70 Evolution 



Leaving behind this very varied and interesting world 

 of the one-celled animals, we have to see how the higher 

 (many-celled) animal was evolved. The primitive mi- 

 crobes would tend to cluster together in groups, and live 

 a communal life. Each cell must be in communication 

 with the water to get its food, and the result would be 

 a round cluster of microbes — let us now call them cells 

 — with a hollow interior. In moving through the water, 

 or resting at the bottom, one part of the cluster would 

 be in a better position to take in food than the rest, and 

 would specialise on digestion. The "digestive" part of 

 the ball would tend to sink inwards, until the ball 

 doubled on itself. The edges drew closer together, and 

 at length we get an animal with an inner layer of 

 digestive cells (a stomach), a mouth, and an outer layer 

 of cells more or less sensitive, and armed with cilia for 

 locomotion. We have plenty of examples of this in 

 nature still. 



At this point there were two alternatives for the 

 developing animal. It might attach itself, for security, 

 to the floor of the ocean, and develop arms for reaching 

 out after its food, or an apparatus for making little 

 whirlpools and bringing the food to it; or it might swim 

 about in search of its food. The sponges, polyps, corals, 

 hydrae, and anemones chose the sedentary life, and 

 developed organs of the type suggested. As early as the 

 Cambrian strata we find relics of the coral, sponge, and 

 hydrozoan. The sponges seem to have had a different 

 protozoan ancestor (a Choanoflagellate) from the other 

 higher animals. The lowest specimen (Proterospongia) 

 differs comparatively little from a group of Choanoflagel- 

 lates — there is a good deal of social life even amongst the 

 one-celled animals — and the other types are developed 

 from this. "There is," says Professor Minchin, "no 

 group which so strikingly illustrates the theory of 



