In 1880 he published "Natural Science and Religion," two lectures delivered to the Theological School of Yale College, before a critical audience, who listened with the deepest interest to what was, in some points, his most advanced view of natural selection. We need not dwell on a subject about which so much has lately been written by far abler pens than ours. Briefly stated, Gray was probably the best expounder of Darwinian principles, - meaning thereby those actually advocated by Darwin himself, and excluding the wild deductions attached to the original theory by those who deserve the name of Darwinissimists rather than Darwinists, - although he himself regarded natural selection as a less efficient cause than it was assumed to be by Darwin.
His influence as an exponent of Darwinism was due partly to the admirable clearness and candor of his reviews, and his interesting way of putting things; for his fertile imagination was constantly discovering apt similes to illustrate otherwise dry arguments. It was also due in part to his known caution and conservatism, and his professed Christian faith. If an avowed accepter "of the creed commonly called the Nicene" saw nothing in Darwinism which implied atheism, or was opposed to the idea of design on the part of the Creator, surely one might, at least, listen to his account of the development theory with safety. To his hearers at New Haven, in 1880, he said: "Natural selection by itself is not an hypothesis, nor even a theory. It is a truth, - a "catena" of facts and direct inferences from facts .... There is no doubt that natural selection operates; the open question is, what do its operations amount to.
The hypothesis based on this principle is, that the struggle for life and survival of only the fittest among individuals, all disposed to vary and no two exactly alike, will account for the diversification of the species and forms of vegetable and animal life, - will even account for the rise, in the course of countless ages, from simpler and lower to higher and more specialized living beings." He gave it as his opinion that natural selection is, on the whole, a good working hypothesis, but does not explain how wholly new parts are initiated, even if the new organs are developed little by little.
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