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Thinking over extinction of S. American mammals - I thought man might have killed these weighty quadrupeds, but Hyæna, horse & antelope disprove it. But we ought not to feel surprise at extinction, any more than at one species being rarer than other. Why is one whale rarer than other? Why one S. African Rhinoceros than other? Why one of the sloths in S. America than other? Why one of willow wrens than other, birds so like as only to be distinguished by nest & laying same nos of eggs?

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Why long-tailed titmouse rare, when no British bird lays so many eggs? Why in the most recent deposits when every-shell recent have some of those then commonest now become rarest? Although we see extreme difficulty of ascertaining all the conditions of any living being as to say why it it is rare, yet when we know that the diminution of number must be caused by actual death of individuals & these

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individuals, as in Megatherium, are little liable to death from other animals, we can hardly avoid recurring to some overwhelming causes of destruction. We shall aid matters by reflecting that there always is a percentage of death equivalent to the births, & that the increase of this proportion even by 1/1000 would in over 1000 years reduce by so much, the number of individuals.

Our best answer to S. American question is to say, that the

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extinction has been due to the action of analogous & equally imperfectly known causes, as those which make Dasypus gigas uncommon - or one species less common than others or one monkey than others or still better one Sloth than other.

This is legitimate course of argument, for in known cases of extinction. (I after Lyell, thinking that man act like other animals) this is the cause - in S. America & Europe we see the change from all recent to some extinct is made by change in proportional numbers. Some becoming rare.