THE MECHANICAL THEORY SUBSTANTIATED.--Thus, whether biologists were considering the _structure_ or the _behaviour_ of organisms, they were arriving at the same conclusions. The structure was revealed as physical and chemical structure, and the behaviour as the resultant of familiar physical and chemical processes. Hence biology came to be regarded as a compartment of physics and chemistry, for life itself was nothing but a complex physical or chemical phenomenon. Life could thus be satisfactorily expressed in terms of matter and energy. The speculations of Descartes seemed to be established by experimental science. THE FINAL OBSTACLE.--The situation, already satisfactory to those whose hope it was to see the mechanical theory impregnably established, was marred, however, by one untoward circumstance. The phenomena of organic structure, growth, and behaviour having been reduced to order, and expressed in terms of physics and chemistry, certain important facts still resisted explanation, and stood out as a last stronghold of the older view. THE ORIGIN OF SPECIES.--The existence of definite forms of animal and vegetable life, whose infinite variety and complexity was continually being increased by research[34]--still remained a mystery. How did these innumerable species naturally and automatically come into being? was the question that must be satisfactorily answered before the mechanical view could be held to cover all the facts. The direction in which to look for a reply had been indicated by a number of thinkers. The French naturalist Buffon, the philosopher Kant, and the poet Goethe--besides other thinkers--had already in the eighteenth century familiarised the idea that species are not immutable, but that, by some means or other, new forms of life are derived from pre-existing ones. The conception had gained a firm foothold in England, where it was hospitably entertained by Mr. Herbert Spencer, and where it formed the staple of a book which caused a good deal of controversy in its day, and which is not yet forgotten.[35]