DARWIN S AND SPENCER'S THEORIES 199 



2. Hypotheses as to Possible Mechanism of Transmission. 



In the second place, attempts have been made to construct 

 hypotheses by aid of which we might conceive how a modifi- 

 cation of, say, the skin, can exert a specific or representative 

 influence on the germinal material. 



Thus, Darwin suggested his provisional hypothesis of pan- 

 genesis, according to which the parts of the body give off gemmules 

 which pass as samples to the germ-cells. But his suggestion 

 remains a pure hypothesis and an unnecessary one unless new 

 facts come to light and is nowadays maintained by no one 

 except in extremely modified form e.g. in the Pangen-theory 

 of De Vries. 



Spencer deserves credit for at least facing the difficulty of 

 conceiving a modus operandi whereby a particular modification 

 in, say, the brain or the thumb, can specifically affect the ger- 

 minal material in such a way that the modification or a tendency 

 towards it becomes involved in the inheritance. Briefly stated, 

 his theory is as follows : 



Spencer's Theory of the Mechanism of Transmission. Spencer 

 made the legitimate postulate that, intermediate between the 

 biological unit or cell and the chemical molecule, there were " con- 

 stitutional units," the vehicles of specific characters, ancestral and 

 parental traits, and the individual peculiarities of the organism 

 itself. 



He supposed that they were very stable in their " fundamental 

 traits," but plastic as regards their " superficial traits." 



He supposed that they had " such natures that while a minute 

 modification, representing some small change of local structure, is 

 inoperative on the proclivities of the units throughout the rest of 

 the system, it becomes operative in the units which fall into the 

 locality where that change occurs." 



He supposed " an unceasing circulation of protoplasm throughout 

 an organism," such that, " in the course of days, weeks, months, 

 years, each portion of protoplasm visits every part of the body " a 

 wild assumption. 



Finally, " we must conceive that the complex forces of which 



