1 86 THE CAUSES OF VARIATIONS. 



Galton " to inherit, bit by bit, this feature from one 

 " progenitor, that from another .... while the 

 "several bits are themselves liable to some little change 

 " during the process of transmission." The conditions of 

 environment, heat or cold, an absence or excess of moisture, 

 change of food supply, or any circumstance which affects 

 the conditions of life of the parent, influence and lead to 

 variations in the offspring, and these variations may be 

 transmitted through successive generations. 



As animals are generated and produced under mutable 

 conditions the variations which arise may take almost any 

 direction. There are limits of course to their extent, as 

 whatever the differences may be which occur they are 

 governed and restricted in the individual by the laws of 

 heredity. But by the same laws, a variation arising in an 

 individual will probably be transmitted to its offspring in a 

 modified or intensified degree, the variation itself varying 

 in the next generation. It may in many cases disappear 

 altogether, or be so modified as to produce no appreciable 

 effect either detrimental or advantageous to its possessor, 

 and may persist in its modified form, or even be emphasized 

 to a certain extent and still remain what may be termed a 

 passive variation. 



But should a modification or deviation take, and persist 

 in, a form which is in any way, however little, prejudicial 

 to the animal or plant in which it arises, it will lead sooner 

 or later, but inevitably, to its destruction. 



On the other hand variations in any direction, whether 

 of colour, size or form ; keenness of sight, scent or hearing ; 

 swiftness of foot, of flight, or of swimming ; or which, in any 

 way, gives the individuals a distinct advantage over their 

 fellows in the struggle for life, will tend to preserve them 

 from a fate which the others may prematurely meet. And 

 these favoured individuals transmitting the advantageous 



