THE THEORY OF CHANGE OF FUNCTION 55 



certain ones are improvements, are changes for the 

 better, or what we call useful modifications. All 

 the animals possessing these useful modifications 

 will be slightly better off and will have a slight 

 advantage over their companions who have not got 

 them. Now more animals of every species are born 

 than can possibly live. Hence the greater propor- 

 tion of animals that are born die, and die young 

 before they have produced any offspring. It is 

 clear that those animals which have this slight 

 advantage over their brethren will by virtue of this 

 fact have a better chance of living. Now we know 

 as a fact that such variations tend to be inherited 

 i.e., to be handed down from the parents in whom 

 they first appeared to their offspring, and not only 

 so but also that they tend to appear in the offspring 

 in a rather more strongly developed form. 



Hence we get this state of affairs ; certain forms 

 of a particular species, say of deer, tend to survive 

 because they have some slight accidental modifica- 

 tion of their eyes which gives them a slight but 

 distinct advantage over their brethren ; they trans- 

 mit this modification and with it this advantage to 

 their offspring ; certain of their offspring present the 

 modification in a greater degree not only than their 

 brethren but also than their parents. These will 

 get a slight additional advantage and will tend to 

 survive, and so on for successive generations. The 

 gradual accumulation of these slight modifications 

 will in time cause a perceptible change in the 

 structure of the eye, and as each successive modifi- 

 cation is preserved only because it is useful it is 



