142 THE BIOLOGY OF BIRDS 



metabolism ; or those proceeding from the reproductive organs 

 which have to do with the development or non-development 

 of secondary sex-characters, or with the preparation of the 

 mother for the child both before and after birth. The 

 complexity of the situation is increased by the fact that 

 one organ of internal secretion may produce more than 

 one hormone, and that there may be reciprocal inter- 

 relations between glands, so that they corroborate or 

 counteract one another. It is difficult to answer the 

 question : What, if anything, is the common characteristic 

 of hormones ? They vary greatly in chemical composition ; 

 they have specific effects, but except when a structural 

 result ensues these effects do not last unless more hormone 

 is produced. They are not ferments, yet they operate in 

 minimal quantities. Perhaps their only common charac- 

 teristic is the negative one, that they never evoke anti- 

 bodies or counteractives, as many substances do when 

 introduced into the blood. 



To the evolutionist there is much about hormones that 

 is interesting, {a) One type of animal sometimes differs 

 from another in the length of different areas on its life- 

 curve, one having a long-drawn-out senescence, and another 

 a telescoped juvenility ; and we can see, as Professor Arthur 

 Dendy points out, how this might be fixed in the course of 

 ages by variations in the activity of the ductless glands at 

 different periods of life, (b) It has been shown that a 

 strengthening of the musculature of the legs, e.g. in mountain- 

 climbing or in dancing, may be correlated with an increase 

 in the strength of the muscles of the arms. This probably 

 means that some chemical messenger, exciting to the 

 formation of muscle substance, is distributed throughout 

 the system by the blood. And this, as has been suggested 

 by Mr. J. T. Cunningham and others, opens up the possi- 

 bility that specific hormones produced in the establish- 

 ment of individual modifications as the direct result of 

 peculiarities of function and environment, might affect 

 the reproductive elements in a representative manner, and 

 thus likewise affect succeeding generations, (c) As to the 



