12 INTRODUCTION 
Hildebrand summarizes the results of his investigations in the following 
sentences (pp. 81, 82):— 
1. The arrangements in most flowers are of such a kind that there is no 
self-pollination, but transference of the pollen from flower to flower is effected. 
2. For this transference insects are in most cases necessary. 
3. The prevention of self-pollination necessarily implies the prevention of 
self-fertilization. 
4. In cases in which self-pollination is possible, or even inevitable, the possi- 
bility of cross-pollination is usually not excluded. 
5. In these cases, also, insects are active agents for effecting cross-pollination. 
6. There are probably no flowering plants in which cross-pollination is not 
possible, at least in some of the flowers, and in which constant self-pollination alone 
is possible; and therefore there are probably no flowering plants which furnish proof 
against the law that continuous self-pollination and self-fertilization are avoided, 
7. Experiments have shown that in certain cases in which self-pollination 
was inevitable, or was artificially effected, there was nevertheless no self-fertilization ; 
or if this did occur, the production of seed was less abundant than with cross- 
fertilization—a fact that agrees with the law to which reference has just been 
made. 
8. A progressive series may be made out, starting from cases in which 
self-pollination and therefore self-fertilization are absolutely impossible, and leading 
to those in which it is possible, or even actually takes place, but in which the 
possibility of cross-pollination is not excluded. 
g. The mode of distribution of the sexes and the kind of fertilization do 
not always agree in flowers that show morphological relationship. In certain 
families all the species are alike in sexual relations; but there are other families, 
or even genera, of which the species differ from one another altogether in this 
respect. Sexual relationships, accordingly, have not developed at the same rate 
and in the same way as morphological relationships, in the course of the meta- 
morphosis and development of the flowering plants. 
Hildebrand in his work gives to the Knight-Darwin law a somewhat different 
meaning, for he says (p. 5): Zhere are no sexual plants which can constantly reproduce 
themselves by self-fertilization alone ; cross-fertilization is possible in all ; tn most cases 
self-fertilization ts prevented by special adaptations, or 2s impossible, or at least is not 
advantageous, while cross-fertilization alone can occur, does actually occur, or has good 
results. 
Two years later (in 1869) Severin Axell’ published a work ‘Om anordningarna 
for de fanerogama vaxternas befruktning.’ In this he gave a summary of floral 
‘ According to information received by me from Professor A. G. Nathorst, of Stockholm, 
Johann Severin Axell was born on October 22, 1843; was a student in Upsala in 1861, and became 
Doctor of Philosophy and ‘ Docent’ there in 1869. In 1868 he published in the ‘ Bot. Notisar’ 
a treatise ‘Om det fargade hyllets betydelse for vaxten,’ and afterwards, in 1869, published as his 
degree thesis on taking the doctorate, the above-named work ‘Om anordningarna for de fanerogama 
vaxternas befruktning.’ He soon, however, gave up his scientific career on the death of his father, 
whose business as a timber-merchant in Sundsvall he took over. Subsequently he became a member 
of the Swedish Reichstag, and died at Wiesbaden on January 1, 1892. 
