n8 THE GROWTH OF GROUPS 



the distinction between species and varieties. Sports, 

 common enough in the animal world, are not quite like the 

 mutants of (Enothera. Any class of animals examined over 

 a wide area is seen to contain sports and groups of sports 

 at all times. There is little evidence of periodic mutation 

 among animals. A single case suffices to illustrate this. 

 How is it that Lingula has been exempt from mutation, 

 if that process is periodic ? Thus the zoologist may 

 criticise De Vries' opinions, and has done so in certain 

 instances without remarking, perhaps even without 

 noticing, that his criticism is not directed against the 

 Mutation Theory, as denned by its author. 



The teachings of De Vries concerning the elementary 

 nature of species, the distinction he draws between species 

 and varieties as well as the idea of periodic mutation 

 seem to be distinct from the Mutation Theory as denned 

 by him. In that theory he has pointed out to us that 

 living things possess many of their attributes as separate 

 entities. Since this fact has been pointed out, some of 

 us can see it plainly ; so plainly that we feel it must be 

 for long a guiding principle in biology and even in daily 

 life. 



Let us pass now to another subject. It is difficult to 

 compare the higher animals and plants, since the latter 

 are comparatively simple in structure, and do not move 

 from place to place, hence it will be more satisfactory to 

 compare what we have seen among the rats with similar 

 observations made in other parts of the animal kingdom. 

 Few persons have attempted to make a general survey 

 of a generic group of animals as it lies spread over a wide 

 area. Fortunately, one such attempt has been made 

 with great success. I refer to Mr. W. L. Tower's work 



