468 THE GENERAL PRINCIPLES OF ZOOLOGY 



hibited the variation of having these appendages less developed 

 than their fellows possessed an advantage ; natural selection 

 thus resulted in a gradual diminution in the size of the pelvic 

 limbs until they became entirely lost. In the bodies of nearly 

 all animals there are many rudimentary structures which are 

 undergoing the action of panmixia ; some seventy such struc- 

 tures have been noted in the human bod}', which one scientist 

 describes as a " museum of relics " ; they serve to show some- 

 thing of the past history of the organism, just as the silent 

 letters in many words reveal the history of such words. 



The theory of natural selection as accounting for the origin 

 of species is probably accepted to-day by the majority of 

 scientists, although not a few regard it as inadequate, believ- 

 ing that there are still other principles affecting the origin of 

 species which have not as yet been discovered. Some impor- 

 tant objections have been raised to the theory, which demand 

 attention. It has been said that the variations which occur in 

 nature are far too slight to give rise to differences of structure 

 sufficiently great to produce new species. This is an objection 

 of some weight, and yet there are instances which tend to show 

 that under certain circumstances at least it is not valid. An 

 interesting example is furnished by a litter of European rabbits 

 which was placed early in the fifteenth century on the island 

 of Porto Santo. They have multiplied rapidly and have become 

 so altered in structure, color, and habit as to constitute a new 

 species, different from the European rabbit, with which they 

 will no longer interbreed. Here natural selection had full 

 play, but under the condition of isolation, the rabbits being 

 separated from the rest of their species ; and this factor is of 

 great importance in all theories of evolution. 



Another important objection is that there is nothing in nature 

 to prevent free crossing between the various members of a 

 species, and any variation would thus be promptly obliterated 

 through crossing with individuals which did not possess it. We 

 know that domesticated varieties will not breed true if allowed 

 to cross with other varieties. An extension of the factor of 

 isolation just referred to has been brought forward to meet 

 this objection, and is sometimes called the migration theory. 

 As the number of individuals of a given species increases there 



