Even Bugon, seeing how naturally he could regard 'the wolf, the fox, and the jackal' as 'degenerate species of a single family,' concluded we could not go wrong in supposing that 'nature could have with time drawn from a single being all other organised beings.' Erasmus Darwin (1794) had insisted on the power of 'appetency' in the organs of a living creature to create and acquire new structures which it handed down to its posterity. G. R. Treviranus[7] in his _Biology_ (1802-5) had noted the influence of environment, and Jean Lamarck in his _Philosophie Zoologique_ (1809) had--after assuming that 'nature created none but the lowest organisms'--maintained that need and use (or disuse) can so effectively modify a creature that it may even produce new organs, and give rise by imperceptible degrees to a variety of creatures as widely divergent as they now appear. E. g. 'The giraffe owes its long neck to its continued habit of browsing upon trees.' And gradually it had become recognised by speculators on this subject that, as Mr. H. Spencer wrote in 1852, 'by small increments of modification any amount of modification may in time be generated.' Finally, in 1859, Darwin, with an ample resource of illustrative examples, enforced the doctrine that the existing fauna and flora of the earth represent the result of a struggle for existence, protracted during vast ages, in which those creature's have been preserved (selected to live) which, among all the variously-endowed offspring of any kind, were best fitted to appropriate the means of subsistence in the circumstances in which they for the time found themselves placed. The circumstances of life on the globe are perpetually varying from place to place and time to time: progeny never exactly reproduce their parents, and diverge widely from each other: hence each form of life is perpetually sliding on from phase to phase, and only those survive which are best adapted to the new conditions of life.