WHEN DEVELOPMENT GOES WRONG 131 



It must not be assumed, however, that development always 

 proceeds in this regular manner nor that the results are all 

 perfect. A great variety of departures from the usual plan may 

 occur, each with its attendant consequences, some of which are 

 worth mentioning here, not so much for their own sake as to 

 make the student intelligent upon the really complicated proc- 

 esses involved in development, and with which we must reckon 

 in all attempts at improvement. 



Underdevelopment, or dwarfing. In order to produce a per- 

 fect individual, differentiation must not only occur in proper 

 form, but each of the various parts must grow to the normal 

 size. If growth stops short of this point, it is a case of dwarf- 

 ing. The dwarf is like the normal individual except as to size. 1 

 The separate cells of the body of the dwarf are of the usual 

 magnitude, but the number is fewer ; that is, cell division 2 has 

 not continued the normal length of time. 



Nearly all animals are subject to dwarfing, though it is ap- 

 proximately rare, if not unknown, in some. It is common with 

 human beings as well as with horses, dogs, and chickens, in 

 which dwarf species have developed. 



The dwarf is very rare, if not unknown, among cattle and 

 sheep, though, as in all species, a good many individuals are 

 "undersized." The "titman," or "runt," in. the litter of pigs 

 is really a dwarf, the dwarfing process often being due to 

 insufficient food at the start. 



Dwarfing due to this cause will sometimes disappear with 

 improvement in the conditions of life, though, in general, size 

 and development lost in early life are seldom fully restored. 

 Cell division and growth are more rapid at birth than ever after- 

 wards, a steady decline setting in that does not permit full repa- 

 ration for early checks to normal development, — a fact which 



1 So-called dwarfs are often misshapen things, but in these instances other 

 accidents than dwarfing have occurred, as will be shown later. 



2 For a description of cell division in connection with growth, see" Principles 

 of Breeding," pp. 145-152. 



