THE WONDER OF LIFE 577 



and not blind, as it was to begin with. Another great 

 invention was the blood itself, a fluid tissue, transporting 

 digested food, carrying oxygen and carbon dioxide, drain- 

 ing away the nitrogenous waste, and distributing the 

 regulative hormones produced by organs of internal 

 secretion. 



Looking backwards we see that there has been a wonder- 

 ful twofold progress in differentiation and in integration. 

 On the one hand, bodies have become more complicated ; 

 on the other hand, more unified and controlled. In parti- 

 cular, we see that life has become richer and freer as the 

 nervous system became more complex and more unified. 

 A fresh start was doubtless made when backboned animals 

 emerged, it is difficult to say whence or how, for with them 

 the possibilities of a distinctly higher life began, with more 

 intelligence and less instinct, with more mastery of the 

 medium. We think of birds, of mammals, and of man ; 

 of the detailed colonization of the earth and the exploitation 

 of its resources ; and of the consummate adaptations 

 seen at every turn. 



One of the big impressions is the gradual emergence of 

 nobler forms of life. Millions of years passed before any 

 backboned animals appeared. The earliest fossil fishes 

 are obtained from Silurian strata ; the first Amphibians 

 are much later in the Carboniferous ; the Reptiles 

 probably began in the Permian ; the oldest known bird, 

 Archseopteryx, is Jurassic. Some races reach their climax 

 and begin to wane, but if we take the Vertebrate series, 

 we may say in general terms that the rock-record reveals 

 a slowly increasing perfection. 



To refer to a concrete detail, it is strange to think of 

 the fact that it was not till millions after millions of years 



PP 



