THE WONDERS OF LIFE 



the Jurassic, and the first placentals in the Cretaceans. 

 The great wealth of varied and highly organized forms 

 which are contained in this third and last sub-class of 

 the mammals appear only in the succeeding Tertiary 

 Period. The numbers of well-preserved skulls which 

 these placentals have left behind in fossil form are 

 particularly important, because they give us an idea of 

 the quantitative and qualitative formation of the brain 

 within the various orders; thus, for instance, in the 

 modern carnivora the brain is from two to four times, 

 and in the modern ungulates from six to eight times, as 

 large (in proportion to the size of the body) as in their 

 earliest Tertiary ancestors. It is also found that the 

 cortex (the real organ of mind) has developed in the 

 Tertiary Period at the expense of the other parts of the 

 brain. The duration of this Csenozoic Period has lately 

 been calculated at three million years (according to other 

 geologists twelve to fourteen or more million years). 

 It was, at all events, sufficient to make possible the 

 gradual development of the human mind from the lower 

 intelligence of our ape-ancestors and the instincts of 

 the older placentalia. 



We have given the physiological name of the "phro- 

 nema," as the real organ of mind or the instrument of 

 reason, to that part of the cortex on the normal anatomic 

 condition of which the action of the human mind 

 depends. The remarkable investigations during the 

 last few decades of the finer texture of the grey cortex 

 (or cortical substance of the cerebrum) have shown 

 that its structure — a real anatomic "wonder of life" — 

 represents the most perfect morphological product of 

 plasm; and its physiological function — mind — is the 

 most perfect action of a "dynamo-machine," the highest 

 achievement that we know anywhere in nature. Millions 

 of psychic cells or neurona — each of them of an extremely 

 elaborate fibril molecular structure — are associated as 



328 



