38 HEREDITY 



ance of modifications are doubtless mere coincidences. 

 It is a rather unconvincing explanation of an 

 individual case, perhaps, to call it a coincidence; but 

 such coincidences must occasionally occur, and the 

 probability is that a large proportion of them are 

 recorded. 



It must be said, then, that evidence of the inheritance 

 of mutilations is extremely scanty and unsatisfactory. 

 The extreme rarity of supposed cases, in the first place, 

 must make one somewhat sceptical. If modification 

 inheritance were possible we should expect it to occur 

 in a fair percentage of cases. Of alleged cases, many 

 will not bear close inspection, and in most there is some 

 other explanation at least possible. It seems fair to 

 conclude that such few cases as are not open to objec- 

 tion may probably be coincidences. 



A large group of modifications may be classed as 

 the effects of use and disuse of organs, and it is perhaps 

 with regard to the possibility of the inheritance of such 

 effects that the greater part of the whole controversy 

 has arisen. Many biologists are of the opinion that 

 mutilations, having often but a very slight effect on the 

 general system, will not as a rule be inherited. But 

 the effects of use and disuse, they claim, burrow deep 

 into the whole organisation, and are thus much more 

 likely to affect the germ cells. 



In the process of evolution, many things have occurred 

 which are easy of explanation if we assume that the 

 effects of use and disuse are inherited, and some of 

 them are hard to explain on any other hypothesis. 



The hind limbs of the whale long ago ceased to be 

 used, and they have now all but disappeared. The 

 wings of the ostrich have ceased to be used for flight, 

 and they have become greatly reduced. The remote 

 ancestor of our present-day horse began to walk on the 

 tips of his toes, with the result that the middle toe of 

 the five began to bear most of the weight. Since then 

 the middle toe has become very much larger, and the 

 others have disappeared except for two small rudi- 

 ments. 



