8 Charles GELDERD 



whether this concordance is îvantîng; or,from a phylogenetic point of i>ieiv, 

 n'hether thc causes which hâve broitght about the external différences, that 

 characterise the varions species and groups ai the présent day, hâve acted ai 

 the same time iipon their internai organisation, and hojpfar they hâve nio- 

 dijied or respected that internai organisation. 



Many naturalists seem to spend their lives in the study of infinitely 

 minute surface détails, with the view of finding spécifie characters, and not 

 to pay the slightest attention to the détails of the inner structure. Allowing 

 that the external characters niay be more important, we consider that the 

 Systems of classification in which the internai anatomy is utterly neglected 

 may be found some day to be erroneous in many points and will necessitate 

 further rearrangement in the future. At ail events, such Systems must remain 

 under suspicion as long as anatomical researches hâve not shown that no 

 discordance in this internai construction exists between forms bundled 

 together in groups based on purely external characters. We do not sug- 

 gest that minute internai anatomical différences are more important than 

 minute external ones, and sufficient to make up groups in spite of the 

 existence of the latter. 



Yet, it is reasonable to ask ourselves, whether, we may neglect at the 

 présent day, without being wanting in scientific exactitude, the variations 

 in thc internai organisation, in the study of the affinities, and in the hypo- 

 thetical establishment of the natural groups. 



The reply to this question of scientific method still calls for vast 

 research, and can only be settled after the accumulation of a great number 

 of data, bearing upon the question of the structure of the internai organs. 

 On account of technical difficulty, perhaps, they hâve been very much less 

 studied than the surface détails. 



A séries of memoirs, undertaken for this purpose, hâve already ap- 

 peared in this review. Besides the interest they possess individually as 

 morphological descriptions, we may say that they form already an impor- 

 tant collection of material, ready to be used by future investigators for the 

 study of this question of parallelism or nonparallelism, m the évolution of 

 the internai and external organs of the Arthropoda. 



We think it may be useful to give hère a list of thèse works : 



Les glandes odorifères du Blaps niortisa^a et d'autres espèces. Prof. 



G. GiLSON. IH88. 



Le poumon des arachnides. L. Berteaux. 1889. 



