858 



TIT.-VNOTHBRES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



[p. 169]. * * * We have before stated that, during seasons 

 of extraordinarj' severity, many northern birds and, in some 

 countries, many quadrupeds migrate soutliwards. If tliese 

 cold seasons were to become frequent, in consequence of a 

 gradual and general refrigeration of the atmosphere, such 

 migrations would be more and more regular, until, at length, 

 many animals now confined to the Arctic regions would become 

 the tenants of the temperate zone; while the inhabitants of the 

 latter would approach nearer to the Equator [p. 169]. * * * 

 But although some species might thus be preserved, every 

 great change of climate must be fatal to many which can find 

 no place of retreat, when their original habitations become unfit 

 for them. For if the general temperature be on the rise, then 

 is there no cooler region whither the polar species can take 

 refuge; if it be on the decline, then the animals and plants 

 previously established between the Tropics have no resource 

 [p. 170]. * * * Let us now consider more particularly the 

 effect of vicissitudes of climate in causing one species to give 

 way before the increasing numbers of some other [p. 172]. 



* * * That they would be supplanted by other species at 

 each variation of climate may be inferred from what we have 

 before said of the known local exterminations of species which 

 have resulted from the multiplication of others. Some minute 

 insect, perhaps, might be the cause of destruction to the huge 

 and powerful elephant [p. 174]. 



[Moving sands.] — If we attribute the origin of a great part 

 of the desert of Africa to the gradual progress of moving sands, 

 driven eastward bj' the westerly winds, we may safely infer 

 that a variety of species must have been annihilated by this 

 cause alone. The sand flood has been inundating, from time 

 immemorial, the rich lands on the west of the Nile, and we 

 have only to multiplj' this effect a sufficient number of times 

 in order to understand how, in the lapse of ages, a whole group 

 of terrestrial animals and plants may become extinct [p. 166]. 



* * * In a small portion of so vast a space, we may infer, 

 from analogy, that there were many peculiar species of plants 

 and animals which must have been banished by the sand, and 

 their habitations invaded by the camel and by birds and insects 

 formed for the arid sands [p. 166]. * * * If it be imagined, 

 for example, that the aboriginal quadrupeds, birds, and otiier 

 animals of Africa emigrated in consequence of the advance 

 of drift sand and colonized Arabia, the indigenous Arabian 

 species must have given way before them and have been 

 reduced in number or destroj'ed [p. 167]. 



[Repopulation through migration.] — So great is the instability 

 of the earth's surface that if Nature were not continually 

 engaged in the task of sowing seeds and colonizing animals, 

 the depopulation of a certain portion of the habitable sea and 

 land would in a few years be considerable [p. 158]. * * * jf^ 

 therefore, the Author of Nature had not been prodigal of those 

 numerous contrivances before aUuded to, for spreading all 

 classes of organic beings over the earth — if he had not ordained 

 that the fluctuations of the animate and inanimate creation 

 should be in perfect harmony with each other, it is evident 

 that considerable spaces, now the most habitable on the globe, 

 would soon be as devoid of life as are the Alpine snows, or the 

 dark abysses of the ocean, or the moving sands of the Sahara 

 [p. 1591. 



Summary. — Summing up Ly ell's opinions of 1831 

 we find them remarkably modern and Darwinian, 

 both in. observation and in reasoning. The chief 

 points as to extinction are the following: (1) The 

 destructive effect of physiographic changes in Ter- 

 tiary time (p. 308) is to be interpreted through the 

 extinction of species at the present time; (2) tempera- 

 ture barriers are important factors in distribution 



(p. 172); (3) the destructive action of floods (p. 199) 

 may be understood through the disappearance of large 

 numbers of horses in South America (pp. 249, 312) 

 and of buffaloes in India (p. 250); (4) animals may 

 have also perished in large numbers in bogs (p. 217); 

 (5) changes of climate induced by physiographic 

 changes (p. 308) influence both distribution (p. 169) 

 and competition between species (p. 172); (6) com- 

 petition between related species is illustrated by that 

 between the horse and ass, as introduced into South 

 America; (7) competition between unrelated animals 

 is illustrated by the unchecked increase of goats 

 (p. 153), of asses (p. 153), of cattle and horses (pp. 

 152-153), and of insects (p. 320); (8) increase, how- 

 ever, suffers many checks, as was first noted by Buffon 

 (p. 154); (9) there is constant competition between 

 established species (p. 142) and newly introduced 

 species; (10) the balance of nature (pp. 133, 138, 139) 

 is brought to an equilibrium through the relations of 

 plants and insects (p. 132), and unlimited increase is 

 subject to other checks (pp. 133, 138, 139); (11) many 

 species might be destroyed through the devastation of 

 plant life caused by locusts (p. 137), or by the advance 

 of sands and sandstorms in desert regions (p. 166), as 

 illustrated in the burial of camels and other animals; 

 (12) plagues of ants would be highly destructive of 

 certain types of animals (p. 137). Under the influence 

 of Lamarck, Lyell observed that extinction might be 

 averted by "accommodation" to a new environment, 

 but he argued that under many conditions accommo- 

 dation would not be rapid enough, as Lamarck sup- 

 posed, to avert extinction. 



Especially interesting is Lyell's calculation (p. 183) 

 of the time required for the extinction of one species. 

 If the earth were divided into twenty regions of equal 

 area one of these would about equal the dimensions 

 of Europe and might contain a twentieth part of the 

 million species which we would suppose to exist. In 

 such a region one species, according to the assumed 

 rate of mortality, would perish in 20 years, or 5 

 species out of 50,000 in the course of a century. In 

 the class Mammalia it would require more than 8,000 

 years to lose one group in a region of the dimensions 

 of Europe. 



BAIANCE OF NATURE (lYEII, DARWIN, WALLACE) 



Lyell. — Referring to that subtle adjustment of the 

 sum of all internal and external causes known as the 

 balance of nature, Lyell (1872.1, vol. 2, pp. 455-456) 

 observed: 



Every new condition in the state of the organic or inorganic 

 creation, a new animal or plant, an additional snow-clad moun- 

 tain, any permanent change, however slight in comparison to the 

 whole, gives rise to a new order of things and may make a 

 material change in regard to some one or more species. Yet a 

 swarm of locusts, or a frost of extreme intensity, or an epidemic 

 disease may pass away without any great apparent derange- 

 ment; no species may be lost, and all may soon recover their 

 former relative numbers, because the same scourges may have 



