September, 1928 



EVOLUTION 



r*CE Seven 



the old ways may be all wrong. Then new ways nuist 

 be thought out. That is the front-brain's job. It can 

 do the job well only if it gets exact facts to work on. 

 As the eye gets better facts than the nose, the thinking 

 front-brain never could amount to much until it used 

 eye-facts instead of nose-facts. It just had to quit being 

 led around by the nose. 



So it was really tough luck that the eye nerves of the 

 lower animals always got the wrong 

 connections. In the fishes and frogs, they 

 connected with the rear-brain which 

 could not think, and in the reptiles and 

 birds with a part of the front-brain that 

 was cramped and could not grow. But 

 at last, in the mammals, they plugged 

 in on the right switchboard and then 

 things began to happen. For good con- 

 nections do count, — especially in evolv- 

 ing brains. 



Before this, the nose nerves always 

 had the best connections and dominated 

 front-brain thinking — what little there 

 was of it. There could not be much, for 

 the nose was a poor provider of exact 

 facts. But though the nose-facts were not 

 exact, they were really very important, 

 — about the things one could smell — 

 food, friends and foes, mates and rivals. 

 At first, there was a good reason for the 

 nose getting and keeping the best brain 

 connections. The eyes just had to wait 

 for their big chance. 



There was a second reason. The eyes 

 were not yet good enough. Eyes, to help 

 much, must be really good, and the first 

 ones were pretty crude. They probably 

 only distinguished light from darkness, 

 movement from stillness, without tell- 

 ing anything about color, shape and 

 distance. To develop them took time. 

 So the nose got there first and took over 

 the front-brain job, with the eyes as mere helpers. 



In the reptiles and birds, the eyes got a first, slim 

 chance. In helping out the smell sense with stray bits 

 of exact knowledge (which worked into the front-brain 

 associations), the eye nerves got a foothold in the front- 

 brain. But unluckily they got into a crowded spot, un- 

 derneath and inside where real growth was impossible. 

 So nothing much came of it until one of the reptile^ 

 evolved into a mammal. 



Among the mammal improvements was the connect- 

 ing of the eye-nerves with the roomy outside of the 

 front-brain. That helped a lot. The eye-facts, because 

 they were more exact and useful in thinking, gradually 

 displaced the nose-facts as rulers over tlie front-brain. 

 New associations, dominated by sight sensations, were 

 acquired, and the front-brain grew a lot of new nerve 

 connections in its complicated switchboard. Of course, 

 the new sight-center grew. But so did other centers too. 

 of muscle control, of touch, of association, because clear 

 seeing helped them do their jobs. Only the smell cen- 

 ters (olfactory lobes, etc.) dwindled. In man lliey 



SIGHT CENTERS 

 ■■ IN THE BRAIN 



Mechanism of stereoscopic vision. By 

 branching and crossing of eye-nerves, 

 each half of brain sees through cor- 

 responding halves of both eyes. 



Drawing after J. R. Angell. 



have completely lost their dominating importance. To- 

 day we are predominantly eye-minded. 



In the very highest mammals an additional eye-power 

 developed, — stereoscopic vision, — seeing an object as a 

 solid by viewing it with both eyes at once. It works like 

 the old-time stereoscope that made two flat pictures stand 

 out as solid objects. In most animals, the eyes work 

 separately and can watch on both sides. In man and his 

 recent ancestors, the two eyes look at 

 the one object, converging (sort of 

 cross-eyed) to do it. The closer the ob- 

 ject, the more they converge. Our feel- 

 ing of the convergence then tells us the 

 distance. Try it. Close one eye and try 

 to thread a needle. Open it again and 

 note how it brings out distance. Your 

 tree climbing ancestor, swinging and 

 leaping from branch to branch, had to 

 know his distances, for a miss was as 

 l)ad as a mile. He needed a quick and 

 accurate range-finder, and the new eye- 

 power was just that. 



To do this stereoscopic trick, the eyes 

 needed new brain connections. For one 

 thing, they had to move accurately to- 

 gether and this required better accord 

 in muscular control. But something even 

 more fundamental had to change. Be- 

 fore this, each hemisphere ( side half ) 

 of the brain connected with just one 

 eye and the two images entering through 

 the two eyes had no common brain 

 center in which to blend into one image. 

 With the new eye-power, each hemi- 

 sphere connects with the corresponding 

 halves of both eyes. Each half of the 

 brain therefore sees through both eyes, 

 and the two images merge and become 

 one to the mind. That one image now 

 has, besides height and width, also 

 depth (distance). It appears in three 

 dimensions, looks solid and stands out. You can disting- 

 uish substance from shadow. That helps skilful handling 

 and clear thinking. 



Also some changes had to be made outside on the 

 face. The eyes had to be right out front where they 

 could both see the one object at one time. That meant 

 that the big snout for smelling had to go, for it inter- 

 fered with the view. To your tree-dwelling ancestor 

 this loss did not matter, for he no longer tracked his 

 prey by nosing out the smells they left along the ground, 

 and from his high outlook he could see danger as easily 

 as he could smell it. With eyes front and snout reduced, 

 liis face began to have the human look. Best of all, 

 peering out through those knowing eyes was a growing 

 mind, dominated by clear vision and muscular mastery. 

 It took millions of years to develop, but it was worth 

 it. For it gave man his lead on the animal world and 

 made him the big boss on eartli. 



But to win man needed also touch and muscle con- 

 trol, hearing and sense of balance. Some of the story 

 of how thev grew up together will be told in our next. 



