8 PRINCIPLES OF BREEDING 



of heredity which fall naturally into two categories : (a) 

 the doctrine that variations are due, as a general rule, to the 

 immediate and direct action of the environment on the 

 germ-plasm, and (fr) the doctrine that they arise as a rule 

 'spontaneously.' The two doctrines, as commonly formu- 

 lated, are not entirely exclusive. Most biologists admit the 

 existence of both, but differ greatly in the relative im- 

 portance they attach to the two factors. Some biologists 

 believe that the environment does not now influence 

 the germ-plasm, but that it did so in a very distant past. 

 Useful variations in proportion to their usefulness are pre- 

 served and, in succeeding generations, are accentuated by 

 Natural Selection. Of the useless variations some may be 

 rendered latent, but the vast majority are immediately planed 

 away by reversion directed by bi-parental reproduction the 

 intermixture of somewhat similar germ-plasms." 



In discussing the theory of recapitulation, Reid says: 

 "The development of every individual, except for his own 

 variations, is a complete recapitulation of the parents' 

 development, but it is not a complete recapitulation of the 

 life-history of the race. No child could develop unless there 

 were recapitulation ; but no race could undergo extensive 

 evolution unless the recapitulation were incomplete and 

 inexact. . . . ' As it is, the dim and fleeting resemblances to 

 lower animals, displayed by the embryos of all higher types, 

 present the strongest evidence of the truth of the doctrine of 

 evolution that exists in the whole range of science ' (Roi/iniics\ 

 We have here a real history retold in every generation with 

 the additions and omissions made by the preceding generation 

 a history which ever grows longer with the lapse of years, 

 and ever more and more inaccurate and incomplete in its earlier 

 parts. . . . The theory of recapitulation is essentially physio- 

 logical. It supposes that under fit conditions the embryo 

 tends to develop by repeating the life-history of the race, thus 

 growing into an organism much like the parent form. No 

 attempt, impossible of verification in the present state of our 

 knowledge, is made to pry into the morphological details of 

 the germ-plasm. . . . All the organs and tissues of which an 

 individual is compounded possess the power of independent 

 variation. Every single cell may possess this power. . . . All 

 evolution in shape and function depends on this power of 

 independent variation ; without it, descendants might become 

 larger or smaller than ancestors, but otherwise would remain 

 exact copies. In studying heredity our attention is apt to be 

 distracted by the multitude of variations, and the consequent 

 complexity of the subject, from the main fact of development, 



