On Aquatic Carnivorous Coleoptera or Dytiscidce. 955 



iiifeqiialis we see the other extreme in these respects, and we find a small and 

 acuminate prosternal process, with very approximate coxae and exposed posterior 

 coxal articulations. If we take a third form of the Hydroporides, Dytiscus 

 duodecimpustulatus of the genus Deronectes for example, and continue our com- 

 jiarison, we find in it a general shape and swimming legs more adapted for 

 aquatic locomotion ; the head is completely without margin and the labrum is 

 quite exposed. The middle and hind legs are less approximate than in D. intequalis, 

 the prosternal process is much broader and the posterior coxal cavities more pro- 

 tected. As the result of the comparison of the three species we find then, that 

 having regard to the general form and the development of the swimming legs, a 

 supposed line of descent, thus — 1. Hydrovatus ; 2. Coelambus ; 3. Deronectes; 

 while if we look to the distance of the coxae, the form of the prosternal process, 

 and the protection of the hind coxal cavities, the order would be — 1. Elydrovatus ; 

 2. Deronectes ; 3. Coelambus ; and if again we look to the structure of the head, we 

 find that Hydrovatus is the intermediate form, and the line of descent would be 

 — 1. Coelambus {D.insequalis) ; 2. Hydrovatus; 3. Deronectes. It ajjpears, there- 

 fore, that a natural classification in which all the structures are dealt with o-ives no 

 support to the idea of a genetic relationship between these three forms ; while in 

 an artificial classification — a classification in which some one or two characters 

 should be taken into account to the exclusion of others — the three forms would 

 stand in different relationships to one another according to what jsoint of structure 

 should be selected as the basis of the classification. It does not however follow 

 that in a natural classification it would be incorrect to speak of any one of these 

 three species as being higher than the others, for if we were acquainted with all 

 the points of structure, and knew accurately their functional value, we would be 

 able to assign to each organ a numerical value, indicative of its orade of develop- 

 ment (as for instance, in the case of the swimming legs, 1 to Hydrovatus, 3 to 

 Coelambus, and ."3 to the Deronectes), and by adding all these together we would 

 be able to say with certainty which was absolutely the higher form ; but because 

 we could do this it would not in the least follow that the lower form was ancestral to 

 the higher. 



This is equally applicable to the relations of the anomalous Pelobius and 

 Amphizoa to the other Dytiscidse ; although they are less highly developed as 

 Dytiscidre there does not appear any reason for supposing them to be truly 

 ancestral forms to their more highly developed allies, and thus the fact that certain 

 of the lower forms have numerous points in common does not indicate a genetic 

 relationship between them; the similarities found are indeed indicative not of such 

 relationship, but rather of similarity in the stage of development of some one or 

 more of the points of structure. 



TRANS, nor. DUB. SOC., N S., VOL. II. g Q. 



