xl FACTS AND OBSERVATIONS. 



The causes of extinction are in universal operation. They are cosmical. Silurian life was 

 discontinued everywhere at the same time, proximately. 



There is no example, as far as I know, of a Silurian community rising, by migration or other- 

 wise, into Devonian or Carboniferous strata ; but single species do, and somewhat largely, just as we 

 see in every epoch up to the present. 



Little appears to suffice for extinction, so obscure, delicate, and slow are some of its causes. 

 They are of course of a mixed nature, as was observed at the beginning of these remarks. The chief 

 of those which are mechanical is oscillation. Oscillation alters the form and the proportions of 

 land and sea, changes sea-bottoms, depths, currents, and temperatures, and this in various degrees 

 of intensity, from merely occasioning uneasiness, to the infliction of immediate death. Examples of 

 all this are plentiful. Under the last mentioned state of things the oscillation is irresistible. After 

 a period of confusion and agony, existence ceases, except in cases where the summons to depart was 

 instantly obeyed. This fatality happened to the beautiful Devonian Crinoid, Hypanthocrinus, on 

 the Pennsylvanian shore of Lake Erie ; it was suffocated by a sudden mud -flow. In palaeozoic times 

 changes in climate were probably rare ; but a lowering of temperature is always a powerful cause of 

 extinction. When the change of level is moderate, life may be continued with diminished energies. 

 Readjustments, reparations, and slow accretions of new life now take place. 



Oscillation does not permanently lessen the amount of Silurian or other life ; it changes its 

 forms, and perhaps the precise locality. It may confer new food and shelter, take away or modify 

 either. As long as levels are stationary, genera and species make healthy and happy use of their 

 instincts, with but few intrusions or desertions ; but a change of level brings both. 



When sea-levels are being depressed, all the zones of life are in distress, in proportion to the 

 rate and .extent of the process. It is a process which is always visibly going on now, in some 

 part of the earth or other. The whole marine population then move upwards, with some few 

 exceptions. In times of elevation the general life-movement is downwards, the littoral mollusks 

 being left high and dry to perish ; the red- weed-loving animals are ill at ease on the new littoral, 

 covered with rotting algse, and the nullipore-browsers are equally so in their new place ; probably 

 the deep-sea mollusks lose in quietude, nourishment, and temperature ; so that let there be oscillation, 

 and all animated existence is set in motion, not only within, but beyond the disturbed area ; for 

 wanderers will inconveniently crowd the outer residents in quiet seas. 



But this unstable area may become a place of rest, when it will be gradually peopled by suitable 

 organizations, driven from troubled homes, and glad to occupy the void. This new peopling sea- 

 bottoms from a distance need not perplex the naturalist ; it is an affair of causes, all within the 

 epoch. No form of life alien from the existing epoch can enter, except a few recurrents. There is 

 no mingling of epochs. 



The first Great Cause has granted to all His creatures great liberty of action. Zones of 

 residence are very broad, except for a few. Neither depths nor sediments are adhered to very 

 strictly by the Silurian or any other fauna, the sediments themselves (sea-bottoms) being formed 

 at almost every level. 



After this digression, we add a few words on the vital or physiological causes of extinction. 

 The genus, species, or individual may exhaust its term of life. We see this term (average viability) 

 in all animals. In conformity with it each dies, if permitted by external events. The life-term of 

 species varies exceedingly. By far the larger number of them we have seen to have a very brief 

 existence. The species simplest in point of structure, the Amorphozoa, Coelenterata, Polyzoa, &c., 

 do not enjoy any peculiar longevity, if we are to believe the ' Thesaurus 3 with certain exceptions. 



The mutual relations of the members -of a molluscan community have great influence on its 

 preservation or destruction. The carnivorous portion may be too active, or the herbivorous too few ; 

 and the same may happen to other branches of it. Epidemics arise, touching only one form of life, 

 but nevertheless fatal to the whole. The population may become so large as to press severely on 

 subsistence, when one of two things, or both, will take place an unusual fatality, or a forced 



