C.—GEOLOGY. 105 
3. Not less remarkable than the period of ‘ retreat * is that of booming 
development which at last came to each successful modification. In this 
connexion we can instance the ‘ pleine évolution’ of the graptolites, the 
 euechinoids, ammonoids, and belemnoids, the fishes, reptiles, birds, and 
mammals, each in its own time. Each slowly but surely built up its supre- 
macy, and then wantoned through long ages as the lord of creation in its 
own element and in its own day. Both the period of sanctuary and the 
subsequent boom can be closely paralleled by the case of many human 
inventions and in the occupations and history of mankind. 
4. But while there are outstanding cases in which a line of advance 
is taken that is capable of successive improvements and leads on to con- 
tinuous success, there are many other instances in which the line of 
advance, though temporarily advantageous, has only been carried through 
a limited number of stages, and eventually failed either by its inherent 
inadequacy or by imposing so heavy a burden on the economy of the 
_ organism that it was unable to bear the cost. 
The only instance I need quote, though there are many others, is the 
use of defensive armour, spines, plates, hooks, horns, &c. These provide 
an obvious method of resistance to attack, and this defensive attitude has 
been practised by one group of organisms after another, but always with the 
same disastrous result, the imposition of a fatal strain on the organism to 
meet renewed, perfected, and more vigorous attack. The spinose grapto- 
lites and trilobites, the armoured fishes and reptiles, are cases in point, 
and in the last of these instances, at least, victory rested with the acquire- 
ment of swiftness in movement, accompanied by increasing power of 
attack such as is given by the development of teeth or claws or both. 
Again and again in the Tertiary Era one group of mammals after another, 
before, or more usually after, the attainment of great size, has taken to 
some means of sedentary defence, and in every case the cost of upkeep 
has been too great and the group has gone under. Every time the race 
has been to the swift, active, and strong, and those that trusted in 
“passive resistance,’ in ‘defence and not defiance,’ have gone under in 
competition with those that have been prepared to face the risks involved 
in attack. The fact that turtles and armadilloes have survived to the 
present endorses rather than vitiates the principle. 
Other cases of rapid decline or sudden disappearance are more difficult 
to account for. The waning of the brachiopods but not yet their dis- 
appearance, the disappearance of the pteridosperms, the rugose corals, 
_ the belemnoids and ammonoids synchronising with the vanishing of many 
orders of reptiles, will long furnish subjects for research by biologists 
and geologists. And it may well be that the explanation will often lie 
along biological rather than physical lines, such as those suggested for 
_ the graptolites ; Lapworth pointed out that their disappearance—in spite 
of a brave effort of passive resistance—synchronised with the great develop- 
ment of fishes, and the assumption by them of many of the functions pre- 
viously discharged by the trilobites. In other cases the explanation may be 
more in the direction of that given for the reptiles to be referred to later. 
The rarity in the geological record of some of the stages in evolution, 
and the absence of others which must surely have existed, may receive 
some explanation from what has frequently occurred in the history 
of human invention. If variants arise and are subjected to intense 
