REPRODUCTION AND SEX 



489 



logist admits that in the individual any irritation or stimulation 

 regularly repeated produces some definite physiological effect, some 

 local and special change of tissue in the way of either growth or 

 absorption, enlargement or decrease, or change of shape. Thus not 

 only hypothetically at some former time, but actually at present in 

 every individual, the unisexual organs or appendages are subjected 

 in their functional activity to special strains, contacts, and pres- 

 sures, that is, to stimulation, which must and does have some physio- 

 logical effect on their development and mode of growth." To explain 

 the restriction of sex-characters to one sex, to the period of maturity, 

 and often to one period of the year, Cunningham supposes that 

 "heredity causes the development of acquired characters for the 



Fig. 70. 



Sex Dimorphism illustrated by the Trachea (TR) or Windpipe in the Male 

 (M) and Female (F) of a Bird of Paradise {Phonygammus gouldi) The 

 trachea grows down into the breastbone or sternum (ST), and is coiled 

 many times in the male (A), but only slightly in the female (B), After 

 Pavesi. 



most part only in that period of life and in that class of individuals 

 in which they were originally acquired". Unisexual characters are 

 largely of the nature of excrescences which originated from mechani- 

 cal or other irritation in the male or the female at particular times 

 and in particular states of body. They are now part and parcel of 

 the inheritance, but they are not expressed in the body except in 

 association with physiological conditions the same as those under 

 which they were originally produced. 



Cunningham seeks to show that sex-characters may be inter- 

 preted as the hereditary outcome of special irritations. The legitimacy 

 of this interpretation depends (i) on the experimental evidence that 

 can be adduced to show the origin of callosities, excrescences, 

 proliferations, etc., as the direct result of stimulation, and (2) on 

 the case that can be made out, on experimental or logical grounds. 



