224 EVOLUTION OF TO-DAY. 



Variations Eliminated by Crossing. 

 A still more serious difficulty amounts to a dem- 

 onstration that, as originally conceived, natural 

 selection is not adequate to explain the facts. It is 

 the tendency of varieties to disappear by crossing. 

 A reviewer in the North British Review in 1867 

 demonstrated the necessity of assuming that varia- 

 tions appear in many individuals at once, showing 

 that it is mathematically impossible for a single 

 variation to be preserved, no matter how valuable it 

 be. He takes, for an example, the instance of an 

 animal which produces two hundred young, of 

 which only two survive the struggle for existence 

 long enough to produce young. He assumes that 

 one of the two hundred has, by a chance variation, 

 some feature not possessed by the others, and which 

 is highly valuable (it must be remembered that vari- 

 ations, as a rule, are so slight as to be of little use). 

 Now, in the above case, the chance for the survival 

 of this particular individual is not very great. 

 Doubtless this animal would have a better chance of 

 survival than any other individual, but the chances 

 are much in favor of some other of the two hundred. 

 This single individual, while having superior facili- 

 ties, must contend with great numbers, and its 

 chances for preservation are therefore not very great. 

 But even supposing it to survive and produce 

 young, it will, of course, be obliged to mate with 

 some individual not possessing its favorable peculi- 

 arities, and its offspring will inherit the peculiarities 

 in a less marked degree. " It will breed and have a 

 progeny of, say, one hundred. Now this progeny 



