loo POEMS OF LIFE CHANGING [Chap. XL 



ally 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 correspond- 

 ingly rapid manner; and the forms which thus yield 

 their places will commonly be allied, for 'they will par- 

 take 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 mo- 

 ment that we understand the many complex contin- 

 gencies 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 econ- 

 omy 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 an- 

 other can be naturalised in a given country; then, and 

 not until then, we may justly feel surprise why we can- 

 not account for the extinction of any particular spe- 

 cies 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^ 



