78 THE FOUR PRINCIPLES OF SEGREGATION. 



fluctuating variations and culminating in a mutation that becomes 

 stable. 



22. Selection and the Inheritance of Acquired Characters. 



The inheritance of functional variation, if proved in any case, does not 

 prove that selection has had no influence in shaping the characters of the 

 same species. In a recent volume by J. T. Cunningham, entitled 

 "Sexual Dimorphism," a large collection of very interesting facts on 

 the subject of secondary sexual characters has been presented. It, 

 however, seems to me that but little proof has been given of his con- 

 tention that these characters have been produced, not by selection, 

 but by the action of direct stimulation on the individual, facilitated 

 and strengthened in successive generations by an increasing inher- 

 itance of the effects of stimulus in previous generations. Whether 

 acquired {i. e., functional) characters and tendencies can be inherited 

 demands most careful investigation ; but if it is found to be a fact it 

 will not disprove the importance of the different forms of selection in 

 determining the special kinds of response that the function awakens. 

 Hertwig is undoubtedly right when he says that neither selection nor 

 the inheritance of functional variations determines whether a given 

 bee's egg shall develop into a drone, a queen, or a worker, for the 

 determining influence is the power of responding in different ways to 

 different conditions. The question, however, remains as to why the 

 workers of the Italian bee develop in such a way as to produce a long 

 tongue, while certain other bees develop a comparatively short tongue. 

 Hertwig admits that selection has had much to do with this diversity 

 in the powers of response. 



In Prof. C. B. Davenport's two volumes on "Experimental Mor- 

 phology," a large mass of facts illustrating the different responses of 

 organisms to external conditions has been brought together, and in 

 certain cases the effects are found to increase in successive generations 

 when there is reason to believe that the increase is not due to selection. 

 Though the inheritance of acquired characters may be proved by 

 these experiments, I judge that Professor Davenport does not doubt 

 that the different forms of selection have an important influence in 

 shaping the specific characters of the same organisms. 



