GREAT STEPS IN ORGANIC EVOLUTION 869 



centre (combining a succession of activities into an effective 

 sequence). Given a ganglion, there is greater efficiency in storing, 

 shunting, combining, and inhibiting nervous impulses; given a brain, 

 there is the beginning of a higher grade of behaviour, and the 

 beginning of the emergence of mind. The centralising of the nervous 

 system, often re-effected, was the main method of progressive 

 integration of life ; and it evolved hand in hand with improvements 

 in the sense-organs. 



Step by Step. — But our object in this book is merely to illus- 

 trate and suggest, and we can only select here and there from the 

 long series of steps of great evolutionary significance. 



How important it was when a body-cavity fluid was replaced or 

 supplemented by a definite blood circulating in vessels ! The blood 

 forms a common medium of the body from which all the cells take 

 and to which they all give; it carries digested food and captured 

 oxygen to the tissues; it sweeps away carbon dioxide and other 

 poisonous forms of waste ; it distributes phagocytes and anti-toxins 

 and hormones; it secures a chemical balancing of the salts in the 

 body — salts whose history leads us back to the composition of a 

 pre-Cambrian sea. 



What acquisitions were implied in having a segmented or meta- 

 meric body and in gaining paired limbs ! How interesting are the two 

 or three hints among Invertebrates of hormone-making cells and 

 the momentous specialisations of these in the various endocrinal 

 glands of Vertebrates ! 



One of the greatest steps was the origin of Vertebrates — a step 

 still wrapped in almost complete obscurity. What a series of 

 advances they have made in hundreds of millions of years ! After the 

 supporting skeletal axis came the skull, and after the skull the jaws 

 and the paired limbs. Long afterwards we discern in pioneer 

 amphibians the first digits and lungs and voice. 



How momentous the adventure — several times repeated — of 

 leaving the water for the dry land, and leaving terra firma for the 

 air and the trees ! Of far-reaching importance also were the several 

 tentatives towards viviparity, which eventually found its best 

 expression in mammals. 



But take two or three instances in more detail to illustrate the 

 value of adding to the more familiar and conventional retrospect 

 of racial evolution an envisaging of the great steps and their 

 consequences. 



SOME OTHER GREAT ADVANCES IN ORGANIC 



EVOLUTION 



THE ORIGIN OF LAND PLANTS.— The more one thinks about 

 the conquest of the dry land by adventurous animals of aquatic 



