Page Four 



EVOLUTION 



July, 1929 



the spikelet surrounds a pulp-like cavity and is formed 

 of hardened, bone-like skin tissue which contains no 

 cells. The sensitive connective tissue in the pulp-cavity 

 sends processes into the dentine, the dentine canals. 



Such is the description of the spikelet on the shark's 

 body. But it is also the description, so far as it goes, 

 of the tooth of man, from the enamel down. And if 

 we follow these spikelets over the shark's nose and 

 down into his mouth — being sure, of course, that the 

 shark is well dead — we will find that there they will 

 develop from the skin inside the mouth just as they did 

 on the shark's sides. Through millions of years of 

 variation and natural selection they have gradually 



flattened (some at least) into blade-like cutting teeth 

 with serrated, or saw-like, edges. Often, too, several 

 teeth are fused into one, developing from a deep fold 

 of the skin lining the mouth. 



In mammals there is also a deposit of cement on the 

 root and sometimes also on parts of the crown. But in 

 no animals do the teeth form any real part of the bony 

 framework. Like hair, fur, scales, feathers, horns and 

 nails, they originated in and developed from the skin. 

 And in all animals higher than the shark, the teeth, 

 however profoundly modified by millions of years of 

 evolution, still bear clear traces of their descent from 

 the tooth-like scales of a family of Devonian sharks. 



Brains — How Come? 



Bv ALLAN STRONG BROMS 

 VII 



MAN brags about his gray-matter. Rightly so, for 

 it's his big asset. But the other animals, apes 

 especially, have a lot too, only not enough. Gray-matter 

 is the switchboard of man's nervous and thinking out- 

 fit, and he has won over the other animals by adding 

 a lot to the board. 



Under the microscope, gray-matter shows up as a 

 myriad nerve-cells bristling with fine branches. These 

 branches make the contacts for plugging in connec- 

 tions on this most complicated of all switchboards. 

 Complicated ! Man has nine thousand million nerve- 

 cells in his brain alone, each with a lot of branches. 



Another kind of nerve stuff, the white-matter, con- 

 sists of long fibres, really long-distance wires to and 

 from the gray-matter nerve-cells. Their business is to 

 carry messages in and out, fact messages inward from 

 sense organs (eyes, ears, nose, tongue, skin) and or- 

 ders outward to muscles and glands that do the body's 

 work. 



An unborn human baby starts off with a brain and 

 nervous system consisting of a mere tube or cord run- 

 ning the length of its body and tail. Some early fish 

 ancestor had no more, just a simple spinal nerve cord 

 to keep head and tail in touch and working together. 

 But the head end, with its mouth, nose, eyes and many 

 contacts, had much business to keep in order, so its 

 end of the nerve cord swelled into a brain-knob, a 

 central headquarters. As time went on, headquar- 

 ters made new outside contacts, through ears for in- 



"Kadcus-''' 



of nerve cell 



"Dendritus , or 

 BrMxches 



€.hrc 



White. 



jxif cvlinder process 



, - "-Brazvchcj of nzn^c fihrz 



stance, and took on new jobs, such as talk. So the 

 l)rain-knob grew big and complex and expanded in 

 new directions. And the baby brain, in its few months 

 of growth, sums up this evolution of our ancestral 

 brains which gave man such a swell head. 



But the brain did not take on all the jobs. Some 

 were just local and so simple that the gray-matter down 

 in the spinal nerve cord could better handle them. 

 When your finger touches fire, a white nerve brings in 

 the alarm. Usually the local spinal-cord station in 

 charge of your arm takes care of it, shoots out an 

 order over a second white nerve to "take it away," and 

 the muscles do just that. Meanwhile a third white 

 nerve relays the alarm to headquarters and you feel it. 

 Of course, if it's a real four-alarm fire, headquarters 

 takes charge, to do some tall cussing, put out the fire, 

 or yell for help. But usually the alarm is just reported 

 with the good word, "already handled." This local. 

 automatic handling, just in and out, just sensing and 

 doing, is called reflex action. The lowest animals de- 

 .pend on it ; even we use it a lot. 



Gray-matter at headquarters is spread out thin over 

 the wrinkled surface of the big top brain we call the 

 cerebrum. Really in sections, each with its own busi- 

 ness to handle, the gray-matter all looks alike. Within 

 each section the short nerve-cell branches make local 

 hook-ups, but between sections long-distance white 

 nerves make the connections. These nerves are inside 

 and behind the big switchboard, masses of white fibres, 

 some joining the board parts together, others bundling 



Grey maitw^ 

 Whjte medier^ 

 Dorsal root of-- 



Spiiud, 



CoTiral c^nal 



SKIK 



Typical Necve-Cell or Neurone 



Ventral roat^ 

 oi spirit nervt: 



SYNRPStS or 

 HOOK-UP 



Cross-Section of Spinal Cord with 

 Diagram of Simple Reflex Action 



MUSCLE 



