36 Cunningham, Unisexual Inheritance. 



reference to this character. Yet as I have said there would seem to 

 be some sexual dimorphism in eye colour in mankind. I am not con- 

 cerned however to interpret Professor Pearson's results, but merely 

 to point out that they tend to disprove the assumption that there is a 

 connection between sexual selection and sexual dimorphism, and also 

 to point out that Professor Pearson has failed to show how the 

 causes of sexual dimorphism are to be investigated by his mathematical 

 methods. 



If the theory I have formulated in my book on Sexual Dimor- 

 phism 1 ) were true we should have a complete explanation of all the 

 peculiarities of unisexual characters. That theory is founded on the 

 truth that in a great number of the best known cases the special struc- 

 tures which constitute the unisexual characters are exposed in every 

 generation to definite special stimulations produced by the behaviour 

 of the animals in their sexual relations. The theory is merely that 

 such stimulations cause excessive growth of the tissues affected, in 

 other words, produce local hypertrophy, and that such processes of 

 growth after a number of generations are inherited. The modifications 

 of growth having been set up in mature individuals of only one sex 

 during sexual maturity and activity, are inherited only in correlation 

 with the same condition of the body. If the inherited process of deve- 

 lopment were thus a repetition more or less exact of the process set 

 up by external stimulation, we should have an intelligible explanation 

 of the well known and remarkable peculiarities in the development of 

 secondary sexual characters. For the original process was limited 

 to one sex, and was set up when the testes were functionally active, 

 and when the nervous system was in a state of excitement which 

 affected all the functions of the body. The hereditary repetition would 

 therefore be similarly limited, whether the limitation was merely to 

 one sex, or as in many cases restricted to one season of the year in 

 that sex. Moreover as the original process of growth was associated 

 with the functional activity of the testes, and the nervous excitement 

 produced by that activity, the hereditary repetition would be wanting 

 in an individual from which the testes had been removed, and thus we 

 should have an explanation of the suppression of secondary characters 

 in castrated males, otherwise one of the most inexplicable facts in all 

 physiology. 



Professor Meldola, in criticising my book 2 ), declared that he 

 could see no force in this argument. He stated that there was ab- 

 solutely nothing in the Lamarckian explanation to account for the non- 

 transmission of the male characters to the female. He asked why 



1) Sexual Dimorphism in the Animal Kindom: London, A.&C.Black, 1900. 



2) n Nature u Vol. 63, p. 197. Dec. 27, 1900. 



