58 PRINCIPLES OF PALAONTOLOGY. 
ment of a great formation die out at or before its close, whilst 
those which are introduced for the first time near the middle 
or end of the formation may either become extinct, or may 
pass on into the next succeeding formation. As a general 
rule, it is the animals which have the lowest and simplest 
organisation that have the longest range in time, and the 
additional possession of microscopic or minute dimensions 
seems also to favour longevity. Thus some of the Forami- 
nifera appear to have survived, with little or no perceptible 
alteration, from the Silurian period to the present day ; whereas 
large and highly-organised animals, though long-lived as zmdz- 
viduals, rarely seem to live long sfeczfically, and have, there- 
fore, usually a restricted vertical range. Exceptions to this, 
however, are occasionally to be found in some “persistent 
types,” which extend through a succession of geological 
periods with very little modification. Thus the existing 
Lampshells of the genus Zzzgzla are little changed from the 
Lingule which swarmed in the Lower Silurian seas; and the 
existing Pearly Nautilus is the last descendant of a clan 
nearly as ancient. On the other hand, some forms are singu- 
larly restricted in their limits, and seem to have enjoyed a 
comparatively brief lease of life. An example of this is to 
be found in many of the Ammonites—close allies of the Nau- 
tilus—which are often confined strictly to certain zones of 
strata, in some cases of very insignificant thickness. 
Of the causes of extinction amongst fossil animals and 
plants, we know little or nothing. All we can say is, that the 
attributes which constitute a sfeczes do not seem to be intrin- 
sically endowed with permanence, any more than the attri- 
butes which constitute an zzdivzdual, though the former may 
endure whilst many successive generations of the latter have 
disappeared. Each species appears to have its own life- 
period, its commencement, its culmination, and its gradual 
decay ; and the life-periods of different species may be of very 
different duration. 
From what has been said above, it may be gathered that 
our existing species of animals and plants are, for the most 
part, quite of modern origin, using the term ‘‘ modern ” in its 
geological acceptation. Measured by human standards, the 
majority of existing animals (which are capable of being 
preserved as fossils) are known to have a high antiquity; 
and some of them can boast of a pedigree which even the 
geologist may regard with respect. Not a few of our shell- 
fish are known to have commenced their existence at some 
point of the Tertiary period; one Lampshell (Zerebratulina 
