By what standard, then, can we measure development? We have already seen, from the "formula," as it is called, or definition, of evolution, that it implies an advance to a state of increased coherence, definiteness, and heterogeneity, by the double process of differentiation of parts, and integration of these parts into a whole by the formation of definite relations to one another. The notions of coherence amongst parts and of increased definiteness of function and structure are easily understood. But the heterogeneity postulated is a more complex notion,--has, in the first place, a double reference, "is at the same time a differentiation of the parts from each other and a differentiation of the consolidated whole from the environment;"[231] and secondly, is manifested in living beings in increased complexity of every kind--of structure, form, chemical composition, specific gravity, temperature, and self-mobility.[232] Can we then apply this at once to ethics, and say that the most developed--that is, the most moral--conduct is that which is most definite, coherent, and heterogeneous? This doctrine has at least the merit of not leaving out of sight so fundamental a characteristic of evolution as the tendency to variation; and, without being consistently held to, it is the burden of much of Mr Spencer's 'Data of Ethics,' where it is illustrated and defended with great ingenuity. [Sidenote: found in degree of complexity of act and motive.] That moral conduct is distinguished by definiteness and coherence--that it works towards a determinate end, and that its various actions are in agreement with one another and parts of a whole--may be admitted. But this is at most a merely formal description of what is meant by morality in conduct. To say that conduct must be a coherent whole, and must seek a determinate end by appropriate means, leaves unsettled the question as to what this end should be, or what means are best fitted to attain it. But, when we go on to say that as conduct is more varied in act,[233] more heterogeneous in motive,[234] it is higher in the moral scale, we seem to have got hold of something which may be a guide for determining the ethical end. The mark of what is higher in evolution, and consequently in morality, will be greater heterogeneity or complexity.[235] [Sidenote: Difficulties of the theory:]