remoulding activity of the organism upon the environment, but in its failure to explain what is that subtle power whereby an organism is enabled to make these prophetic adjustments that characterize vitality. In a chapter added to later editions, Spencer was forced to discuss "The Dynamic Element in Life," and to admit that his definition had not really revealed the nature of life. "We are obliged to confess that Life in its essence cannot be conceived in physico-chemical terms."[1] He did not realize how damaging such an admission was to the unity and completeness of his system.
As Spencer sees in the life of the individual an adjustment of internal to external relations, so he sees in the life of the species a remarkable adjustment of reproductive fertility to the conditions of its habitat. Reproduction arises originally as a readaptation of the nutritive surface to the nourished mass; the growth of an amoeba, for example, involves an increase of mass much more rapid than the increase in the surface through which the mass must get its nourishment. Division, budding, spore-formation, and sexual reproduction have this in common, that the ratio of mass to surface is reduced, and the nutritive balance is restored. Hence the growth of the individual organism beyond a certain point is dangerous; and normally growth gives way, after a time, to reproduction.
On the average, growth varies inversely with the rate of energy-expenditure; and the rate of reproduction varies inversely with the degree of growth. "It is well known to breeders that if a filly is allowed to bear a foal, she is thereby prevented from reaching her proper size.…As a converse fact, castrated animals, as capons and notably cats, often become larger than their unmutilated associates."[2] The rate of reproduction tends to fall as the development and capability of the individual progress. "When, from lowness of organization, the ability to contend with external dangers
