THE DOGMA OF EVOLUTION 



young which will not only inherit this trait but also 

 have a tendency to have a second additional inch in 

 length. Now we see, at once, that if only a few gi- 

 raffes have this added length, then almost all the 

 others must die or there will be little chance of longer- 

 necked giraffes finding other longer-necked giraffes as 

 mates: or else we must assume that not only a large 

 proportion suddenly varies in this fashion but also 

 this variation occurs simultaneously with this unu- 

 sual scarcity of food. 



Now the mathematical probability that a great 

 proportion of a species will have the same variation 

 at the same time is a very small fraction, and the 

 probability of a scarcity of food being so great that, 

 at many different times, the leaves of the trees are 

 stripped to a height from the ground so uniform that 

 a difference of an inch in length of neck decides 

 whether giraffes may obtain food or die, is also a very 

 small fraction. And the probability that these two in- 

 dependent events will occur at the same time is the 

 product of the two separate probabilities; that is, it 

 is the product of two very small fractions which is a 

 very, very small fraction. For example, if each prob- 

 ability were one-thousandth, which is manifestly far 

 too great, then the probability of both of them occur- 

 ring at the same time is only one-millionth. 



In the first edition of the Origin of Species^ Dar- 

 win, whose attention was fixed on artificial selection 

 where man forces selected individuals to mate, as- 



n 226 3 



