Chap. XL] EXTINCTION. 297 



With respect to the apparently sudden extermination of whole 

 families or orders, as cf Trilobites at the close of the palseozoic 

 period and of Ammonites at the close of the secondary period, we 

 must remember what has been already said on the probable wide 

 intervals of time between our consecutive formations ; and in these 

 intervals there may have been much slow extermination. More- 

 over, when, by sudden immigration or by unusually rapid develop- 

 ment, many species of a new group have taken possession of an 

 area, many of the older species will have been exterminated in a 

 correspondingly rapid manner; and the forms which thus yield 

 their places will commonly be allied, for they will partake of the 

 same inferiority in common. 



Thus, as it seems to me, the manner in which single species and 

 whole groups of species become extinct accords well with the theory 

 of natural selection. We need not marvel at extinction; if we 

 must marvel, let it be at our own presumption in imagining for a 

 moment that we understand the many complex contingencies on 

 which the existence of each species depends. If we forget for an 

 instant, that each species tends to increase inordinately, and that 

 some check is always in action, yet seldom perceived by us, the 

 whole economy of nature will be utterly obscured. Whenever we 

 can precisely say why this species is more abundant in individuals 

 than that ; why this species and not another can be naturalised in 

 a given country ; then, and not until then, we may justly feel sur- 

 prise why we cannot account for the extinction of any particular 

 species or group of species. 



On the Forms of Life changing almost simultaneously throughout 

 the World. 



Scarcely any palseontological discovery is more striking than the 

 fact, that the forms of life change almost simultaneously throughout 

 the world. Thus our European Chalk formation can be recognised 

 in many distant regions, under the most different climates, where 

 not a fragment of the mineral chalk itself can be found ; namely, 

 in North America, in equatorial South America, in Tierra del 

 Fuego, at the Cape of Good Hope, and in the peninsula of India. 

 For at these distant points, the organic remains in certain beds pre- 

 sent an unmistakeable resemblance to those of the Chalk. It is 

 not that the same species are met with ; for in some cases not one 

 species is identically the same, but they belong to the same fami- 

 lies, genera, and sections of genera, and sometimes are similarly 

 characterised in such trifling points as mere superficial sculpture. 

 Moreover, other 'orms, which are not found in the Chalk of Europe 



