GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 303 



of the sufferers may often be preserved for a long time, from be- 

 ing fitted to some peculiar line of life, or from inhabiting some 

 distant and isolated station, where they will have escaped severe 

 competition. For instance, some species of Trigonia, a great genus 

 of shells in the secondary formations, survive in the Australian 

 seas; and a few members of the great and almost extinct group 

 of Ganoid fishes still inhabit our fresh waters. Therefore, the utter 

 extinction of a group is generally, as we have seen, a slower proc- 

 ess than its production. 



With respect to the apparently sudden extermination of whole 

 families or orders, as of Trilobites at the close of the palaeozoic 

 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 in- 

 dividuals than that; why this species and not another can be 

 naturalized in a given country; then, and not until then, we may 

 justly feel surprise 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 palaeontological 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 recognized in many distant regions, under the most different 

 climates, where not a fragment of the mineral chalk itself can 



