the formation of vegetable mould-第42章
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defined layer or mantle。 Hensen ｛83｝ placed two worms in a vessel
18 inches in diameter； which was filled with sand； on which fallen
leaves were strewed； and these were soon dragged into their burrows
to a depth of 3 inches。 After about 6 weeks an almost uniform
layer of sand； a centimeter （0。4 inch） in thickness； was converted
into humus by having passed through the alimentary canals of these
two worms。 It is believed by some persons that worm…burrows； which
often penetrate the ground almost perpendicularly to a depth of 5
or 6 feet； materially aid in its drainage； notwithstanding that the
viscid castings piled over the mouths of the burrows prevent or
check the rain…water directly entering them。 They allow the air to
penetrate deeply into the ground。 They also greatly facilitate the
downward passage of roots of moderate size； and these will be
nourished by the humus with which the burrows are lined。 Many
seeds owe their germination to having been covered by castings； and
others buried to a considerable depth beneath accumulated castings
lie dormant； until at some future time they are accidentally
uncovered and germinate。
Worms are poorly provided with sense…organs； for they cannot be
said to see； although they can just distinguish between light and
darkness； they are completely deaf； and have only a feeble power of
smell； the sense of touch alone is well developed。 They can
therefore learn but little about the outside world； and it is
surprising that they should exhibit some skill in lining their
burrows with their castings and with leaves； and in the case of
some species in piling up their castings into tower…like
constructions。 But it is far more surprising that they should
apparently exhibit some degrees of intelligence instead of a mere
blind instinctive impulse； in their manner of plugging up the
mouths of their burrows。 They act in nearly the same manner as
would a man； who had to close a cylindrical tube with different
kinds of leaves； petioles； triangles of paper； &c。； for they
commonly seize such objects by their pointed ends。 But with thin
objects a certain number are drawn in by their broader ends。 They
do not act in the same unvarying manner in all cases； as do most of
the lower animals； for instance； they do not drag in leaves by
their foot…stalks； unless the basal part of the blade is as narrow
as the apex； or narrower than it。
When we behold a wide； turf…covered expanse； we should remember
that its smoothness； on which so much of its beauty depends； is
mainly due to all the inequalities having been slowly levelled by
worms。 It is a marvellous reflection that the whole of the
superficial mould over any such expanse has passed； and will again
pass； every few years through the bodies of worms。 The plough is
one of the most ancient and most valuable of man's inventions； but
long before he existed the land was in fact regularly ploughed； and
still continues to be thus ploughed by earth…worms。 It may be
doubted whether there are many other animals which have played so
important a part in the history of the world； as have these lowly
organized creatures。 Some other animals； however； still more lowly
organized； namely corals； have done far more conspicuous work in
having constructed innumerable reefs and islands in the great
oceans； but these are almost confined to the tropical zones。
Footnotes：
｛1｝ 'Lecons de Geologie Pratique；' tom。 i。 1845； p。 140。
｛2｝ 'Transactions Geolog。 Soc。' vol。 v。 p。 505。 Read November 1；
1837。
｛3｝ 'Histoire des progres de la Geologie；' tom。 i。 1847； p。 224。
｛4｝ 'Zeitschrift fur wissenschaft。 Zoologie；' B。 xxviii。 1877； p。
361。
｛5｝ 'Gardeners' Chronicle；' April 17； 1869； p。 418。
｛6｝ Mr。 Darwin's attention was called by Professor Hensen to P。 E。
Muller's work on Humus in 'Tidsskrift for Skovbrug；' Band iii。 Heft
1 and 2； Copenhagen； 1878。 He had； however； no opportunity of
consulting Muller's work。 Dr。 Muller published a second paper in
1884 in the same periodicala Danish journal of forestry。 His
results have also been published in German； in a volume entitled
'Studien uber die naturlichen Humusformen； unter deren Einwirkung
auf Vegetation und Boden；' 8vo。； Berlin； 1887。
｛7｝ 'Bidrag till Skandinaviens Oligochaetfauna；' 1871。
｛8｝ 'Die bis jetzt bekannten Arten aus der Familie der
Regenwurmer；' 1845。
｛9｝ There is even some reason to believe that pressure is actually
favourable to the growth of grasses； for Professor Buckman； who
made many observations on their growth in the experimental gardens
of the Royal Agricultural College； remarks （'Gardeners' Chronicle；'
1854； p。 619）： 〃Another circumstance in the cultivation of grasses
in the separate form or small patches； is the impossibility of
rolling or treading them firmly； without which no pasture can
continue good。〃
｛10｝ I shall have occasion often to refer to M。 Perrier's
admirable memoir； 'Organisation des Lombriciens terrestres' in
'Archives de Zoolog。 exper。' tom。 iii。 1874； p。 372。 C。 F。 Morren
（'De Lumbrici terrestris Hist。 Nat。' 1829； p。 14） found that worms
endured immersion for fifteen to twenty days in summer； but that in
winter they died when thus treated。
｛11｝ Morren； 'De Lumbrici terrestris Hist。 Nat。' &c。； 1829； p。 67。
｛12｝ 'De Lumbrici terrestris Hist。 Nat。' &c。； p。 14。
｛13｝ Histolog。 Untersuchungen uber die Regenwurmer。 'Zeitschrift
fur wissenschaft。 Zoologie；' B。 xix。； 1869； p。 611。
｛14｝ For instance； Mr。 Bridgman and Mr。 Newman （'The Zoologist；'
vol。 vii。 1849； p。 2576）； and some friends who observed worms for
me。
｛15｝ 'Familie der Regenwurmer；' 1845； p。 18。
｛16｝ 'The Zoologist；' vol。 vii。 1849； p。 2576。
｛17｝ 'Familie der Regenwurmer；' p。 13。 Dr。 Sturtevant states in
the 'New York Weekly Tribune' （May 19； 1880） that he kept three
worms in a pot； which was allowed to become extremely dry； and
these worms were found 〃all entwined together； forming a round mass
and in good condition。〃
｛18｝ 'De Lumbrici terrestris Hist。 Nat。' p。 19。
｛19｝ 'Archives de Zoologie experimentale；' tom。 vii。 1878； p。 394。
When I wrote the above passage； I was not aware that Krukenberg
（'Untersuchungen a。 d。 physiol。 Inst。 d。 Univ。 Heidelberg；' Bd。
ii。 p。 37； 1877） had previously investigated the digestive juice of
Lumbricus。 He states that it contains a peptic； and diastatic； as
well as a tryptic ferment。
｛20｝ On the action of the pancreatic ferment； see 'A Text…Book of
Physiology；' by Michael Foster； 2nd edit。 pp。 198…203。 1878。
｛21｝ Schmulewitsch； 'Action des Sucs digestifs sur la Cellulose。'
Bull。 de l'Acad。 Imp。 de St。 Petersbourg； tom。 xxv。 p。 549。 1879。
｛22｝ Claparede doubts whether saliva is secreted by worms： see
'Zeitschrift fur wissenschaft。 Zoologie；' B。 xix。 1869； p。 601。
｛23｝ Perrier； 'Archives de Zoolog。 exper。' July； 1874； pp。 416；
419。
｛24｝ 'Zeitschrift fur wissenschaft。 Zoologie；' B。 xix； 1869； pp。
603…606。
｛25｝ De Vries； 'Landwirth。 Jahrbucher；' 1881； p。 77。
｛26｝ M。 Foster； 'A Text…Book of Physiology；' 2nd edit。 1878； p。
243。
｛27｝ M。 Foster； ut sup。 p。 200。
｛28｝ Claparede remarks （'Zeitschrift fur wisseuschaft。 Zoolog。'
B。 19； 1869； p。 602） that the pharynx appears from its structure to
be adapted for suction。
｛29｝ An account of her observations is given in the 'Gardeners'
Chronicle；' March 28th； 1868； p。 324。
｛30｝ London's 'Gard。 Mag。' xvii。 p。 216； as quoted in the
'Catalogue of the British Museum Worms；' 1865； p。 327。
｛31｝ 'Familie der Regenwurmer；' p。 19。
｛32｝ In these narrow triangles the apical angle is 9 degrees 34
seconds； and the basal angles 85 degrees 13 seconds。 In the
broader triangles the apical angle is 19 degrees 10 seconds and the
basal angles 80 degrees 25 seconds。
｛33｝ See his interesting work； 'Souvenirs entomologiques；' 1879；
pp。 168…177。
｛34｝ Mobius； 'Die Bewegungen der Thiere；' &c。； 1873； p。 111。
｛35｝ 'Annals and Mag。 of N。 History；' series ii。 vol。 ix。 1852； p。
333。
｛36｝ 'Archives de Zoolog。 exper。' tom。 iii。 1874； p。 405。
｛37｝ I
