212 DIVERSIONS OF A NATURALIST 



Later, and added to these inherited mechanisms 

 — often interfering with them and putting an end to 

 them — are the mechanisms of the second step. These 

 are mechanisms arising from individual experience ; 

 they depend on memory — the inscription on " the tablets 

 of the mind," of the experience that this follows that. 

 They control movement and action, usurping the privi- 

 lege of the previously omnipotent inherited mechanisms 

 or instincts. This second step in the development of 

 mind requires an excessive quantity of brain-cells. It 

 only makes its appearance at all in animals with large 

 brains, and reaches a far greater development in man 

 even than in the apes, his brain being from twice to 

 three times the size of that of the largest living ape. 

 This use of memory and individual experience — instead 

 of an inherited mechanism, which is the same in every 

 member of the species — is obviously a great advantage 

 in the struggle for existence. There are traces of it in 

 some of the cuttlefish and insects, but even in the fishes 

 and reptiles among living vertebrates it is of small 

 account, and the small brain carries on its work by good, 

 sound, inherited mechanisms or instincts, but learns 

 nothing, comprehends nothing! In the birds we see 

 a little — a very little — more capacity for " learning by 

 individual experience," and it is only in the larger and 

 later mammals that educability, or the power of learning 

 by individual experience, becomes of serious importance. 

 All the larger living mammals — horse, cattle, sheep, 

 rhinoceros, tapir — have acquired an enormous increase in 

 the size of their brains — as much as six or eight times 

 the volume of that of their extinct ancestors whose bones 

 and brain cavities we find fossilized in the Tertiary 

 strata. Man has by far the biggest brain of all these 

 animals, and has a unique degree of educability, together 

 with the fewest instincts or in-born hereditary mechan- 



