Träns la tion of 

 Introduction, to top of pa^e 80, 



Dr, d'6virL£:. 



For the study of the natural relationship 

 between different forns of animals a conplete 

 knowledfe of their develop-rient would no doubt 

 abol ish that lack of explicitness by which 

 e^ren the nasters of science of ten seem to be 

 ernbarrassed. Ttiose forms, howe^7er, "which under- 

 po their tb tamorphoses - as iTiany of the Dip te ra 

 do - in swampy and 'noist localities are the re by 

 not easily traced through all stapes. Hence 

 when tryin^; to liiiit the fanilies and groups 

 of tliese s"iall creatures, ve/fc have to confine 

 our investigations to the last stape of their 

 development and stud^y the orf;ans as they appear 

 in. that staf:e. In this respect it is particular- 

 ly important to pay attention to those external 

 organs, which serve the essential life functions 

 because the structural sinilarities or dis- 

 si nilarities exhibited in such ordans correspond 

 to cl öser or -lore re-.iote affinities of t'm insects 

 in questlon. 



It may, hoT^^ever, as a rule he advisable not 

 to affir-p- an,y relationship acoording to the de- 

 •veloprrent of a single organ or a single body part 

 even if this should exhibit the tios t astonishing 

 si :iil arities or dissi nilarities in its structural 

 de tails. 



As realizcd long ago in other spheres of 

 creation so here we f ind that isoinorphisia ex- 

 pressed in a singlc. organ of ten is caused by bare 

 analogous develop^nent of that very' organ in in- 

 sects vrhich really belong to entirely different 

 groups. 



If - on the other hand - the systematic 

 position of a form is clear''- ''-^■'::-' ' r ^he 

 de\'eloprnent of thf: essentia^ but a 



B JUL 3 1 mi i 



