of con- 

 vergence show that this process is more apparent than real, and 

 may generally be unmasked upon careful examination, though 

 even here the cases of the Ammonites, presently to be men- 

 tioned, show the need of extreme caution. The mutual rela- 

 tionships of the various species of two successive genera are 

 usually very obscure. In a given case each of the five species 

 of genus A may seem to have been derived from a different 

 species of genus B, and yet the assumption of such a mode of 

 derivation may prove to be altogether erroneous, because of the 

 tendency so frequently to be observed for each member of a 

 descending series to develop a similar cycle of variations. 

 Neumayr (No. 40, p. 61) has observed this among invertebrates : 

 " Ein anderer bisweilen beobachteter Fall von grosser Wich- 

 tigkeit ist der, dass die verschiedenen Glieder einer Reihe 

 Variationen derselben Art zeigen ; wahrend also ein Theil die 

 Merkmale gleichmassig nach einer Richtung im Laufe der Zeit 

 mutirt, zeigen andere Charaktere regellose Abanderungen und 



