THE THEORY OF ORGANIC EVOLUTION. 435 



from use and disuse, such as callosities on the 

 fingers. These do indeed presuppose a constitution 

 capable of being changed, but we can relate each of 

 them (sometimes with certainty, sometimes only with 

 probability) to some definite influence either of func- 

 tion or of environment which has brought about a 

 structural change transcending the limits of organic 

 elasticity. We call these conveniently " modifica- 

 tions." Now, though organic "modifications " may 

 be of much importance to the individuals possessing 

 them, and may serve as a temporary shield for in- 

 cipient variations in the same direction, they are 

 not proved to be of any direct importance in the 

 evolution of the race, for the simple reason that there 

 is no convincing evidence that they can be as 

 such or in any representative degree transmitted 

 to the offspring. 



So far then we have seen that the raw materials 

 of evolution consist of constitutional or germinal 

 variations, and that we are not justified in including 

 modifications or acquired characters because their 

 transmissibility is unproved. Let us now pass to a 

 brief consideration of the secondary or directive fac- 

 tors operating upon the variations which crop up. 



Natural Selection. The first of these directive 

 factors is natural selection, and it is well known that 

 the most distinctive contribution which Darwin and 

 Wallace made to aetiology was to emphasize its im- 

 portance. The theory admits of brief statement. 



Variability is a fact of life, the members of a 

 family or species are not born alike ; some have qual- 

 ities which give them a little advantage both as to 

 hunger and as to love; others are relatively handi- 

 capped. But a struggle for existence is also a fact 

 of life, being necessitated especially by two facts, 



