172. In connection with this subject, Mr. Darwin has remarked that certain plants are more fertile with the pollen of another species than with their own; and Professor Huxley tells us that there are certain _fuci_ whose male element will fertilise the ovule of a plant of distinct species, while the males of the latter species are ineffective with the females of the first. So obscure in some of its branches is the working of the reproductive system. Again, the following remark by Mr. Darwin is very suggestive:— ‘First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication, by the selection of mere external differences, and not of differences in the reproductive system. In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels.’ 173. The result of all these speculations is to render it probable that there may be in nature, give it time enough, a process which leads to the transmutation of species. The accumulation of successive differences, each representing some element of success in the struggle for life, may easily be imagined to be capable of producing, in the course of ages, a very great change. Reasoning out this hypothesis, the more advanced followers of Mr. Darwin do not hesitate to describe all the varieties of living things, including man, as the results of development from some primordial germ taking place throughout the course of immeasurable ages. And Mr. Darwin himself, in his work on the Descent of Man, lays great stress on the occurrence of homologous structures in man and the lower animals, as well as on the development in man of rudimentary structures, which are either absolutely useless to their possessor, or of very slight service indeed, but which appear to serve as an index of the various stages through which the human species has passed in its progress upwards from lower forms of life. 174. Mr. Wallace, however, sees in the production of man the intervention of an external will. He remarks that the lowest types of savages are in possession of a brain, and of capacities far beyond any use to which they could apply them in their present condition, and that therefore they could not have been evolved from the mere necessities of their environments. 175. Finally, Professor Huxley imagines the possibility of the Darwinian hypothesis requiring modification. Alluding to the assumed circularity of the planetary orbits which followed the establishment of the Copernican hypothesis (Art. 69), he remarks:—