PROGRESS SHOWN IN EVOLUTION 



ancestor of a new group or type, such being the privilege 

 of biologically balanced or generalized creatures. 



The most important progressive steps in the evolutionary 

 ascent of animal life perhaps deserve mention. Starting from 

 the single-celled type, life made its first great advance 

 through the aggregation of many single cells into a colony; 

 this advance was followed by division of labour for dif- 

 ferent functions among different kinds of cells, which gave 

 new possibilities of size and balanced specialization of func- 

 tion. Next came the organization of the community of cells 

 into a two-layered creature with a mouth at one end, a stage 

 preserved to-day in sea-anemones and their relatives. Then 

 came the intercalation of a third layer, and the development 

 of a centralized (though primitive) nervous system and prim- 

 itive kidneys. Then the development of a blood system, a 

 posterior opening to the digestive tube, better locomotor 

 organs, and elaborated sense organs in a region which might 

 properly be called a head. Leaving all but the vertebrates 

 out of consideration for lack of space, we would next come 

 to the enlargement of the brain, the development of a strong 

 internal skeleton, and then to that of paired limbs. These 

 improvements are followed by partial emancipation from the 

 water, as in the amphibians, then total emancipation, as in 

 the reptiles. Still later we find the attainment of the condi- 

 tion of constant temperature, called warm-bloodedness ; 

 the improvement of the nourishment and care of the young, 

 both before and after birth; and the rapid improvement 

 of memory, associative power, and animal intelligence. 

 Finally, in man, comes the new step in brain power 

 which we call reason — the power of generalizing, and 

 consequently of giving names to things, and so speech, which 

 has brought in its train the other enormously important 

 progressive development, the possession by the human species 



[333] 



