CHAPTER IX. 



Causation of Structure (continued) — Heredity at Corresponding Ages — Heredity 

 at Corresponding Seasons — Sexual Heredity — Some Comparisons between 

 Man and Woman. 



Having thus briefly touched upon the important subject 

 of vital rhythms, we must now pass in review some of the 

 special facts of heredity. I shall first consider the principle of 

 heredity at corresponding ages ; next, sexual heredity. I shall 

 then allude to the principle of, what I shall term, " structural 

 mean." Whether or not I am right in saying that the off- 

 spring tend towards a certain structural mean between the 

 two parents, which mean would never be departed from, were 

 they all exposed to exactly the same E, we shall, I think, find 

 this to be a very good working hypothesis. While dealing 

 with this blending of the two parental tendencies into a certain 

 mean, I shall naturally be led to speak of pathological and 

 physiological blendings — that is to say, of the blending in 

 the offspring of certain tendencies derived from the parents : of 

 disease tendencies with other disease tendencies, or with physio- 

 logical peculiarities ; and under this head it will be convenient 

 to touch briefly upon the subject of crossing. I shall next 

 consider the " fixity of structural characters " — namely, the 

 degree of tenacity with which they cling to a race from 

 generation to generation, or, when acquired by an individual, 

 the fixity of the acquisition in the individual himself. The 

 principle of reversion will next come in for consideration. 

 This subject will suggest a few remarks on potentiality. I 

 shall then notice the influence of the male element upon the 

 female, and finally, the inheritance of acquired characters. 



I. Heredity at Corresponding Ages. — The fact that the 

 developmental and other changes occur at the same rate in the 

 offspring as in the parents, leads to the reproduction of similar 



