282 



KNOWLEDGE. 



[December 1, 1894. 



have already been visited. Such contrivances for saving 

 their visitors' time benefit the flowers in the long run, 

 since the visitors are thereb}' enabled to overtake a larger 

 number of blossoms ; these facilities, in fact, increase their 

 rates of industry. There are, however, other flowers 

 which instead of promoting the industry of their visitors 

 imprison their guests, detaining them for a day or longer. 

 Such is the eifect of the well-known mouse-trap arrange- 

 ment of hairs in Aristolochia, Ceropejia, Arum, and a few 

 others. Flowers of this class are frequented in great 

 numbers by minute flies or midges, which hover in clouds 

 near the plants. On account of their numbers, little or 

 nothing would be gained by arrangements to facilitate 

 speedy visitation, and as little is lost by the detention of 

 such insects. These flowers rely on the numbers of their 

 visitors, not on their industry, and hence honey-guides and 

 other conveniences are mostly absent. Aristolochia is 

 protogynous, the stamens not being matured until after the 

 stigma has been pollinated, hence the necessity for detaining 

 the visitors until the pollen is shed ; were they allowed to 

 depart before this happened, they could confer no benefit 

 on the next flower they entered. Further, there is in 

 Aristolochia a special provision against revisitation, for 

 the flower-tube becomes inverted soon after the pollen 

 is discharged. We can hardly be in doubt, therefore, 

 as to what were the conditions out of which this curious 

 mouse-trap contrivance arose ; the number of insects 

 must have been greatly in excess of the flowers, and 

 they must have belonged to the class of unsystematic or 

 careless workers. On the other hand, the arrangements 



for facilitating 

 visits, so common 

 m flowers, could 

 only arise with the 

 flowers greatly in 

 excess of the 

 insects, and with 

 insects of indus- 

 trious habits, 

 given to flower 

 visitation more or 

 less continuous. 

 Open shallow 

 flowers with ex- 

 posed honey or 

 pollen, being 

 accessible to all 

 insects, attract a 

 great variety of 

 visitors but the 

 overlapping of 



1. — First or female stage. — H, mouse-trap visits must be SO 

 hairs ; M, stamens not vet mature ; F, pistils great that only a 

 ' ^ -'■ ■—'■■'- small proportion 



of the visits can 

 beefi'ective. Deep 

 tubular flowers 

 adapted to the proboscis of a bee or moth exclude 

 short-lipped flies, and though attracting fewer visitors, 

 yet attract a more industrious class, so that as regards 

 cross-fertilization, the efticiency is as great or greater 

 than in the case of shallow flowers with more numerous 

 guests. The restriction is, therefore, advantageous, 

 even apart from the circumstance that bees and butter- 

 flies are in respect of size and shape more desirable 

 visitors. A difliculty is presented by very deep tubular 

 flowers adapted exclusively to Lepidoptera, of which 

 Lonicera, Lychnis, Gymnademia, and Habenaria are 

 examples. Awjrcecum sesquipcdidc, an orchid belonging 



Arvm. 



capable of fertilization. 



2.— Seeond or male stage.^H, mouse-trap 

 hairs withered; M, stamens discharging pollen; 

 F. pistils fertilized in preceding stage. 



to Madagascar, with a spur ten or eleven inches 

 long, is an extreme example of the same type. Flowers 

 of this class exclude even bees. Now although as 

 regards adaptation to flowers the Lepidoptera take the 

 first place among insects, they are only of second or 

 third rate importance as fertilizing agents. In this 

 respect bees decidedly take the first place. They have, 

 moreover, generally been accredited with greater diligence 

 than butterflies. It is not easy, therefore, to see how the 

 industry of individual visitors could be a factor in bringing 

 about the adaptation of deep tubular flowers to the 

 Lepidoptera. A little reflection, however, serves to show 

 that the principle applies even here. It hardly admits 

 of doubt that these very deep flowers are derived from 

 ancestors which had their honey accessible to bees. Now 

 we only need to suppose that the bees got into the way of 

 avoiding the deepest flowers on account of the difliculty 

 experienced in obtaining the nectar ; not that fewer bees 

 visited the deep-tubed flowers, but that • each bee paid 

 fewer visits to them. By this means not only would the 

 total visitation of the deeper flowers be reduced and 

 diligence in consequence gain ascendancy over numbers, 

 but in relation to the deep flowers bees would become 

 intermittent or casual visitors, and the value of their visits 

 would be greatly reduced. Finding they enjoyed a 

 monopoly of the nectar, butterflies would frequent the deep 

 flowers with increased diligence, and their visits, from less 

 overlapping, would become more valuable. Honey and pollen 

 bestowed on casual visitors is for the most part wasted ; 

 and natural selection must inevitably lead to the exclusion 

 even of bees, notwithstanding their exceptional industry, 

 when they cease to visit any species of flower dihgently. 



The penalty of tmused power is its removal ; a door which 

 a person does not often enter is likely to close against him 

 permanently. This punishment has overtaken the bees, 

 who now find the deep-tubed flowers their predecessors 

 neglected closed against them for ever. This has apparently 

 taken place in the Alps, where MiiUer found that many 

 flowers, which in the lowlands are adapted to bees, have in 

 the uplands become specialized for Lepidoptera. This 

 result he ascribed to the scarcity of bees in the higher 

 regions, but the scarcity of an insect can hardly account 

 for its exclusion : rather we should expect this condition 

 to bring about keener competition for the services of the 

 insects, and increased attractions on the part of the flowers. 

 The scarcity of bees cannot, therefore, be the direct cause 

 of this change ; it may, however, bring it about indirectly. 

 As the num'ber of bees diminishes they have a greater 

 choice of flowers, and each honey-gatherer will naturally 

 go to the deeper and more difficult flowers as seldom as 

 possible. In relation to these flowers bees will thus acquire 

 the character of casual or indolent visitors. At the same 

 time, on account of diminished visitation, the balance is 

 turned against numbers in favour of diligence. The 

 butterflies meantime becoming more diligent so far as the 

 deeper flowers are concerned, and the bees less so, we 

 have the conditions fulfilled which lead to the exclusion 

 of the occasional visitors, and more perfect adaptation 

 to the habitual frequenters of the flowers. The same 

 process in course of time would lead to still further 

 specialization, such as is seen in Angnecum, from which 

 the honey can only be removed by a moth having a 

 proboscis nearly a foot in length. Butterflies and moths 

 in some respects are, no doubt, preferable to bees as 

 pollen-carriers ; they are more purely suctorial, and do 

 not consume such quantities of pollen as bees, neither do 

 they use the mouth -parts for other purposes ; a greater 

 differentiation of these in relation to flowers has, therefore, 

 been possible than in the case of bees. Miiller states 



