A New Tlieory of Heredity. 89 



which are not pcarts of the living substance of the or- 

 ganism, bat which simply owe their existence to the 

 properties of its living snbstance, we can readily under- 

 stand that the complexity of an adult animal may be 

 vastly greater than the complexity of the <igg. 



In the second place we must recollect that there are 

 many race characteristics which are of constant occur- 

 rence without being hereditary. 



Organisms are often greatly modified by the direct 

 action of external conditions; for instance, a tree may 

 be dwarfed by insufficient food, or the muscles of a limb 

 may be greatly enlarged by unusual work. If all the indi- 

 viduals of a species are similarly exposed to conditions 

 of this sort, they will all be acted upon in the same way, 

 and the modification which is thus produced will be 

 characteristic of the species, without being hereditary. 



To take one of the simplest cases: Trees which grow 

 upon mountain-tops, where they are exposed to extreme 

 changes of climate, and to constant and violent winds, 

 have a very characteristic appearance, which is familiar 

 to all mountain climbers. In some eases ttiis peculiar 

 form is hereditary, and persists in seedlings which are 

 grown in more favored regions, but in other species the 

 transplanted trees show, by losing their peculiarities, 

 that these are due to direct modification. 



If a certain species occurs naturally nowhere except in 

 such situations, this species will be characterized by its 

 dwarfed size and bv its twisted and distorted branches; 

 but if individuals reared under favorable influences 

 grow and flourish and become regular and symmetrical, 

 we may conclude that the characteristics of each Avild in- 

 dividual are caused by its scanty food and constant ex- 

 posure, and that they are not represented in the ^gg, and 



are not congenital. 



