io64 LIFE : OUTLINES OF GENERAL BIOLOGY 



the chromatophores directly, as in the frog. But more than one kind 

 of influence may operate in the same animal. (See Hogben, 1924.) 



The regulating of functions by means of hormones (q.v.), the 

 quality of immunity {q.v.) to poisons of frequently recurrent menace, 

 the production of poison as a protection or as an aid in attack, the 

 sensitiveness of phagocytes to intruding microbes, the suscepti- 

 bilities of unicellular organisms to nutritive substances at a distance, 

 the power of the alimentary tract to change foreign proteins into 

 harmless ones, the attraction of the spermatozoon to the ovum and 

 its movements against currents in the oviduct, or in circular orbits 

 around an insect's hard egg-shells which have only one minute 



Fig. 183. 



The Head of a Thigh-bone, split to show the internal architecture, adapted 

 to withstand strains and stresses in different directions. (After Hesse). 



entrance (the micropyle), or but a few — these are among the hun- 

 dreds of physiological adaptations. 



Passing to structural or anatomical adaptations, we admit that 

 the adaptiveness may not be evident until the organ in question 

 enters into activity, so that the demarcation between structural 

 and functional fitnesses is necessarily in many cases vague. The 

 neatly fashioned valves of the heart, which keep the blood flowing in 

 the right direction, are of course adaptive in functioning, but their 

 structural details place them in a different group from the bio- 

 luminescence by which the female glow-worm attracts the attention 

 of a male. The luminescence is more of a functional adaptation ; the 

 valves are more of a structural adaptation, though the proof of their 

 adaptiveness is in their functioning. 



