ON HEREDITY. 101 



abandon any attempt at a natural explanation. But, in my 

 opinion, there is no reason why natural selection should be con- 

 sidered inadequate to the task. It is true that the objection has 

 been lately urged, that it is inconceivable that all the wonderful 

 adaptations of the organism to its surroundings can have arisen 

 through the selection of individuals ; and that for this purpose an 

 infinite number of individuals and infinite time would be required ; 

 and stress is laid upon the fact that the wished-for useful changes 

 can only arise singly and very rarely among a great number 

 of individuals. 



This last objection to the modern conception of natural selec- 

 tion has apparently some weight, for, as a matter of fact, useful 

 variations of a conspicuous kind seldom appear, and are often 

 entirely absent for many generations. If we expect to find 

 that qualitative changes take place by sudden leaps, we can 

 never escape this difficulty. But, I think, w.e must not look 

 for conspicuous variations such as occur among domesticated 

 animals and plants in the process of the evolution of species 

 as it goes on in nature. , Natural selection does not deal with 

 qualitative but quantitative changes in the individual, and the 

 latter are always present. 



A simple example will make this clearer. Let us suppose that 

 it was advantageous to some species for instance the ancestors of 

 the giraffe to lengthen some part of the body, such as the neck : 

 this result could be obtained in a relatively short time, for the 

 members of the species already possessed necks of varying length, 

 and the variations which form the material for natural selection 

 were already in existence. Now all the organs of every species 

 vary in size, and any one of them will undergo constant and 

 progressive increase, as soon as it acquires exceptional usefulness. 

 But not only will the organ fluctuate as a whole, but also the 

 parts composing it will become larger or smaller under given con- 

 ditions, will increase or diminish by the operation of natural selec- 

 tion. I believe that qualitative variations always depend upon 

 differences in the size and number of the component parts of 

 the whole. A skin appears to be naked, when it is really covered 

 with a number of small fine hairs : if these grow larger and increase 

 in number, a thick covering is formed, and we say that the skin 

 is woolly or furry. In the same way the skin of many worms and 



