No. 554] ADAPTATION AND THE PHYSIOLOGIST 100 



tion of increasing adaptability or increasing perfection 

 of irritability. This law may be put by the side of the 

 law for the evolution of universes : all spontaneous change 

 is in the direction of increasing entropy. 



It is not by form, by color, by increasing complexity or 

 simplicity, that animals may be classified in the order of 

 their evolutionary appearance. It is by this property of 

 adaptability and this alone. At the summit is man ; now 

 consciously attempting to carry on what nature has been 

 unconsciously attempting these millions of years, and to 

 secure mastery of his environment. Below him are the 

 other placenta! mammals of lower intelligence; beneath 

 them the marsupials, less adaptable than the mammals, 

 because of lower brain power ; then the reptiles independ- 

 ent of water, but not of temperature ; the amphibia, only 

 partially independent of water, but not of temperature; 

 the teleosts able to live in salt and fresh water; the 

 selachians, most without osmotic control and limited to 

 the sea ; the arthropods living on land and sea, but depend- 

 ent on temperature, food and climate, cramped by an 

 external skeleton, and with the fatal defect of running 

 the alimentary canal through the nervous system, so that 

 for higher brain power, either a new nervous system or a 

 new alimentary canal would be needed; lower still the 

 molluscs and annelids, closely limited to their environ- 

 ments ; and last the echinoderms and protozoa. No adap- 

 tation or power of the body has been so consistently 

 attacked by natural selection as this ; and it is this prop- 

 erty which seems to have been the determining factor in 

 the general course of evolution and to have determined 

 the steady development of the psychic powers. 



I come now to the second part of my subject, namely, 

 correlation. By the first part I have attempted to show 

 that the selection of variations in adaptability is respon- 

 sible for at least a part of the steady progress in one 

 direction of many kinds of animals; and explains that 

 unity of progress which has been one of the main causes 

 for assuming orthogenesis. In this second part of the 



