188 The Outline of Science 



actually standing before an apparently inexhaustible fountain of 

 change. This is evolution going on. 



The Sporting Jellyfish 



It is of interest to consider a common animal like the jelly- 

 fish Aurelia. It is admirably suited for a leisurely life in the 

 open sea, where it swims about by contracting its saucer-shaped 

 body, thus driving water out from its concavity. By means of 

 millions of stringing cells on its four frilled lips and on its mar- 

 ginal tentacles it is able to paralyse and lasso minute crustaceans 

 and the like, which it then wafts into its mouth. It has a very 

 eventful life-history, for it has in its early youth to pass through 

 a fixed stage, fastened to rock or seaweed, but it is a successful 

 animal, well suited for its habitat, and practically cosmopolitan 

 in its distribution. It is certainly an old-established creature. Yet 

 it is very variable in colour and in size, and even in internal struc- 

 ture. Very often it is the size of a saucer or a soup-plate, but 

 giants over two feet in diameter are well known. Much more im- 

 portant, however, than variation in colour and size are the inborn 

 changes in structure. Normally a jellyfish has its parts in four or 

 multiples of four. Thus it has four frilled lips, four tufts of di- 

 gestive filaments in its stomach, and four brightly coloured repro- 

 ductive organs. It has eight sense-organs round the margin of its 

 disc, eight branched and eight unbranched radial canals running 

 from the central stomach to a canal round the circumference. 

 The point of giving these details is just this, that every now and 

 then we find a jellyfish with its parts in sixes, fives, or threes, and 

 with a multitude of minor idiosyncrasies. Even in the well-estab- 

 lished jellyfish there is a fountain of change. 



1 



Evolution of Plants 



It is instructive to look at the various kinds of cabbages, such 

 as cauliflower and Brussels sprouts, kale and curly greens, and 





