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769
MOZLEY ON EVOLUTION.

trine of the correlation of heat and motion, this doctrine had not become current; and no conception, even, had arisen of the more general doctrine of the correlation and equivalence of the physical forces at large. Still more recent was the rise and establishment of the associated abstract doctrine commonly known as the "conservation of energy." Further, Yon Baer's discovery, that the changes undergone during development of each organic body are always from the general to the special, was not enunciated till some eight years after the time at which Mr. Mozley was a pupil of my father, and was not heard of in England until twenty years after. Now, since these three doctrines are indispensable elements of the general theory of evolution (the last of them being that which set up in me the course of thought leading to it), it is manifest that not even a rude conception of such a theory could have been framed at the date referred to in Mr. Mozley's account. Even apart from this, one who compared my successive writings would find clear proof that their cardinal ideas could have had no such origin as Mr. Mozley's account seems to imply. In the earliest of them—"Letters on the Proper Sphere of Government"—published in 1842 and republished as a pamphlet in 1844, the only point of community with the general doctrine of evolution is a belief in the modifiability of human nature through adaptation to conditions (which I held as a corollary from the theory of Lamarck) and a consequent belief in human progression. In the second and more important one, "Social Statics," published in 1850, the same general ideas are to be seen, worked out more elaborately in their ethical and political consequences. Only in an essay published in 1852 would the inquirer note for the first time a passing reference to the increase of heterogeneity as a trait of development, and a first recognition of this trait as seen in other orders of phenomena than those displayed by individual organisms. Onward through essays published in several following years, he would observe further extensions in the alleged range of this law; until, in 1855, in the "Principles of Psychology," it begins to take an important position, joined with the additional law of integration, afterward to be similarly extended. Not until 1857, in two essays then published, would he find a statement, relatively crude in form, of the law of evolution, set forth as holding throughout all orders of phenomena, and joined with it the statement of certain universal physical principles which necessitate its universality. And only in 1861 would he come to an expression of the law approximating in definiteness to that final one reached in 1867. All which facts the scientific reader who took the trouble to investigate would see are conclusive against the implication contained in Mr. Mozley's statement; since, were this implication true, my early writings would have contained traces of the specific doctrine set forth in the later ones. But, as I have said, even a reader of my books can not be trusted to recall and consider these facts, but will certainly, in many