IX.] RETROGRESSIVE DEVELOPMENT IN NATURE. 23 



existence, and when that point is reached natural selection will 

 interfere to prevent further degeneration. To return to our 

 former example, it is quite conceivable that the percentage of 

 persons with hereditary short sight may steadily increase, 

 without seriously affecting the general standard of vision of 

 mankind as a whole, or even that of a single nation or class, 

 because degeneration below a certain point would become a 

 fact of decisive importance to the individual, leading to failure 

 in the struggle for existence. Thus we need not fear the com- 

 plete loss of our eyes through degeneration, like that which 

 has affected the animals living in the dark and the above- 

 mentioned parasites ; and we need not anticipate any serious 

 diminution of our muscular strength, or powers of endurance, 

 or many other qualities. 



Hitherto I have only treated of the degeneration of physical 

 characters in consequence of disuse and Panmixia, but the same 

 thing takes place with mental qualities, a fact which need not 

 surprise us when we remember how close is the connection 

 between all mental and physical processes, how the relative 

 size and complexity of the brain is a measure of the degree of 

 intelligence, and how every instinctive action of an animal pre- 

 supposes a corresponding arrangement of the nervous system 

 which compels a certain action to follow upon a certain stimulus. 

 Hence degeneration of an instinct in an animal must always 

 have been preceded by degeneration of that network of nerve- 

 cells and nerve-fibres in the brain in which the instinctive 

 action had its rise. Retrogression, then, in physical structure 

 is not antagonistic to retrogression in instinct and mental 

 faculty, but mental and physical degeneration rather go hand 

 in hand. Very definite and extensive physical degeneration 

 always implies a corresponding mental deterioration. Those 

 Entoniscidae which have lost their eyes, antennae, legs, and 

 jaws, have also degenerated in intelligence, as is but natural in 

 animals which only require to remain still and imbibe nourish- 

 ment : the whole nervous system of these Crustacea has been 

 reduced to a remarkable degree. 



Certain examples are most interesting as tending to prove 

 that retrogression may be confined to one particular instinct, 

 leaving the animal and its powers as a whole quite unaffected. 

 The loss by domestic animals of the instinct to escape is one of 



