100 POEMS OF LIFE CHANGING [Chap. XI. 



unusually rapid development, many species of a new 

 group have taken possession of an area, many of the 

 older species will have been exterminated in a corre- 

 spondingly 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 con- 

 tingencies 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 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 paheontological 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 Xorth America, in equatorial South America, 



