94 INTRODUCTION 
the odour of orange-flowers, which also occurs in the flowers of species of Citrus, 
and in a few species of Magnolia. The citron-odour of Thymus citriodorus, 
Th. montanus and Dictamnus is due to citron-oil contained in the flowers, that of 
Lavendula to the oil of lavender. 
Kerner’s classification of odours is a very praiseworthy constructive effort, as 
here, for the first time, grouping has been attempted on a sczentific foundation, 
according to the chemical composition of the odoriferous substances. That it is 
only an attempt Kerner himself emphasizes. There is great difficulty in determining 
the odours of flowers, more especially because it is largely a subjective matter: 
as Kerner remarks—one observer thinks he recognizes the odour of vanilla where 
another perceives that of the violet. Both may be right, because, as a matter of 
fact, two kinds of odour may simultaneously emanate from the same flower. But 
it must be added that the sense of smell is very liable to be deceived; while taste and 
sight may both be concerned with such illusion (Kerner, op. cit., p. 203). When we 
see a carnation, the odour of carnations at once comes to mind. This may happen 
before the odour proceeding from the carnation reaches the organs of smell. It is 
therefore recommended that in determining an odour the flower should not be looked 
at at all, the investigation being made with closed eyes. 
Odours agreeable to bees, lepidoptera, and hover-flies are also, as a rule, 
acceptable to man, while many (e.g. indoloid and aminoid odours) which are 
pleasing to flies are disagreeable to human beings. Carrion-flies and dung-flies 
in particular (Sarcophaga, Calliphora, Scatophaga, Lucilia, and others) take pleasure 
even in odours that are disgusting to us, licking and probing products of decom- 
position (dunghills, decomposing flesh, liquid manure, pus, carrion) the colour and 
smell of which fill us with loathing. There are also certain small flies and gnats, espe- 
cially moth-flies (Psychodidae), which are everywhere common in closets, and delight 
in the above-named disgusting substances. All these insects prefer to visit flowers 
possessing odours or colours repulsive to us, and which have therefore already been 
described (see p. 67) as Mauseous Flowers. Kerner (op. cit., p. 206) also advances 
the view that many odours which are not perceived by man, are perceptible to 
certain insects. As already mentioned (see p. 87), Kerner explains the regular visits 
of Andrena florea to the small greenish flowers of Bryonia dioica, which, though 
half hidden among foliage, it knows how to find, by supposing the existence of an 
odour which is only perceived by this particular bee. The flowers are practically 
odourless to human beings. The inconspicuous green flowers of Ampelopsis quin- 
quefolia, scentless to us, are visited by bees, according to Kerner, with great 
eagerness and industry. These insects may be seen flying to the plant from all 
sides in a way leaving no doubt that its flowers are recognized at a considerable 
distance. ‘As it is not sight, it must certainly be the smell that leads to this 
recognition. ‘The flowers are odourless to human beings.’ Kerner (op. cit.) gives 
the following additional examples of inconspicuous flowers, which, though apparently 
scentless to man and many animals, are eagerly sought out by particular insects— 
Aristolochia Clematitis, Vaccinium Myrtillus, Chamaeorchis alpina, and Listera 
ovata. Kerner also thinks that the characteristic odour of many brightly coloured 
flowers explains the constancy with which certain insects visit the same species 
more or less exclusively. According to Kerner’s view the odours of such flowers 
