[BK] _Origin of Species_, 4th ed. p. 141. [BL] It may be well to mention here that the _third_ American edition is from the _fifth_ English edition. [BM] _Origin of Species_, 6th ed., p. 289. [BN] _Origin of Species_, 6th. ed. chap. iv. p. 100. [BO] _Contributions to the Natural History of the United States_, Introduction, Boston, 1857; and _Essay on Classification_, p. 15, published separately; London, 1859. See Appendix VI. [BP] _Origin_, 4th ed. p. 143. [BQ] _Origin of Species_, 6th. ed. chap. iv. p. 100. LECTURE V. RELATIONS OF LOWER AND HIGHER ORGANISMS. From the general aspects of the theory of species, we pass to the consideration of distinct groups of organism, with the view of ascertaining their relations to each other. In doing so, it is better to begin at the lower end of the scale, leaving for a more advanced stage of inquiry the higher types of organism. In this department of the subject, special obligations are due to the wide range of investigations either occasioned or stimulated by the theory of evolution. For, whatever may be the ultimate award passed on this theory, there will be a unanimous recognition of the great value to science attending on the varied forms of inquiry stimulated by the writings of Mr. Charles Darwin. And one obvious and strong reason for such acknowledgment is that so many of the results of these researches have an inherent value quite distinct from their testimony in favor of the theory that the struggle for existence is the principal factor in the origin of new species. One of the most interesting fields of observation thus opened, is that concerned with the fertilization of plants by the intervention of insects and birds. A beginning in this department was made by the German naturalist, Christian Konrad Sprengel, who published in 1793 the report of his observations. In this he has been followed by Darwin, in 1862; by Dr. Hooker, Professor Asa Gray whose contributions appeared in the _American Journal of Science and Art_ in 1862, and 1863, Moggridge, Fritz Müller, and Sir John Lubbock. The facts now accumulated, rank as an important contribution to botany and zoölogy, and naturally fall within the circle of recent advances to which it is desirable that attention be turned.