154 



KNOVV^LEDGE 



[March 1, 1886. 



feet in front, and the tail ending in a spine, and, from 

 their size, it -will be easily understood that they work 

 creat havoc in fir-trees in which they have established 

 themselves, devouring, as they do, the solid timber. 

 "Wlien the insect reaches the end of its larval life, it 

 fm-ms a silken cocoon in its burro^y, and in this changes 

 to a pujja, which, as is customarily tbe case amongst the 

 Hymenoptera, looks like a mummy of the perfect insect. 

 In this same burrow it enters on its perfect life by 

 casting its pupal skin : but, when thus freed, it has still 

 to make its way into the open air ; its burrow has already 

 been carried as far as the bark of the tree, and it therefore 

 has now to perforate the bark in order to escape from its 

 prison. This it does by gnawing through it, and then 

 creeping through the opening thus made. It often 

 recruits its strength after its exertions by sitting on the 

 tree-trunk ju.st outside for a time, before starting on its 

 noisy flight. 



When domiciled with man, however, its escape from 

 its prison-house is not always .so easy. At a militarj' 

 clothing-store in France, one of the shelves on which the 

 clothes were jailed contained a pair of Sirices, presumably 

 in the larval cnnditiou when first introduced. On 

 arriving at maturity, the insects proceeded to work their 

 way out of the wood as usual, but when they reached 

 the surface, they fou.nd their further progress barred b}- 

 the piles of clothing, which happened to consist of a 

 number of pairs of woollen trousers. Nothing daunted, 

 however, they set to work upon these also, and pierced 

 them in several directions, as they had previously done 

 the wood, until at last they reached daylight, when, as a 

 rather disapjiointing reward for their perseverance, they 

 fell into the hands of one of the officers, who was himself 

 an entomologist. 



Like one of the longicorn beetles before alluded to, this 

 insect has sufficient strength and perseverance not to be 

 hindered in its burrowing operations, even by so for- 

 midable an obstacle as sheet lead — or, indeed, by a still 

 thicker layer of the same metal. Two instances of this 

 have been reported to the Entomological Society of 

 France by M. Lucas. In one instance it was a lead- 

 covered roof that wns perforated, the lead being about 

 one-eighth of an inch in thickness. The other was a very 

 curious case. It occurred in an arsenal at Grenoble. A 

 box of cartridges was discovered in which some of the 

 bullets had been pierced by tliese insects, the explanation 

 apparently being that the larva^ had been in the wood of 

 which the box was composed, and that the perfect insects, 

 in endeavouring to work their way out, had directed their 

 course inwards instead of outwards, and had thiis en- 

 countered the cartridges, through which they had been 

 compelled to eat their way ; some of them, however, had 

 perished in the attempt, and they were found dead in the 

 box, with their beautiful yellow bodies blackened with 

 the lead and powder. 



There is an allied species, called S. jurencus, in which 

 the female has a shorter ovipositor, and is entirely of a 

 splendid steel blue colour. This also occurs in houses, 

 similarly to S. giijas, which it equals in destructiveness 

 as well as in size. Some years ago, no less than two 

 hundred fir-trees were destroyed by this insect on a large 

 estate in Norfolk. It seems probable, however, that the 

 Sirex must not be charged altogether with this wholesale 

 destruction ; the insects appear to have a tendency to 

 attack trees that are already enfeebled by disease or 

 damage, instead of those that are vigorous and healthy, 

 and, therefore, perhaps in some cases they merely 

 accelerate a death which could not have been long 

 delayed. Still, of course, when they do attack a tree, 



they often utterly spoil the wood as timber, by their 

 niimerous burrows in all directions. As an instance of 

 this, we may take a tree that was found in Bewdley 

 Forest some years ago. Twenty feet of the length of this 

 tree was so perforated by this insect as to be completely 

 useless as timber, and serviceable for nothing but fire- 

 wood. It was transferred to an oiithouse, and while 

 lying there for some months, the insects emerged from 

 their burrows at the rate of some five or six a day. It is 

 curious to note that the first specimens hatched were 

 chiefly males, but, as time went on, the females became 

 more numerous and the males less so, till at last only 

 females appeared. 



(To be coniinued.) 



- THE EARTH'S PAST. 



By Richabd A. Peoctok. 



HE earth's surface has long been recognised 

 as presenting a stupendou.sly difiicult 

 series of problems — jiroblems indeed 

 which can never be fully solved. So 

 soon as men gave up the old idea that 

 the crust had been fashioned originally 

 much as it is now — so soon as, turning 

 over the leaves of the great earth-volume, 

 they began to read what is recorded there, they found, in 

 the first place, that the record runs back over millions cf 

 past years, and in the second place, that it is full of gaps, 

 of blurred pages, of scarce interpretable passages. Yet 

 imperfect though the record is in many places, and hard 

 to read in others, it at least tells us clearly the general 

 history of the earth from the time when first there were 

 lands and seas in her surface as now, and when the rival 

 forces of denudation on the one hand and of land-making 

 on the other began the contest which has continued for 

 millions of years in the past, and will last for millions of 

 years yet to come. 



We no longer, indeed, look back over such a uniform 

 series of changes as the earlier students of geology con- 

 templated. We no longer regard the layers of the earth 

 as comparable with those of an onion, or formed in uni- 

 form succession as to time. We see, for examiale, that 

 even as, in our own age, the denuding forces are forming 

 new strata out of the materials of Quaternary rocks here, 

 out of Tertiary rocks there, of Cretaceous, Jurassic, and 

 Triassic rocks elsewhere, and in other vast regions, even 

 out of the Primary rocks down to the Lower Silurian 

 and Cambrian, nay even to the Archtean rocks them- 

 selves, so it has been all the time. The crust of the 

 earth has never presented features purely Pleistocene, or 

 Pliocene, or Miocene, or Eocene — or presented, indeed, 

 any uniform aspect at all ; and as the formations have 

 never been uniformly presented, so also the strata have 

 never been uniformly laid down. We can no more say 

 the earth was at one time carboniferous and at another 

 cretaceous, than we can say that the soil of England was 

 in such and such an era waste, at another time pasture 

 land, at another crop land. 



Tet we can look back over the past history of the earth 

 and recognise her constant, though not uniform, pro- 

 gression from her Archwan condition to her present state. 

 The problems thus presented by the earth's history, 

 while stupendously difficult in detail, are yet so far 

 soluble that we can find in the action of air and water 

 on the one hand, and subten-anean forces on the other, the 

 explanation of the general jn-ogression of the earth to 



