WORMS TO VERTEBRATES 79 



possible or improbable. " No thoroughfare " is often 

 written across paths previously followed by blood or 

 nervous impulses, when other paths have been found 

 more economical or effective. 



But where did the notochord come from ? I do not 

 know. It always forms in the embryo out of the en- 

 toderm or layer which becomes the lining of the intes- 

 tine. Now this is a very peculiar origin for cartilage, 

 and the notochord is a very strange cartilage even if 

 we have not made a mistake in calling it cartilage 

 at all. My best guess would be that it is simply a 

 thickened portion of the upper median surface of the 

 intestine to keep the "balls'" of digesting nutriment 

 or other hard particles in the intestine from " grind- 

 ing " against the nerve-cord as they are crowded along 

 in the process of digestion. Once started its elasticity 

 would be a great aid in swimming. 



Professor Brooks has called attention to the fact 

 that the higher a group stands in development, the 

 longer its ancestors have maintained a swimming life. 

 Thus we have noticed that the sponges were the first 

 to settle ; then a little later the mass of the coelen- 

 terates followed their example. But the ctenophora, 

 the nearest relatives of bilateral animals, have re- 

 mained free swimming. Then the flat worms and 

 mollusks took to a creeping mode of life, while the 

 annelids and vertebrates still swam. Then the anne- 

 lids settled to the bottom and crept, and all their de- 

 scendants remained creeping forms. The vertebrates 

 alone remained swimming, and probably neither they 

 nor their descendants ever crept until they emerged 

 on the land, or as amphibia were preparing for land 

 life. If this be true, it is a fact worthy of our most 



