252 THE INDUCTIONS OF BIOLOGY. 



among classes of people who tax their brains much. Among 

 these classes, we daily see this constitutional modification 

 produced by excess of function, in men whose progenitors 

 were not nervous ; and the children of such men habitually 

 inherit more or less of the modification. 



§ 83. Two modified manifestations of Heredity remain to be 

 noticed. The one is the re-appearance in offspring, of traits 

 not borne by the parents, but borne by the grandparents or by 

 remoter ancestors. The other is the limitation of Heredity by 

 sex — the restriction of certain transmitted peculiarities to 

 offspring of the same sex as the parent possessing these 

 peculiarities. 



Atavism, which is the name given to the recurrence of 

 ancestral traits, is proved by many and varied facts. In the 

 picture-galleries of old families, and on the monumental 

 brasses in the adjacent churches, are often seen types of 

 feature that are still, from time to time, repeated in members of 

 these families. It is matter of common remark that some con- 

 stitutional diseases, such as gout and insanity, after missing a 

 generation, will show themselves in the next. Dr Strutters, 

 in his above-quoted paper on " Variation in the Number of 

 Fingers and Toes, and of the Phalanges, in Man," gives cases 

 of malformations that were common to grandparent and 

 grandchild, but of which the parent had no trace. M. Girou 

 (as quoted by Mr Sedgwick) says — " One is often surprised 

 to see lambs black, or spotted with black, born of ewes and 

 rams with white wool, but if one takes the trouble to go 

 back to the origin of this phenomenon, it is found in the an- 

 cestors." Instances still more remarkable, in which the re- 

 moteness of the ancestors copied is very great, are given by 

 Mr Darwin. He points out that in crosses between varieties 

 of the pigeon, there will sometimes re-appear the plumage of 

 the original rock-pigeon, from which these varieties descend- 

 ed ; and he instances the faint zebra-like markings occasion- 

 ally traceable in horses, as having probably a like meaning. 



