XVII] THE COMPARISON OF RELATED FORMS 1095 



continuous variations are a natural thing, that "mutations" — or 

 sudden changes, greater or less — are bound to have taken place, and 

 new "types " to have arisen, now and then. Our argument indicates, 

 if it does not prove, that such mutations, occurring on a com- 

 paratively few definite lines, or plain alternatives, of physico- 

 mathematical possibility, are likely to repeat themselves: that the 

 "higher" protozoa, for instance, may have sprung not from or 

 through one another, but severally from the simpler forms ; or that 

 the worm-type, to take another example, may have come into being 

 again and again. 



