Sir Charles Lyell, who made a careful study of Lamarck's great work, notes especially that the whole of the argument is vague and general, and that no cases are given in which is shown how the alleged causes can be supposed to have acted so as to bring about the innumerable changes that must have occurred. What is more important, however, is the failure to explain how the numerous minute adaptations of each species to its environment could have arisen by the direct action of that environment—in plants, the infinitely varied forms of leaves, flowers, and fruits; in animals, the forms and sizes of the teeth of mammalia and of the beaks, wings and feet of birds to the food they obtain; while the enormous range of colour and marking in most groups of animals are such as no amount of desire or exertion on the one hand, or direct action of external causes on the other, could possibly have brought about. It is not, therefore, surprising that, although a vast amount of evidence was adduced to show that changes had taken place leading to the evolution of species from pre-existing species, yet causes adequate to bring about the changes, and especially those necessary to produce the marvellous adaptations continually being discovered, had not been shown to exist.
It is necessary to point this out, because the difference between the almost universal rejection of Lamarck's attempted solution of the problem of evolution, and the almost immediate and universal acceptance of that adduced by Darwin, is otherwise unexplained. The belief in the doctrine of evolution as the only rational explanation of the gradual development of the innumerable forms of living things became more and more general. The great body of arguments in its favour were admirably set forth by Robert Chambers in his _Vestiges of Creation_, published anonymously in 1844; while Herbert Spencer's masterly exposition of the argument for universal evolution convinced a large number of naturalists and men of science. But still the nature of the laws and forces by which the evolution of the organic world in all its variety and beauty, could have been brought about remained not only unknown but unimagined, so that even so great a thinker as Sir John Herschel termed it "the mystery of mysteries." I will now state as briefly as possible the essential features of Darwin's solution of the mystery in his epoch-making work, _The Origin of Species_.