VERTEBRATES: BACKBONE AND BRAIN 111 



cognizance of ever subtler forces and movements. 

 Memory comes into activity very early. The animal 

 begins to learn by experience. The brain is becoming 

 not merely a steering bnt a thinking organ. More 

 and more nervous material is crowded into it and de- 

 tailed for its work. Wits and shrewdness are begin- 

 ning to count for something in the battle. Not only 

 the animal with the strongest muscles, but the one with 

 the best brain survives. And thus at last the brain 

 began to develop with a rapidity as remarkable as its 

 long delay. Thus each higher function is called into 

 activity by the next lower, serves this at first, and 

 only later attains its supremacy. 



And yet the advance of the different functions is 

 not altogether successive. Muscle and nerve do not 

 wait for digestion and reproduction to show signs of 

 halting before they begin to advance. They all ad- 

 vance at once. But the progress of reproduction and 

 digestion is most rapid at first, and it appears as if 

 they would outrun the others. But in the ascending 

 series the others follow after, and soon overtake and 

 pass by them. And these lower functions, when out- 

 marched, do not lag behind, but keep in touch with 

 the others, forming the rear-guard and supply-train of 

 the army. And notice that each organ holds the pre- 

 dominance about as long as it shows the power of 

 rapid improvement. The length of its reign is pretty 

 closely proportional to its capacity of development. The 

 digestive system reaches that limit early, the muscular 

 system is capable of indefinitely higher complexity, as 

 we see in our hand. But the muscular system has 

 nearly or quite reached its limit. The body had seen 

 its day of dominance before man arrived on the globe. 



