EECENT CRITICISMS AND HYPOTHESES. 673 



Among dogs may be observed the trick of rolling on some 

 mass having a strong animal smell: commonly a decaying 

 carcase. This trick has probably been derived from the trick 

 of rolling on the body of an animal caught and killed, and 

 so gaining a tempting odour. A male dog vrhich first did 

 this, and left a trail apt to be mistaken for that of prey, 

 would be more easily found by a female, and would be more 

 likely than others to leave posterity. Now such a trick 

 could have no relation to better maintenance of the moving 

 equilibrium, and might very" well arise in a dog having no 

 superiority. If it arose in one of the worst it would be 

 eliminated from the species, but if it arose in one of medium 

 constitution, fairly capable of self-preservation, it would tend 

 to produce survival of certain of the less fit rather than the 

 fittest. Probably there are many such minor traits which are 

 in a sense accidental, and are neither adaptive nor specific 

 in the ordinary sense. 



§ 17 Ah. But now let it be confessed that though all pheno- 

 mena of organic evolution must fall within the lines above 

 indicated, there remain many unsolved problems. 



Take as an instance the descent of the testes in the Mam- 

 malia. Neither direct nor indirect equilibration accounts 

 for this. We cannot consider it an adaptive change, since 

 there seems no way in which the production of sperm-cells, 

 internally carried on in a bird, is made external by adjust- 

 ment to the changed requirements of mammalian life. Nor 

 can we ascribe it to survif al of the fittest ; for it is incredible 

 that any mammal was ever advantaged in the struggle for 

 life by this changed position of these organs. Contrariwise, 

 the removal of them from a place of safety to a place of 

 danger, would seem to be negatived by natural selection. Nor 

 can we regard the transposition as a concomitant of re-equili- 

 bration; since it can hardly be due to some change in the 

 general physiological balance. 



An example of another order is furnished by the mason- 



