July. 1929 



E \- O L U T I 



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E~.''y Devonian Land Plant: (a) Spore Upper Devonian 

 Cases, lb) Spine-like Leaves, (c) Seed-Fern 



Wood-Cells of Stem 



Devonian period notable advances were made, such as 

 the development of woody stems for support (with 

 their water conducting "pipe-lines"), and true leaves. 

 From the simpler woody-stemmed, but quite leafless 

 ancestors of the ferns in Devonian strata, we can trace 

 the steps to the forests of giant Qub Mosses and 

 Horsetails of the fern family found so abundantly in 

 the Carboniferous coal measures. Already in Upper 

 Devonian times, seed-ferns had developed from the 



Tiji-ical Forest of Carboniferous Giants 



true ferns, and these seed-ferns through a definite 

 fossil series gave rise to the modern cone-bearing 

 Pines and other Gymnosperms. The flowering plants 

 (Angiosperms) first appear in the middle Cretaceous 

 only eighty million years ago. The fossil remains of 

 our modern major plant groups therefore appear in 

 the same geological order as the complexity of allied 

 present forms would demand. The fossil record con- 

 vincingly establishes the fact of plant evolution. 



How the Shark Gave Man His Teeth 



By MAYNARD SHIPLEY 



AS the sharks are the lowest family of true verte- 

 brates in the line of man's ascent, it is not sur- 

 prising that man inherited the shark's teeth along with 

 his spinal column, which in the shark's case is made 

 up of segments of cartilage. 



Perhaps this article should have been entitled, "How 

 the Shark Got His Teeth." As all the higher verte- 

 brates got their teeth from the shark, what we need 

 to learn is where he got the teeth he bequeathed to us. 



We know no animal before the shark that had true 

 teeth, such as could develop, through the ages, into 

 those of the apes and man. So the earliest sharks could 

 not inherit their teeth but had to develop them. 



But everything in this world has developed from 

 something pre-existing. There could not have been 

 land animals with lungs unless there had been fishes 

 with s\vim-l)ladders ; and if reptiles had not had scales, 

 the birds that followed would not have had feathers. 

 And so, to get back to teeth, had there been no fish 

 with tooth-like (placoid) scales, there might have been 

 no fish or other animals with teeth like ours. 



It happened some four hundred million years ago 

 in Devonian times, when much of the interior United 

 States was an inland sea. Abundant fossils found there 

 tell us the waters were prolific with sharks, especially 

 with one family which the scientists give the jaw- 

 breaking name Cladoselachidae. There still flourish, 

 along the Atlantic Coast, sharks closely akin to them. 

 These modern survivors have also been given a name 

 to tax the articulating mechanism of the tired business 

 man, l)ut we shall call them dog-fishes for short. It 



was the ancestor of the spiny dog-fish who developed 

 real teeth, all over his body, from his placoid scales. 



"But," someone objects, "teeth don't grow all over 

 the body— thank heaven!" No, but the early Cladose- 

 lachian grew his scales all over the body and right 

 over the snout and into his mouth where they became 

 teeth. In fact these scales were real teeth. For if you 

 could conveniently coax a shark ashore, extract one 

 of those teeth and compare it under the microscope 

 with one of your own, this is what you would see : 



These teeth within the shark's mouth are of course 

 greatly modified, but in origin and structure they are 

 similar to the scale spikelets all over the body. Be- 

 ginning with the horny layer that covers the spikelet, 

 we come next to a layer of tall, column-like cells, set 

 at right angles to the surface of a lower layer of 

 genuine enamel— in fact these columnar cells secrete 

 the enamel— the hardest substance in animal bodies. 

 In the higher animals, this enamel is only found as the 

 covering of the dentine of the teeth. The dentine of 



Spikelet Teeth, (Shagreen Denticles) of Modern Sliark 

 Breaking Through Skin of Mouth 



