VII 

 THE LOGIC OF EVOLUTION 



WE cannot fail to notice that life is continually rising 

 from lower to higher planes, and that it shows new 

 powers and new meaning from stage to stage. The 

 upward road seems to pass from table-land to table-land, and 

 the transition to be a sudden and rapid ascent. This is prob- 

 ably not the case. We might compare these transitions to 

 the changes at the melting and boiling points in a mass of ice. 

 The rise of temperature may be continuous, but the form and 

 behavior of the mass becomes entirely different and new at 

 certain critical points. 



The progress from a bacterium to an amoeba and thence to 

 a group of cells is great, but we cannot dwell upon it. The 

 second stage is that of the sacklike coelenterate, whose cells 

 are arranged in two layers. It has a digestive cavity, gains a 

 fair amount of food, uses comparatively little for fuel, hence 

 reproduces rapidly. All the surplus goes to the reproductive 

 organs. There are nervous and muscular tissues, and some 

 feeling and locomotion. But these functions play a very small 

 part in the life of the animal. Temporary survival of the 

 individual, existence and continuance of the species is prac- 

 tically all it asks. 



The fiat worm shows us a transition to something higher. 

 It still lives to eat and reproduce. The digestive system has 

 improved, the reproductive organs are exceedingly highly 

 developed and very complex. But muscles, or rather masses 

 of muscular fibrils, nerves and nerve-centers and sense organs 

 have appeared. The animal is moving and feeling its way. 



Certain results of this change are apparent in the highest 



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