THEORY OF SEX-DIMORPHISM 75 



L 3 erhaps the most serious difficulty is that 

 some of the modern experimental work in 

 regard to selective breeding is beginning to 

 shake the confidence with which many have 

 held the Darwinian view that marked aug- 

 mentations can be accounted for by the 

 gradual accumulation of minute increments 

 generation after generation. It may be that 

 such selection as is afforded by combats and 

 preferential mating is quite inadequate to 

 build up slight fluctuations into extraordinary 

 differentiations like the stag's antlers and the 

 peacock's tail. 



A LAMARCKIAN THEORY. In his interesting 

 Sexual Dimorphism, J. T. Cunningham argues 

 in support of a Lamarckian interpretation : 

 " In either sex unisexual characters have, as 

 a general rule, some function or importance 

 in the special habits or conditions of life of 

 the sex in which they occur." " But the 

 important truth, which appears to have been 

 generally overlooked, is that in the case of 

 each special organ its special employment 

 subjects it to special, usually mechanical, 

 irritation or stimulation, to which other 

 organs of body are not subjected. Every 

 naturalist and every physiologist 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 hypotheti- 

 cally at some former time, but actually at 

 present in every individual, the unisexual 

 organs or appendages are subjected in their 



