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 

 Lingulae 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 



