The obstacle in the way of the acceptance of the derivative origin of species was the then prevalent notion concerning the nature of life. We must briefly sketch the change which has taken place in the last forty years in our ideas on this subject.
Until about forty years ago, the different forces of Nature, such as gravity, electricity, magnetism, light, heat, chemical affinity, etc., were supposed to be entirely distinct. The realm of Nature was divided up into a number of distinct and independent principalities, each subject to its own sovereign force and ruled by its own petty laws. About that time it began to be evident, and is now universally acknowledged, that all these forces are but different forms of one, universal, omnipresent energy, and are transmutable into one another back and forth without loss. This is the doctrine of correlation of forces and conservation of energy, one of the grandest ideas of modern times. But one force seemed still to be an exception. Life-force was still believed to be a peculiar, mysterious principle or entity, standing above other forces and subordinating them; not correlated with, not transmutable unto, nor derivable from, other and lower forces, and therefore in some sense supernatural. Now, if this be true of living forces, it is perfectly natural, yea, almost necessary, to believe that living forms are wholly different from other forms in their origin. New forms of dead matter may be derived, but new living forms are underived. Other new forms come by natural process, new organic forms by supernatural process. The conclusion was almost unavoidable. But soon vital force also yielded to the general law of correlation of natural forces. Vital forces are also transmutable into and derivable from physical and chemical forces. Sun-force, falling on the green leaves of plants, is absorbed and converted into vital force, disappears as light to reappear as life. The amount of life-force generated is measured by the amount of light extinguished. The same is true of animal life. As in the steam-engine the locomotive energy is derived from the fuel consumed and measured by its amount, so in the animal body the animal heat and animal force are derived from and measured by the food and tissue consumed by combustion. Thus, vital force may be regarded as so much force withdrawn from the general fund of chemical and physical forces, to be again refunded without loss at death. This obstacle is, therefore, now removed. If vital force falls in the same category as other natural forces, there is no reason why living forms should not fall into the same category in this regard as other natural forms. If new forms of dead matter are derived from old forms by modification, according to physical laws, there is no reason why new living forms should not also be derived from old forms by modification according to physiological laws. Thus, at last, the obstacle was removed—the ground was cleared.
But Science is not content with removal of a priori objections. She must also have positive proofs. The ground must not only be