On Aquatic Caniivorous Cohoptera or DytisciJcje. 183 



have attempted to make on a purely synthetical method. Linnaeus and the more im- 

 mediate of his followers made their classifications by a deductive or analytical 

 method. Natural objects were taken for granted as forming three separate kingdoms, 

 animal, vegetable, and mineral — and then each of these hypothetical or axiomatic 

 kingdoms was divided and subdivided until the genus and species were reached 

 and defined. This method — most useful in the early stages of scientific develop- 

 ment — is being giadually replaced by an inductive or synthetic system. In other 

 ■words species are studied and defined and then treated as units whose relations to 

 one another may be exhaustively considered. This method has been clearly 

 recognized by fTuxley as will be seen by consulting his paragraph on morphological 

 groups ("Man. Anat. Invertebrated Animals," p. 17) : and it is quite evident that 

 only by this synthetical method can we hope to ascertain whether genera and the 

 more complex aggregates have any real existence in nature, or whether they must 

 remain — as they undoubtedly are at present — merely conventional arrangements. 

 On tlie Linntean method it may be said with perfect truth that the genera were 

 made by the system, whereas it is clear that on a natural science method the system 

 must be made and built up in accordance with the actual structures ascertained 

 detail by detail. If there be an order in Nature our zoological systems must be 

 made in accordance therewith, and representative thereof; if there be no such order, 

 we may then revert to the Linnafjan method, for this is well adapted for purposes of 

 artificial arrangement. At present I scarcely think there is any conscientious 

 naturalist who after a careful review of facts would say whether there are or not in 

 Nature, independent of classifications, such things as genera. I believe, if we limit 

 our view to the creatures coexisting at the present moment, no naturalist could be 

 found who would venture to deny the^existence of species as real and objective. 

 It is in fact perfectly clear that the hosts of individuals livingaround us are arranged in 

 clusters or groups, isolated from other clusters or groups ; and although there may be 

 doubts as to the actual number of such clusters — and doubts as to whether certain 

 masses of individuals form two isolated clusters or only one, yet no practical naturalist 

 will be found who will deny the reality of the existence and isolation of such clusters, 

 and it is these we call species. The nature of the connection between the individuals 

 of these clusters, and the kind of isolation existing between them are most difficult 

 questions, but their discussion has been commenced by Lamarck, Darwin and others; 

 and these problems are now recognized as legitimate subjects for scientific investiga- 

 tion, although perhaps but few perceive their excessively complicated and difficult 

 nature. 



If, however, it be granted that species have a real existence, and if their characters 

 have been well ascertained, it is clear that we may then deal with them as units to 

 be classified and arranged, preliminary to considering the question whether these 

 species are, like the individuals of which they are composed, arranged in naturally 

 isolated clusters. This is evidently a much more complex question to deal winh 



