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Show 2'T4 GEOLOGICAL SUCCESSION. [CHAP. X. Species of different genera and classes havfu n~t chan ed at the same rate, or in the same deg:ee. . t e oldes~ tertiary beds a few living sh~lls may still be found I· n the mi·a St 0 f a multitude of exti.n ct. forms. Falconer I.' t • r has given a strikinO' instance of a similar 1ac , In an ex-istin crocodile as~ociated with manJ: strange and l?st maJmals and reptiles in the sub-I-IImalayan. ~eposits. The Silurian Lingula differs but little from the hvn:g s:pe· cies of this genus; whereas most of the other Silunan Molluscs and all the Crustaceans have changed gre.atly. The productions of the land seem to change at a quicker rate than those of the sea, of w:hich s. strilnng inst~nce has lately been observed in S'Yitzerland: Ther~ IS. s01ne reason to believe that organisms, considered high In the scale of nature change more quickly than those that are low : thouO'h 'there are exceptions to this rule. The amount of ~rganic change, as Pictet h~s remarked, do~s not strictly correspond with the succession of our gcolo.gical formations ; so that between each two conse?utive formations the forms of life have seldom changed In exactly the s~me degree. Yet if we compare any but the most closely related formations, all the species will _be found to have undergone some change. When a spemes has once disappeared from the fac.e of ~he earth, we ~ave reason to believe that the same Identical form never reappears. The strongest apparent exception to this latter rule is that of the so-called " colonies" of M. Barrande, · whi~h intrude for a period in the midst of an older formation and then allow the pre-existing fauna to reappear; but 'Lyell's explanation, namely, that it is a. case of ~emporary migration from a distinct geographical province, seems to me satisfactory. , These several facts accord well with my theory. I believe in no fixed law of development, causing all the in~ habitants of a country to change abruptly, or simulta~ neously, or to an equai degree. The proc.ess.~f modification must be extremely slow. The vanabihty of each ~p_ecies is quite independent of that of all others. Whether such variability be taken advantage of by natural selection, and whether the variations be accumulated CHAP. X.] GEOLOGICAL SUCCESSION. 2'T5 to a greater or lesser. amo~nt, .thus causing a greater or lesser amount of modificatiO~l In t~e varying species, depe~ ds on many COJ?plex contingenCies,-on the variability bmng of a benefiCia~ nature, on the power of intercrossing, on t~e. rate of breeding, on the slowly changing physical conditions. of the co~1ntry; and ~ore especially on the natu~e of the. other Inha~~tants with which the varying spee1~s. come Into comp~tition. Hence it is by no means surpnsing that one speCies should retain the same identical form much longer than others; or, if changing, that it s~ou!d c~ange l~s~. We se~ the same fact in geographical d1strib~tion; for_ Insta;nce, m. the land-shells and coleopterous Insects of Maderra having come to differ considerably from their n.earest allies on t~e continent of Europe, whereas the marine shells and b1rds have remained unalt~ red. We can per~aps und~rstand ~he appar~ntly quiCk~r rate of ch~nge In terrestna~ and In more highly organis~d productions compared With marine and lower pr?ductlons, ?Y the more complex relations of the higher bmng~ to tJ:!eir organic and inorganic conditions of life, as ~xplained In a former chapter. When many of the inhabItants of a country have become modified and improved, we can understand, on the principle of competition, and on that ~f the many all-important relations of organism to organism, that any form which does not become in some d~gree modified and improved, will be liable to 'be exterminated: lienee we ~an see why all the species in the same r~gion do at last, I~ we look to wide enough intervals of time, become modified; for those which do not change wHl become extinct. Iri memb.ers of the same class t?e average amount of change, durmg long and equal periods of time, rnay, perhaps, ~e near~~ the same; but as the accumulation of longenduri; ng fossilif~rous formations depends on great rnasses of ~ediment having been deposited on areas whilst subSldmg, our formations have been almost necessarily accumulated at wide and irregularly intermittent intervals; consequ~ntly the amount of organic change exhibited by the f~ss1ls embedded in consecutive formations is not equal. Each formation, on this view, does not mark a |