THE EPIC OF EVOLUTION m^y 



instead of one, the possibilities of the offspring of successive genera- 

 tions were more than doubled. 



Take one more illustration of a series of changes that has altered 

 the whole course of subsequent biological events. The bilateral sym- 

 metry of locomotor animals, that is, those having a head end and 

 right and left sides to the body, was preceded by the radial symmetry 

 of attached forms like Hydra, an arrangement making it possible 

 from a point of anchorage to explore the surroundings for food in 

 every direction without the machinery of locomotion. When ani- 

 mals with radial symmetry become free-swimming, like jellyfish 

 and sea-urchins, they go at random in any direction. It is only 

 after one definite part in the circumference of a radially symmetrical 

 animal constantly takes to leading the way that a head end is initi- 

 ated, with a bram center to direct the increasing activities of the 

 changing animal. The connecting hnks in this chain of changes 

 between radial and bilateral animals are to be seen in certain tur- 

 bellarian flatworms, whose fundamental plan is the same as that 

 which would be formed if a radial jellyfish were stretched out length- 

 wise with one horizontal axis elongated, thus forming a polar arrange- 

 ment with head and tail ends. It may be a long call to return thanks 

 to these lowly creatures for discovering the advantages of a head with 

 a directive brain in it, but perhaps it is not too late at least to register 

 oiu' gratitude. 



There is much variation in the degree of plasticity shown by 

 organisms and in the range of changes which they undergo. Some 

 forms, like certain brachiopods and shell-bearing protozoans of the 

 deep sea, are so well fitted to the constant habitat in which they 

 five, where there is no variation of pressure or temperature, and 

 where no day or night intrudes upon their tranquillity, that those 

 living now have not changed perceptibly in appearance from their 

 extremely ancient forebears. 



On the other hand, in the strenuous environment of the tidal zone, 

 where land and restless waters meet, the inhabitants are kept con- 

 stantly busy and alert in matching structural and functional changes 

 with insistent and recurring changes in their surroundings. Thus it 

 is that changes in the environment necessitate continuous adjust- 

 ments on the part of plants and animals. Whatever may happen 

 meantime to the individual actor, the show must go on. This tradi- 

 tion of the dramatic stage is quite as true also for the larger stage 

 of changing life. 



