EVOLUTION 449 



inherited at the same rate as those organs.^ A son 

 inherits to a certain degree the health and strength of his 

 parents ; a brother has to a certain degree the athletic 

 capabilities of his brethren. But should we find this in- 

 heritance maintained if the son or brother were reared in a 

 different environment ? Will the son of the strongest and 

 healthiest man for England be the strongest and healthiest 

 man for India or Equatorial Africa ? The son of phthisical 

 parents may be a weakling here, but a robust man at the 

 Cape. Shortly, change of environment may mean change 

 of the function that such characters as health and fertility 

 are of the organs of an individual, and it by no means 

 follows that parents under one environment and offspring 

 under a second will have the same strength of heredity as 

 when they have a common environment. This I take to 

 be one of the most important unsolved problems in 

 biology, important not only for the theory of evolution, 

 but for the study of the social and economic conditions 

 of an empire like our own, so diverse in the environments 

 of its separated units. Meanwhile the theory of heredity 

 to which we now pass will throw greater light on how 

 such a problem can be successfully dealt with. 



^ 6. — First Notions of Heredity 



We must now proceed to ascertain how a quantita- 

 tive measure may be found for the last great factor of 

 evolution, inheritance. Without heredity no amount of 

 natural, sexual, or reproductive selection would avail to 

 progressively change still less to differentiate living forms. 

 We have already indicated (p. 422) that the offspring do 

 not even in the case of self-fertilisation exactly resemble 

 the parent. In cases of self-fertilisation and of partheno- 

 genetic reproduction, the offspring are not exactly like 

 each other ; they form an array of given variability or 

 standard-deviation, and this array has for mode, mean or 

 type, a value usually divergent from that of the parent. 

 As there is variability in the leaves of one and the same 



1 Philosophical Transac/ioiis, vol. cxcii., A., p. 260. 



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