EVOLUTION 1089 



Savory suggests that the primitive type of web consisted of an 

 expansion of the tube which Unes the burrow. The vibrations caused 

 by tripping insects would be felt by the spider lurking in the hole. 

 Almost every stone wall in Northern France shows a large spider 

 (Segsstria florentina) living in such a retreat. 



As a gaping hole is rather a temptation to inquisitive aggressive 

 creatures, such as centipedes, it is easy to understand the advantage 

 of making a trap-door as many spiders do. But this was a later 

 achievement. The theory of deriving a web from an extension of 

 the mouth of the silk tube lining the burrow seems shrewd; we 

 should like to add the suggestion that some of the rough-and-ready 

 webs may have arisen, apart from any tube, from tangles and snares 

 of silk among the grass and herbage. 



The next stage in evolution was the cobweb, familiar in the 

 house-spiders, Tegenaria. There is still a silken tube for a resting- 

 place, but the extension of the fringe is almost confined to the 

 lower edge, which is spread out horizontally as a hammock-like 

 sheet. Cobwebs are not confined to indoors; the gleaming white sheet 

 spun hy A gelena labyrinthica is common on gorse bushes in autumn. 

 Supporting the hammock there are mooring threads, and as insects 

 are apt to strike the upper ones, an extension of these might well be 

 the next step in evolution. 



When the sheet is raised to a more or less exposed situation, 

 where flying insects are more likely to blunder into it, the tube has 

 to be dispensed with; and the spider takes up its position on the 

 under-surface of the web, as may be seen on every bramble-bush. 

 But an exposed web is apt to be torn by the wind; and thus we 

 can understand that many spiders prefer to do one of two things 

 — to weave small sheets near the ground, where we often see them 

 glistening, or to dispense with the sheet and trust to a tangle of 

 Unes in all directions. 



Some regard these tangles, familiar in the hedges, as primitive webs 

 in the making, but Mr. Savory has given reasons for interpreting them 

 as degenerate rather than primitive, simplified rather than simple. 



The climax is the orb-web, familiarly illustrated by the garden 

 spider's work of art. The characteristic features here are: (i) the 

 reduction of the net to two dimensions, (2) the uniform covering of 

 the area by the simplest sequence of movements, and (3) the making 

 of a net that is unified, so that it can be held relatively taut, and so 

 that vibrations are readily transmitted by a special thread to where the 

 spinner lurks. 



IMPERFECTIONS IN ANIMALS 



In a famous deliverance Walt Whitman maintained that all 

 animals are "equally perfect". But this is not borne out by 

 VOL. II AA 



