CHAP. XI.] FORMS OF LIFE CHANGING. 279 



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. When- 

 ever 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. 



Qn the Forms of Life changing almost simultaneously throughout 

 tfo World. 



Scarcely any pal aeon tological 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 present an unmistakeable resem- 

 blance 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 families, genera, and sections 

 of genera, and sometimes are similarly characterised in such 

 trifling points as mere superficial sculpture. Moreover, other 

 forms, which are not found in the Chalk of Europe, but which 

 occur in the formations either above or below, occur in the same 

 order at these distant points of the world. In the several 

 successive palaeozoic formations of Russia, Western Europe, 

 and North America, a similar parallelism in the forms of life 

 has been observed by several authors; so it is, according to 

 Lyell, with the European and North American tertiary deposits. 

 Even if the few fossil species which are common to the Old 

 and New Worlds were kept wholly out of view, the general 

 parallelism in the successive forms of life, in the palaeozoic and 

 tertiary stages, would still be manifest, and the several formations 

 could be easily correlated. 



These observations, however, relate to the marine inhabitants of 

 the world : we have not sufficient data to judge whether the pro- 

 ductions of the land and of fresh water at distant points change in 

 the same parallel manner. We may doubt whether they have thus 

 changed : if the Megatherium, Mylodon, Macrauchenia. and Toxo- 

 don had been brought to Europe from La Plata, without anv 



