Another condition, that of space, is present as a factor. The Pea-weevil is the largest of our Bruchi. When she attains the adult age she requires a bigger lodging than is demanded by the other seed-destroyers. A pea provides her with a very adequate cell; nevertheless, cohabitation in twos would be impossible: there would be no room, even if the occupants accepted the discomfort. And so the inexorable need returns for reducing the numbers and, in the seed invaded, doing away with all the competitors save one.
On the other hand, the broad bean, which is almost as great a favourite of the Bruchus as the pea, is able to house a whole community. The grub that was but now a solitary becomes a cenobite. There is room for five or six more, without encroaching on the neighbours’ domain. Moreover, each grub finds infant-food within its reach, that is to say, the layer which, being at some distance from the surface, hardens slowly and retains the dainty juices for a greater length of time. This inner layer may be regarded as the crumb of an otherwise crusty loaf. [[200]]
In the pea, which is a small sphere, it occupies the central part, a limited area which the grub has to reach or perish; in the bean, a generous muffin, it includes the large joint of the two flat seed-lobes. No matter where the big seed is tackled, each larva need but bore straight ahead and it quickly reaches the coveted food.
Then what happens? I add up the eggs adhering to a bean-pod, I count the seeds inside, and on comparing the two totals, I find that there is plenty of room for the whole family, at the rate of five or six to each bean. Here we have no surplus larvæ dying of starvation almost as soon as they leave the egg: all have their share of the ample portion, all live and prosper. The abundance of the provisions counterbalances the mother’s extravagance.
If the Bruchus always adopted the broad bean as the establishment of her family, I could very well explain her exuberant emission of germs on a single pod: a rich supply of food, easily acquired, invites a large colony. The pea, on the other hand, puzzles me. What vagary makes the mother abandon her offspring to starvation on this insufficient legumen? Why so many boarders gathered around a seed which forms the ration of one alone?
It is not thus that matters are arranged in life’s general balance-sheet. A certain foresight rules the ovaries and makes them adjust the number of [[201]]eaters to the abundance or scarcity of the thing eaten. The Sacred Beetle, the Sphex-wasp, the Burying-beetle and the other manufacturers of preserved provisions for the family set close limits to their fertility, because the soft loaves of their baking, the baskets containing their game and the contents of their sepulchral retting-vat are all obtained at the cost of laborious and often unproductive efforts.
The Bluebottle, on the contrary, heaps her eggs in bundles. Trusting in the inexhaustible wealth of a corpse, she lavishes her maggots without counting the number. At other times, the provision is obtained by crafty brigandage, exposing the new-born offspring to a thousand fatal accidents. Then the mother makes up for the chances of destruction by an excessive outpouring of eggs. This is the case with the Oil-beetles, who, stealing the property of others under very parlous conditions, are for that reason endowed with prodigious fertility.
The Bruchus knows neither the fatigues of the hard worker, obliged to restrict her family, nor the woes of the parasite, obliged to go to the other extreme. Without costly researches, entirely at her ease, merely by strolling in the sun over her favourite plant, she can ensure an adequate provision for each of her children; she can do this, and yet the mad creature takes it into her head to over-populate the pea-pod, a niggardly baby-farm [[202]]in which the great majority will die of starvation. This folly passes my understanding: it clashes so utterly with the usual perspicacity of the maternal instinct.
I am therefore inclined to believe that the pea was not the Bruchus’ original share in the distribution of the earth’s gifts. It must rather have been the bean, one seed of which is capable of entertaining half a dozen visitors and more. With a seed of this size, the startling disproportion between the number of the insect’s eggs and the foodstuffs available disappears.
Besides, there is not a doubt that, of our various culinary acquisitions, the broad bean is the earliest in date. Its exceptional dimensions and its pleasant flavour have certainly attracted man’s attention since the most remote times. It is a ready-made mouthful, of great value to the hungry tribe, which would have hastened to secure its increase by sowing it in the patch of garden beside the house, a hut of wattled branches plastered with mud. This was the beginning of agriculture.