of the Vertebrate Skeleton. 279 



of special parts in accord with the conditions of mechanical 

 causation which we found to characterize regions of the body. 



Finally, it will conduce to clearness to show the kind of 

 way in which structures are inherited, so as to get the results 

 of persistent modifying causation accumulated. The indivi- 

 duals of that common bond called a species, though nearly 

 resembling each other, as is well known, have differences so 

 marked that it is rare for the eye to be unable to distinguish 

 them ; so that the variation of a species is enormous : and if 

 this variation, instead of being in a multitude of different 

 directions, be in any manner caused to be chiefly of the same 

 kind, obviously the mere summing of the variation will 

 produce most extensive differences. Here it is necessary to 

 remark that in every family there may be seen two kinds of 

 variation among the children, — first, that which depends upon 

 the individual peculiarities of nutrition, and which gives a 

 different aspect to brothers and sisters, and then that kind of 

 inheritance by which the child reproduces the mental and 

 physical form and distinctive peculiarities of the parents. And 

 when the variation in nutrition coincides with the distinctive 

 peculiarities in inheritance, these latter will be specially in- 

 tensified. And it is found by experiment that the accumula- 

 tion of characters by inheritance has an influence in foetal 

 development by which parts may be multiplied. It is proba- 

 ble that the epiphyses of bones thus take their origin ,• and it 

 is certain that increase in the number of vertebrae is thus in- 

 stituted. Upon this point Mr. Charles Darwin's observations 

 upon pigeons are specially instructive. Pigeon-fanciers have 

 gone on selecting pouters for the length of their bodies ; and it 

 is found that their vertebrae are generally increased in number, 

 and their ribs in breadth. The tumhlers have been selected 

 for their small bodies ; and their ribs are generally lessened in 

 number. Fantails have been selected for their large, widely 

 expanded tails with numerous tail-feathers ; and the caudal 

 vertebrae are increased in size and number. From which it 

 seems to me evident that the special exercise of a function in 

 life sometimes produces an increase of structure in reproduc- 

 tion, beyond that which was possible to the parent from the 

 plan of its structures. 



The variation from nutrition in reproduction sometimes 

 goes so far that a tissue is dialyzed with its characters so far 

 intensified as to be both unlike the parent and all others of its 

 species. Two cases quoted by Mr. Darwin illustrate this. 

 First, there is Lambert, the porcupine-man, whose skin was 

 covered with warty projections which ])eriodically moulted, 

 and whose six children and two grandchildren were similarly 



