In these days it is becoming an accepted fact that morphology, physiology, and psychology are intimately related and connected, and that a thorough knowledge of the one implies an equally thorough knowledge of the others. Morphology and physiology, until a comparatively recent time, led divergent paths; but, thanks to such men as Haeckel, Romanes, Huxley, Wolff, and many others, this erroneous method of investigation, to a great extent, has ceased. "The two chief divisions of biological research--Morphology and Physiology--have long travelled apart, taking different paths. This is perfectly natural, for the aims, as well as the methods, of the two divisions are different. Morphology, the science of forms, aims at a scientific understanding of organic structures, of their internal and external proportions of form. Physiology, the science of functions, on the other hand, aims at a knowledge of the functions of the organs, or, in other words, of the manifestations of life."[1] [1] Haeckel, _Evolution of Man_, Vol. I. p. 20. Indeed, physiology has so diverged from its sister science, morphology, that it completely and entirely ignores two of the most important functions of evolution, heredity and adaptation. This has been clearly shown by Haeckel, who has done much towards bringing about a change of opinion in these matters.[2] [2] _Ibid._, p. 21 _et seq._ Morphology and physiology are interdependent, correlated, and connected one with the other; and, as I will endeavor to point out as my argument develops itself, psychology is, likewise, intimately associated with these two manifestations of life. It will be noticed that as forms take on more complexity, and as organs develop new and more complex functions, _psychos_ becomes less simple in its manifestations, and more complex in its relations to the internal and external operations of life. Keeping in view the definition of mind as advanced in the opening paragraph of this chapter, it at once becomes evident that even the very lowest forms of life possess mind in some degree. It is true that in the _monera_, or one-celled organisms, the nerve-cell is not differentiated; consequently, if I were to be held to a close and strict accountability, my definition of mind would not embrace these organisms. Yet, some small latitude must be allowed in all definitions of psychological phenomena, especially in those phenomena occurring in organisms which typify the very beginnings of life. I am confident that, notwithstanding the fact that the nerve-_cell_ is not differentiated in these primal forms, nerve-elements are, nevertheless, present in them, and serve to direct and control life. Mind makes itself evident in two ways--consciously and unconsciously. The conscious manifestations of mind are volitional, while the unconscious, "vegetative," reflex operations of mind are wholly involuntary.