Biological Papers. 125 



much higher forms, such as worms, mollusks, and trilobites. 

 He hence infers that the sponges and coelenterates, the first 

 animals with digestive chambers, appeared on earth before 

 the Cambrian, in the Proterozoic era, at least forty-five million 

 years ago. It seems possible, then, that the stomach-making 

 instinct has required these forty-five million years to evolve a 

 digestive apparatus as complicated as that possessed by the ox 

 and man. 



This digestive chamber was merely a sac with one opening 

 in the coelenterates ; it became a tube with two openings in the 

 worms, if we are to accept as representative of the Proterozoic 

 type of worms the modern, stay-behind examples of these 

 animals; a tube with various enlargements and associated di- 

 gestive glands in the amphibians thirty-five million years 

 later ; and became fully differentiated for animal and vegetable 

 food in the mammals some two million years ago, since which 

 time life has evolved little that is new in the construction and 

 operation of a stomach, unless we add its ability to get out of 

 order in man. 



A system of tubes for conveying blood to remote parts of the 

 body is found now in its simplest form among the worms, and 

 worms of the higher types certainly existed twenty-seven mil- 

 lion years ago, as shown by fossil worm holes and casts found 

 in the Potsdam sandstone of the Cambrian era. These may 

 have had one-cavity hearts, as earthworms do to-day. Fish 

 with two-cavity hearts existed in the Silurian era, over eight 

 million years later; amphibians with three-cavity hearts were 

 abundant twelve million years ago, in the Carbonic era, and 

 primitive mammals with four-cavity hearts were numerous 

 seven million years ago in the Jura-Trias ; since which time the 

 heart-making instinct has not improved much, except in the 

 direction of making larger-hearted individuals. 



Ventral nerve-threads to control the food-digesting and dis- 

 tributing organs probably appeared first in the worms twenty- 

 seven million years ago. This ventral nevve-cord producing 

 instinct is certainly so old that man has no conscious control 

 over this system of nerves, the sympathetic system. 



The dorsal nerve-cord was originated by an instinct which 

 appeared much later in the development of life. As is now 

 known, the dorsal nerve-cord is formed from a dorsal furrow- 

 invagination of the ectoderm of a worm-like animal, and was 

 made primarily to control an increasingly complex system of 



