In spite of the fascination of these theories, however, Lyell was not carried away by them, and it was not for some years that he estimated them at their true value. Meanwhile the new geology made its appearance with the publication of the three volumes of his own _Principles of Geology_, between 1829 and 1833. The significance of the book for biological speculation--for theories of the origin of species--lay in its thesis that the present condition of the earth is the product of geological processes incalculably long. Hitherto the "catastrophic theory" had been dominant--the notion that a series of immense catastrophic events (like the Deluge) had been responsible for the present condition of the earth's surface. For this Lyell substituted his "Evolutionary Theory," according to which the almost invisibly slow geological processes which we may now see operating around us, are typical of the behaviour of the crust of this planet for incalculable periods of time; for even the slowest changes, if sufficient time is allowed them, are capable of producing the most stupendous results. Lyell may be said to have extended the age of the earth _ad infinitum_. Just as Galileo removed all barriers of space, Lyell removed those of time. Their joint achievement was to present to humanity a universe infinite both in space and time--a staggering conception. RESULTS OF LYELL'S THEORY.--Though Lyell's boldness disturbed a good many of his contemporaries, those biologists who were engaged upon seeking the origin of species were thankful to one who had removed the chief obstacle to the solution of their difficulties. They were now relieved of one embarrassment: Lyell gave them the power to draw on the Bank of Time to any extent; bankruptcy was no longer possible.[37] Indeed, Lyell seems himself to have been convinced of the evolutionary origin of species (though the mode of its operation still remained a mystery for him no less than for the biologists themselves). In fact, it became quite evident that the idea of "continuity" which the _Principles of Geology_ had established in the inorganic world, must be equally applicable to the organic world. DARWIN.--The theory of a common descent of species had occurred, as early as 1837, to an enthusiastic student of Lyell's writings, who was also a personal friend. Charles Darwin had collected much geological, botanical, and zoological matter on his voyage with the _Beagle_ round the world, and continued for twenty years to accumulate an immense volume of _data_ to substantiate a theory which had first suddenly suggested itself to him in 1838 as the result of reading for amusement Malthus' _Essay on the Principle of Population_.