TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 99 



often based on very slender differences, so as to have a sufficient 

 number of distinctive features to enable their position in the scheme of 

 classification to be determined without too much difficulty. 



The value of generic characteristics is moreover very different in 

 various groups, and authors also interpret them very differently. More- 

 over, this cannot be otherwise in the present state of diatomography 

 so long as so many forms are still unknown to us, and others are only 

 known by just a few rare valves or even in some cases by solitary ones. 



In the same way I have been logically compelled to adopt certain 

 genera rejected by Professor Smith. Such, for example, as those founded 

 on a cuneate form. The Gomphonema are nothing else but cuneate 

 Navicula. If the genus Gomphonema be admitted, it is impossible to 

 refuse to admit Meridian, Gomphonitzschia, Trachysphenia, etc. 



If difference of beading is of any importance, it is impossible to unite 

 Sceptroneis caduccus with Raphoncis, with which Trachysphenia is also 

 intimately connected, and it is also impossible, in my opinion, to unite 

 Sceptroneis caduceus with Sceptroneis gemmata, which is closely allied to 

 Opephora. 



Such are a few of the reasons which I give for adopting a 

 much larger number of genera than I previously admitted. In the 

 future, when forms are better known, it will be necessary to decide 

 whether many of the present genera, which I have admitted in this 

 work to facilitate study, should be maintained, or whether they should 

 not rather be considered as generic sections. 



With reference to species (') my ideas have not undergone any alteration. 

 If it is difficult to agree on the relative value of higher plants it is, 

 in my opinion, much more difficult to do so in the case of diminutive 

 diatoms, and it is almost impossible in our present state of knowledge 

 at least to fix with any certainty what forms should be considered as 

 primordial species, that is, species that have given birth to those derived 

 forms called secondary and tertiary species, varieties, etc., and which bear 

 a more or less close relationship or resemblance to the primordial form. 



( ' ) I will here refer to a few classical definitions : 



A species is an aggregate of individuals which have been proved to have descended from a 

 common ancestor, or are so similar to one another that they may be presumed to have done so. 

 J. D. Hooker. 



YJespice est la reunion des individus descendus l'un de l'autre ou de parents communs, et de 

 ceux qui leur ressemblent autant qu'ils se ressemblent entre eux. CuviER. 



La variation est une forme passagere et fugace difficile a conserver. 



La race est une variation qui se conserve habituellement par selection artificelle, rarement par 

 selection naturelle. 



La variete est une variation plus profonde qui se conserve tant que durent les causes au milieu 

 desquelles elle s'est produite. Cauvet. 



