_Extermination._ We have seen that in later periods the organisms have disappeared by degrees and [perhaps] probably by degrees in earlier, and I have said our theory requires it. As many naturalists seem to think extermination a most mysterious circumstance{121} and call in astonishing agencies, it is well to recall what we have shown concerning the struggle of nature. An exterminating agency is at work with every organism: we scarcely see it: if robins would increase to thousands in ten years how severe must the process be. How imperceptible a small increase: fossils become rare: possibly sudden extermination as Australia, but as present means very slow and many means of escape, I shall doubt very sudden exterminations. Who can explain why some species abound more,--why does marsh titmouse, or ring-ouzel, now little change,--why is one sea-slug rare and another common on our coasts,--why one species of Rhinoceros more than another,--why is <illegible> tiger of India so rare? Curious and general sources of error, the place of an organism is instantly filled up. {121} On rarity and extinction see _Origin_, Ed. i. pp. 109, 319, vi. pp. 133, 461. We know state of earth has changed, and as earthquakes and tides go on, the state must change,--many geologists believe a slow gradual cooling. Now let us see in accordance with principles of [variation] specification explained in Sect. II. how species would probably be introduced and how such results accord with what is known. The first fact geology proclaims is immense number of extinct forms, and new appearances. Tertiary strata leads to belief, that forms gradually become rare and disappear and are gradually supplied by others. We see some forms now becoming rare and disappearing, we know of no sudden creation: in older periods the forms _appear_ to come in suddenly, scene shifts: but even here Devonian, Permian &c. [keep on supplying new links in chain]--Genera and higher forms come on and disappear, in same way leaving a species on one or more stages below that in which the form abounded. <GEOGRAPHICAL DISTRIBUTION.> Sec. VI. Let us consider the absolute state of distribution of organisms of earth's face.