NATURAL SELECTION 389 



legs and that any decrease in their size, which would be enhanced bv 

 disuse, would be of advantage. This might seem reasonable during 

 the main period of limb reduction, but, after the limb is reduced to a 

 subcutaneous rudiment, there could be little advantage in carrying 

 the rudimentation still farther. Some whales have the hind limbs 

 much more profoundly reduced than others, although they are all 

 thoroughly out of the way and involve no hindrance in swimming. 

 Any number of similar cases of the same kind might be cited. Darwin 

 had no explanation to offer except a resort to Lamarckism; but 

 Weismann, the ablest neo-Darwinian, offered the theory of panmixia 

 to cover this objection, a theory which is mentioned in chapter i and 

 will be discussed later. 



11. It is objected that, unless favorable variations occur in a large 

 number of individuals at the same time, the character would be 

 swamped out by intercrossing with individuals not possessing the 

 favorable variation. The probability that such a swamping-out 

 would occur was shown mathematically by various critics. By way 

 of answer to this objection there arose a number of "isolation theo- 

 ries," according to which favorably varying individuals would be 

 protected from back-crossing with the non-varying individuals. We 

 might also point out that the Mendelian laws of dominance and 

 segregation would serve to prevent loss of any new favorable character. 



12. It is objected that natural selection might explain the "sur- 

 vival, but not the arrival, of the fittest." But Darwin met this 

 perfectly when he said: "Some have even imagined that natural 

 selection induces variability, whereas it implies only the preservation 

 of such variations as arise and are beneficial to the being under its 

 conditions of life." 



13. Criticism has been directed against natural selection because 

 of the fact that some of the supporters of Darwinism, notably Weis- 

 mann, have made the claim that natural selection is the sole cause of 

 evolution. This idea of the Allmachi or all-sufficiency of natural 

 selection was not Darwin's, as is clear from the following statement: 

 "lam convinced that natural selection has been the most important, 

 but not the exclusive means of modification." 



14. It is objected that many, if not most, of the fluctuating varia- 

 tions with which Darwinism deals are purely quantitative or plus- 

 and-minus variations; whereas the differences between species are 

 qualitative. This is a serious objection and difficult to meet, yet a 

 fair defense has been formulated by leading neo- Darwinians. 



