332 THE FORMS OF TISSUES [ch. 



they may be rounded or oval capsules, often very neatly con- 

 structed, out of mud, or vegetable^6re or little stones, agglutinated 

 together with a salivary glue; but they shew, except for their 

 rounded or tubular form, no mathematical symmetry. The social 

 wasps and many bees build, usually out of vegetable matter 

 chewed into a paste with saliva, very beautiful nests of ''combs' ; 

 and the close-set papery cells which constitute these combs are 

 just as regularly hexagonal as are the waxen cells of the hive-bee. 

 But in these cases (or nearly all of them) the cells are in a single 

 row ; their sides are regularly hexagonal, but their ends, from the 

 want of opponent forces, remain simply spherical. In Melipona 

 domestica (of which Darwin epitomises Pierre Huber's description) 

 "the large waxen honey-cells are nearly spherical, nearly equal in 

 size, and are aggregated into an irregular mass." But the spherical 

 form is only seen on the outside of the mass ; for inwardly each 

 cell is flattened into "two, three or more flat surfaces, according 

 as the cell adjoins two, three or more other cells. When one cell 

 rests on three other cells, which from the spheres being nearly 

 of the same size is very frequently and necessarily the case, the 

 three flat surfaces are united into a pyramid ; and this pyramid, as 

 Huber has remarked, is manifestly a gross imitation of the three- 

 sided pyramidal base of the cell of the hive-bee*." The question 

 is, to what particular force are we to ascribe the plane surfaces 

 and definite angles which define the sides of the cell in all these 

 cases, and the ends of the cell in cases where one row meets and 

 opposes another. We have seen that BarthoKn suggested, and it 

 is still commonly beheved, that this result is due to simple physical 

 pressure, each bee enlarging as much as it can the cell which it 

 is a-building, and nudging its wall outwards till it fills every 

 intervening gap and presses hard against the similar efforts of 

 its neighbour in the cell next door|. But it is very doubtful 



* Origin of Species, ch. vni (6th ed., p. 221). The cells of various bees, 

 humble-bees and social wasps have been described and mathematically investigated 

 by K. Miillenhoijf, Pfliiger's Archiv xxxii, p. 589, 1883; but his many interesting 

 results are too complex to epitomise. For figures of various nests and combs see 

 (e.g.) von Biittel-Reepen, Biol. Centralbl. xxxiii, pp. 4, 89, 129, 183, 1903. 



t Darwin had a somewhat similar idea, though he allowed more play to the 

 bee's instinct or conscious intention. Thus, when he noticed certain half-completed 

 cell-walls to be concave on one side and convex on the other, but to become perfectly 

 flat when restored for a short time to the hive, he says: "It was absolutely im- 



