12 EVOLUTION AND GENETICS 



Weismann appealed to common sense. He made 

 few experiments to disprove Lamarck's hypothesis. 

 True, he cut off the tails of some mice for a few 

 generations but got no tailless offspring and while 

 he gives no exact measurements with coefficients of 

 error, he did not observe that the tails of the descen- 

 dants had shortened one whit. The combs of fighting 

 cocks and the tails of certain breeds of sheep have 

 been cropped for many generations and the practice 

 continues today, because sheep's tails are still long, 

 and cocks still grow combs. 



The Unfolding Princijjle 



NAGELI 



I have ventured to put down as one of the four 

 great historical explanations, under the heading of 

 the unfolding principle, a conception that has taken 

 protean forms. At one extreme it is little more than 

 a mystic sentiment to the effect that evolution is the 

 result of an inner driving force or principle which 

 goes under many names such as Bildungstrieb, nisus 

 formativus, vital force, and orthogenesis. Evolu- 

 tionary thought is replete with variants of this idea, 

 often naively expressed, sometimes unconsciously 

 implied. Evolution once meant, in fact, an unfold- 

 ing of what pre-existed in the egg, and the term still 

 carries with it something of its original significance. 



Nageli's speculation may be taken as a typical 



