THE LIVING BIRD 37 



there occur tissues and organs which are known as 

 ductless glands, owing to the circumstance that they 

 do not pour their secretions on a free surface by 

 means of a duct but liberate them directly into the 

 blood stream. The internally secreting organs are 

 sometimes collectively referred to as the endocrine 

 system but morphologically they do not compare 

 with other systems, (e.g. the nervous, or muscular) 

 since they are structurally independent units differ- 

 ing greatly from each other in constitution. How- 

 ever, more is continually being discovered about 

 their function and the fact that many of them are 

 intimately correlated physiologically speaking, i.e. 

 in terms of function rather than structure, permits 

 us, perhaps, legitimately to look upon them as a 

 chemical system of co-ordination. Without going 

 into details, suffice it to point out that their effects 

 on the animal organization are various and ex- 

 tremely profound. They may influence not only 

 growth (bones, etc.) and development of various 

 organs (comb, wattles, antlers, etc.) but also devel- 

 opment of the ''mind" and may even control 

 behavior. They are of vital importance to the 

 general welfare of the individual, whether immature 

 or adult, and the upsetting of a single factor may 

 lead to disturbances of the entire system sufficiently 

 serious to cause death. The "active principle" of 



