Life Progressive. 411 
in existence are noticeable, and therefore corresponding dif- 
ferences in their vertical range, or in the actual amount and 
thickness of strata through which they present themselves 
as fossils, some species being found to extend through two or 
three formations, and even a few have had a more prolonged 
existence. More commonly, however, the species which 
begin in the commencement of a great formation die out at 
or before its close, while those which are introduced for the 
first time near its middle or end may either become extinct 
or pass into the next succeeding formation, animals of the 
lowest and simplest organization as a rule having the longest 
range in time. Microscopic or minute dimensions seem to 
favor longevity, for some of the Foraminifera appear to have 
survived, with little or no perceptible alteration, from the 
Silurian Period to the present day, whereas largely and 
highly-organized animals, though long-lived as individuals, 
rarely seem to live long specifically, and consequently have 
a restricted vertical range. Exceptions to this rule are, how- 
ever, occasionally found in some persistent types, the Lamp- 
shells of the genus Lingula being little changed from the 
Lingulz that swarmed in the Lower Silurian seas, while the 
existing Pearly Nautilus is the last descendant of a clan 
nearly as old. Some forms, on the other hand, the Ammo- 
nites, which are closely related to the Nautilus, and mostly 
restricted to certain zones of strata, seem to have enjoyed a 
comparatively brief lease of life. 
But of the causes that have led to the extinction of plants 
and animals, little or nothing is known. All that can be 
affirmed, in our present knowledge, is that the attributes con- 
stituting a species do not seem to be intrinsically endowed with 
permanence, any more than those constituting an individual, 
though the former may endure whilst many successive gen- 
erations of the latter have disappeared from the earth. Each 
species, it would seem, has its own life-period—its beginning, 
culmination and decay—the life-periods of different species 
being of very different duration. From all that has been 
