[⇐] Many species when once formed never undergo any further change but become extinct without leaving modified descendants; and the periods, during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retained the same form. It is the dominant and widely ranging species which vary most frequently and vary most, and varieties are often at first local — both causes rendering the discovery of intermediate links in any one formation less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they have spread, and are discovered in a geological formation, they appear as if suddenly created there, and will be simply classed as new species. Most formations have been intermittent in their accumulation; and their duration has probably been shorter than the average duration of specific forms. Successive formations are in most cases separated from each other by blank intervals of time of great length; for fossiliferous formations thick enough to resist future degradation can as a general rule be accumulated only where much sediment is deposited on the subsiding bed of the sea. During the alternate periods of elevation and of stationary level the record will generally be blank. During these latter periods there will probably be more variability in the forms of life; during periods of subsidence, more extinction.
___________◊ authp_C_h_a_r_l_e_s_D_a_r_w_i_n
KEY_WORDS
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◊ authp_C_h_D_a_r_w_i_n
◊ authp_D_a_r_w_i_n
◊ cover_O_r_i_g_i_n_o_f_S_p_e_c_i_e_s
◊ edit_J_o_h_n_M_u_r_r_a_y
◊ edit_J_M_u_r_r_a_y
◊ edit_M_u_r_r_a_y
◊ yauth_1_8_5_9, yauth_1_8_7_2, yedit_1_8_7_2
◊ book_quote, lantxt_en, hdr_v2
• keywords_da_inserire
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[] C h. D a r w i n, ‹T h e O r i g i n o f S p e c i e s›, J o h n M u r r a y, 1 8 7 2⁶.
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