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There is, in fact, an obvious limit to the perfection which can be attained by receptor and effector organs. Striated muscles, the modelling of the skeleton and joints for speed in a horse or greyhound, the eye and ear of higher vertebrates, the mammalian sense of smell—no doubt it would be possible for life to have produced more perfect and more efficient mechanisms—but not, apparently, mechanisms much more perfect or much more efficient. They stand near the limit of biological efficiency.

There thus comes a time when it is impossible or extremely difficult to give an organism advantage in the struggle by improving its sense-organs or its locomotor system, or indeed any of its general physical construction, whereas it is still possible to confer the most important advantages upon it by means of improvements in the adjustor mechanism, improvements which involve and imply improvements of mind.

This stage was reached by mammals and birds quite early in the Tertiary period; and one of the most striking spectacles of biology, revealed in the fossils of successive strata, is to see Mind coming into its own after this epoch. Over and over again a group of animals is seen to appear and spread, only to be extinguished and replaced by another type which to all outward appearance is similar, no better adapted to the conditions of life. But the two types differ in one point: the later possessed a larger brain, and so, from all analogy, a better mind. Or, to take another example, man differs from the lower animals in no notable physical specialization except the upright posture.