52 



galls, and that the only purpose of a classification is to make a survey 

 easier. Furthermore he considered this classification as "the thread 

 which connects his observations". 



The doctor thesis contains a large number of interesting observa- 

 tions on numerous gall-formations, illustrated by drawings of the 

 stages of growth and of the anatomical structure. Several of these 

 galls have not been dealt with further in BEIJERINCK'S later publica- 

 tions, and the cecidologist may find still some interesting data in this 

 thesis. Of historical importance is the fact that VAN 'T HOFF made a 

 number of determinations at BEIJERINCK'S request of the tannin 

 content of Cynips Kollari-galls in various stages of development. It 

 appeared that the tannin content of unripe galls, picked at the be- 

 ginning of August, is very high, and decreases on ripening. 



In the years immediately following the publication of his thesis 

 BEIJERINCK'S attention was taken up mainly by the study of plant- 

 galls. Field-observations were constantly made in the neighbourhood 

 of Wageningen and further up along the edge of that part of the Ne- 

 therland province "Gelderland" that is indicated as "de Veluwe". 

 In his house and in his garden too, experiments were started, and 

 these solved the problems which had puzzled him on his botanical 

 excursions. Here is BEIJERINCK'S love for experiment awakened! 



A deep impression was made on BEIJERINCK by the discovery of 

 the heterosis which appeared characteristic of many gall-wasps. B. D. 

 WALSH had already found in 1872 that sometimes a parthenogenetic 

 generation of a gall-wasp is followed by a second generation with 

 male and female insects present, but this publication was unknown 

 to BEIJERINCK at the time of the writing of his thesis. Independently 

 of WALSH, heterosis was rediscovered and published by H. ABLER in 

 Schleswig in 1877, and this publication led BEIJERINCK to publish in 

 1880 a short communication on the interconnection of Biorrhiza 

 aptera and Teras terminalis ' ) . These were only preliminary studies, and 

 BEIJERINCK himself stated later that the heterogenesis obtained its 

 "wissenschaf tliche Begriindung" for the first time in 1881 by the "scho- 

 ne Abhandlung ADLER'S : Ueber den Generationswechsel der Eichen- 

 gallwespen". With tireless exertion BEIJERINCK checked and com- 

 plemented ADLER'S observations during that same year and the 

 following. 



In 1882 appeared BEIJERINCK'S standard work on galls "Beobach- 

 tungen iiber die ersten Entwicklungsphasen einiger Cynipidengallen". 

 It was published as a communication of the Royal Academy of 

 Sciences in Amsterdam 2 ). This paper still commands admiration. 

 W T ith unsurpassable clearness numerous observations on the biology 

 of gall-wasps are described, especially on the method of ovulation, on 



!) Entomologische Nachrichten und Zoologischer Anzeiger, 1880. 

 2 ) Verhandelingen Koninklijke Akademie van Wetenschappen Amsterdam 22, 

 1882 (Verzamelde Geschriften 1, 127-281). 



